the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Ubiquity of human-induced changes in climate variability
- 1Center for Climate Physics, Institute for Basic Science, Busan, South Korea
- 2Pusan National University, Busan, South Korea
- 3National Center for Atmospheric Research, Boulder, USA
- 4Department of Oceanography and International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaiʻi at Mānoa, Honolulu, HI, USA
- 5Korea Polar Research Institute, Incheon, South Korea
- 6Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, USA
- 1Center for Climate Physics, Institute for Basic Science, Busan, South Korea
- 2Pusan National University, Busan, South Korea
- 3National Center for Atmospheric Research, Boulder, USA
- 4Department of Oceanography and International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaiʻi at Mānoa, Honolulu, HI, USA
- 5Korea Polar Research Institute, Incheon, South Korea
- 6Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, USA
Abstract. While climate change mitigation targets necessarily concern maximum mean state change, understanding impacts and developing adaptation strategies will be largely contingent on how climate variability responds to increasing anthropogenic perturbations. Thus far Earth system modeling efforts have primarily focused on projected mean state changes and the sensitivity of specific modes of climate variability, such as the El Niño-Southern Oscillation. However, our knowledge of forced changes in the overall spectrum of climate variability and higher order statistics is relatively limited. Here we present a new 100-member large ensemble of climate change projections conducted with the Community Earth System Model version 2 to examine the sensitivity of internal climate fluctuations to greenhouse warming. Our unprecedented simulations reveal that changes in variability, considered broadly in terms of probability, distribution, amplitude, frequency, phasing, and patterns, are ubiquitous and span a wide range of physical and ecosystem variables across many spatial and temporal scales. Greenhouse warming will in particular alter variance spectra of Earth system variables that are characterized by non-Gaussian probability distributions, such as rainfall, primary production, or fire occurrence. Our modeling results have important implications for climate adaptation efforts, resource management, seasonal predictions, and for assessing potential stressors for terrestrial and marine ecosystems.
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Journal article(s) based on this preprint
Keith B. Rodgers et al.
Interactive discussion
Status: closed
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CC1: 'Comment on esd-2021-50', Richard Rosen, 11 Jul 2021
It is always hard to know how to review articles like this which depend on large groups of climate models which relatively few experts in such models begin to understand. Thus rather than focus on the methodological details of the research which most readers will not understand anyway, it might make the most sense for the authors to focus on the implications of the results for policy makers and other interested readers. The first few sections should be shortened, and the end discussion should be greatly expanded so that policy makers are led through a clear discussion of what the results imply for real problems such as sustainable agriculture that the world faces.
One methodological problem I find in many articles which utilize very high forcing results for a scenario such as 7 watts per square meter like this one does is that by 2100 the temperature increases implied alone might imply the destruction of major parts of the world and human civilization. Thus, this is a very unlikely scenario to occur, especially since policy makers are now focusing on keeping the maximum average global temperature increase to between 1.5 and 2.0 degrees Cby 2050 or 2060, not 4.5 degrees C by 2100 as in this scenario.
Therefore, I think this kind of analysis would make much more sense if it were done for a scenario which achieved net zero carbon emissions by 2060 at the latest, in order to show policy makers why the world should try to achieve net zero carbon emissions even sooner given the implications of this kind of large ensemble modeling effort for the year 2060. Ignore the year 2100 in a revised paper - it is not relevant. Policy makers and the general public need to focus on analytical results for modeling efforts like this paper attempts over a much shorter time frame. The authors might even want to focus on the negative implications of this kind of frequency analysis on years even earlier, such as 2040 and 2050, if that could be done in a scientifically convincing way. If that is done in a revised paper in a convincing way it might get a large readership. It would be important to describe in detail what about the findings are really new in a revised paper. Given the too long run focus of the current paper, it is basically irrelevant for climate change mitigaion policy development, and should not be published as is.
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AC1: 'Reply on CC1', Keith Rodgers, 03 Sep 2021
We appreciate the comments offered through the public comment on our manuscript. The core suggestion of the reviewer concerns our choice of scenario (SSP3-7.0) and our choice to run our simulations through the year 2100 so as to consider variance changes at the end of the 21st century, rather than choose the near-term years 2040 or 2050. Our decisions in these matters were anchored in community-based decisions as reflected for example in the O’Neill et al. (2016) ScenarioMIP paper, that suggested SSP3-7.0 for large ensemble simulations. And more broadly, we chose to follow in most ways the CMIP6 protocols that were developed through broad community decision-making over the last 5 years. We wish in no way to denigrate or dissuade research focusing on nearer-term changes, nor does our work endorse or “choose” most likely outcomes of political decisions or put our money on the most likely scenario for future change. The O’Neill et al. (2016) study was quite specific in its recommendation that as a relatively strong scenario, SSP3-7.0 offers relatively strong forcing, with this being appropriate for studying changes of variance over the 21st century. We’re sorry for any misunderstandings in this regard. In the revised text, we will state more clearly how our model configuration was chosen within the context of broader CMIP6 efforts.
As a matter of procedure, we would encourage the reviewer to participate in the development of protocols for CMIP7, as this is where the protocols that shape studies such as ours are developed and expressed to the climate community. To reiterate, the interests and questions raised by the reviewer are clearly of value and interest for enhancing both public awareness and policy. But procedurally the most constructive way to bring such concerns to the table may not be through arguing posteriori that submitted manuscripts have illegitimate priorities for their chosen timescales (is any timescale illegitimate in climate science?), but rather in shaping community priorities through open processes.
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AC1: 'Reply on CC1', Keith Rodgers, 03 Sep 2021
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RC1: 'Comment on 'Ubiquity of human-induced changes in climate variability' by Rodgers et al.', Anonymous Referee #1, 28 Jul 2021
The manuscript features an evaluation of several key components of the climate system, focusing on statistics of their variability and how they change as a conseguence of anthropogenic climate change. The analysis makes use of the largest (among the recently performed) Large Ensemble (LE) experiment, carried out with CESM2 model. In order to assess changes in the statistics, historical and SSP3-7.0 scenarios are considered, and decadal averages are performed. The authors find that the signature of climate change is apparent, not only in the mean state change, but also in the variance, in the occurrence of extreme events, in the amplitude and frequency of certain periodic oscillations, and in some aspects of co-variability of selected quantities.
Overall, I think that the manuscript is scientifically sound, the methodology is reasonably correct in its implementation, consistently with the aim of exploiting the opportunity given by the huge CESM2-LE dataset to perform an in-depth analysis of climate variability from a global-scale point of view. Nevertheless, I think that the authors miss the chance to provide an interpretation of their findings. As a result, the manuscript is characterized by a collection of outputs loosely connected with each other. Secondarily, a few methodology aspects deserve more careful consideration, such as bringing together runs with different treatment of biomass burning fluxes, selecting the initialisation points according to the phase of the AMOC, or considering 10-year period as a sufficiently long decorrelation timestep for the sampling of initial conditions. Finally, I think that the manuscript would benefit by better referencing the available literature, particularly on the impact anthropogenic climate change has on variance and extremes.
Once these points, that are specifically addressed in the comments below, are taken into account, I think that the mansucript could be accepted for publication.
SPECIFIC COMMENTS
ll. 33-37: I find a bit limiting the notion of fluctuations as "characterized by spectral variance peaks superimposed upon a broad noise background", as I think it does not entails the possibility that modes of spatio-temporal variability are actually influenced by the "noise background" itself. Especially when one deals with processes that have clearly non-Gaussian PDFs, as in the case of this analysis, it is worth mentioning that at least that multiplicative noise processes (and externally driven changes therein) can alter the modes of variability through nonlinear interaction (e.g. Majda et al. 2009; Sardeshmukh and Sura 2009; Sardeshmukh and Penland 2015);
ll.62-64: while I find that mentioning Milinski et al. 2020 objective algorithm for the detection of the required LE size is appropriate, I think that, being the algorithm model dependent, it shall be acknowledged that their conclusions do not a priori apply here. Possibly, a sampling over the pre-industrial simulation, using it to test the internal variability associated with ENSO, would hint at the number of members that is actually required (even though one would have to assume that the same holds when the SSP3-7.0 forcing is applied).
ll. 106-107: the choice of the section of the pre-industrial run, where the model drift is particularly small, shall be better justified. The internal variability of the model might be influenced by the presence (or absence) of such bias, and it would be relevant to assess how relevant this impact is;
ll. 108-116: I am a bit puzzled by the choice of the initialisation dates for the ensemble. 80 members are initialised with 4 initial dates (sampled according to the phase of the AMOC; maximum AMOC, minimum AMOC, ascending AMOC, descending AMOC), then slightly perturbing these initial conditions (20 members per date); for the additional 20 members, initial dates separated by 10 years were chosen. I find hardly justifiable that the members are to be considered as independent and identically distributed, and that, as such, conclusions can be drawn about ensemble mean moments of the distribution. I acknowledge that, as the authors state at ll. 122-123, "further quantitative exploration of the specific duration over which initial condition memory is retained is the subject of a separate ongoing study" but I see two issues in this choice of the initial dates: 1. Members chosen according to AMOC phase are not uncorrelated by construction; 2. when it comes to the internal variability of the ocean, it is quite unlikely that 10 years are a sufficient decorrelation time;
ll. 130-136: I do not think that enough evidence is here provided that the two ensembles with different biomass burning can be assumed as being (or not being) part of the same population. An assessment through statistical tests (e.g. Mann-Whitney?) would here support such an argument;
ll. 175: out of curiosity, I was wondering why the authors chose to take into account the maximum transport at 40 N, instead of 26.5 N (which is often considered as an AMOC metrics);
ll. 201-202: as the authors refer here to variance and extremes, and their changes in future climate, it might be worth noticing that some promising results have been achieved with methods that synthesize several or all moments of the PDF, e.g the Wasserstein distance (cfr. Ghil 2015; Robin et al. 2017; Vissio et al. 2020 for a climate models diagnostics application);
l. 227: I do not have clear why the authors decided to retain the seasonal cycle in this context;
ll. 246-247: this is one of a few sentences I found in the text, that justify my general comment above about the lack of interpretation. In particular, the authors mention a lead-lag relation between precipitation and SST seasonal maxima. The assessment of these relations are challenging in the context of climate models (e.g. Lembo et al. 2017), together with their interpretation (cfr. Su et al. 2005 for this specific context) and the authors might want to discuss what these mean in terms of dynamics of the system;
l. 276: This is in part already known. Several studies (e.g. Screen 2014, Chen et al. 2015, Haugen et al. 2018) have evidenced the relation between Arctic amplification and reduced temperature variance over the mid- and high-latitudes of the Northern Hemisphere, and an interpretation of this has been given from a dynamical point of view (cfr. Sun et al. 2015; Schneider et al. 2015), involving the role of precipitation;
ll. 322-323: same as in my comment to l. 276. I am not surprised that the authors find a reduction in the NEP inter-annual variability, as this is linked to the variability of near-surface temperature. The link has been discussed in previous works (e.g. Yao et al. 2021) and I believe it shall be taken into account here;
ll. 328-330: see my comment at ll. 246-247. I think the authors shall comment on this finding and on how this can be interpreted;
ll. 350-351: this is not a new achievement. It has been long known (see, e.g. Palmer 1993; Corti et al. 1999) that climate change projects on modes of variability in several ways;
l. 360: I wonder if the authors are able to comment on how significant these findings obtained with CESM2-LE are, in relation with the other Large Ensemble exercices described in Maher et al. 2021;
l. 364-367: the lack of interpretation of the findings is here evident. I don’t think that the take-home message is that the Earth system is “far more sensitive in its statistical characteristics to anthropogenic forcing than previously recognized”. There is actually a literature on assessing changes in higher order moments of several aspects of climate variability, often using Large Ensemble exercises, e.g. Swain et al. 2018, for regional precipitation, Tamarin-Brodsky et al. 2020, for NH temperature variability, among others. The authors might compare their results with others, in order to explain how the sensitivity of statistical characteristics was less recognized before. As mentioned above, some of the findings, taken one by one, are confirming, or possibly expanding, what was already somehow kown from previous works. The manuscript might be significantly improved, if the authors would at least qualitatively discuss what drives and what is the relation between e.g. changes in frequency and phasing of ENSO wrt. SSTs and precipitations, cross-ensemble SD for temperature and precipitation, changes in ENSO’s remote correlation with regional mean temperatures and precipitation over some regions, just to mention a few features that might be interpreted in the light of changes occurring to the general circulation.
TECHNICAL CORRECTIONS
- l. 9: replace "runs" with "run";
- l. 150: replace "weaking" with "weakening";
- l. 209: replace "associate" with "association";
- l. 256: replace "n" with "in";
REFERENCES
- Chen, H. W., Zhang, F., and Alley, R. B. (2016). The Robustness of Midlatitude Weather Pattern Changes due to Arctic Sea Ice Loss, Journal of Climate, 29(21), 7831-7849
- Corti, S., Molteni, F. and Palmer, T. (1999). Signature of recent climate change in frequencies of natural atmospheric circulation regimes, Nature, 398, 799–802
- Ghil, M., and Lucarini, V. (2020). The physics of climate variability and climate change}, Rev. Mod. Phys., 92 (3), 035002, 77
- Haugen, M. A., Stein, M. L., Moyer, E. J., and Sriver, R. L. (2018). Estimating Changes in Temperature Distributions in a Large Ensemble of Climate Simulations Using Quantile Regression, Journal of Climate, 31(20), 8573-8588
- Lembo, V., Bordi, I., and Speranza, A. (2017). Annual and semiannual cycles of midlatitude near-surface temperature and tropospheric baroclinicity: reanalysis data and AOGCM simulations, Earth Syst. Dynam., 8, 295–312
- Majda, A., Franzke, C. L. E., and Crommelin, D. (2009). Normal forms for reduced stochastic climate models. Proceedings of the National Academy of Sciences of the United States of America, 106, 3649–3653
- Palmer, T. N. (1993). A nonlinear dynamical perspective on climate change, Weather, 48, 313–348
- Sardeshmukh, P. D. and Sura, P. (2009). Reconciling non-Gaussian climate statistics with linear dynamics. Journal Climate, 22(5), 1193–1207
- Sardeshmukh, P. D. and Penland, C. (2015). Understanding the distinctively skewed and heavy tailed character of atmospheric and oceanic probability distributions. Chaos, 25(3), 036410
- Schneider, T., Bischoff, T., and PÅotka, H. (2015). Physics of Changes in Synoptic Midlatitude Temperature Variability, Journal of Climate, 28(6)
- Screen, J. (2014). Arctic amplification decreases temperature variance in northern mid- to high-latitudes. Nature Clim Change 4, 577–582
- Su, H., Neelin, J. D., and Meyerson, J. E. (2005). Mechanisms for Lagged Atmospheric Response to ENSO SST Forcing, Journal of Climate, 18(20), 4195-4215
- Sun, L., Deser, C., and Tomas, R. A. (2015). Mechanisms of Stratospheric and Tropospheric Circulation Response to Projected Arctic Sea Ice Loss, Journal of Climate, 28(19), 7824-7845
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AC2: 'Reply on RC1', Keith Rodgers, 03 Sep 2021
We thank the reviewer for their careful reading of the manuscript. In our responses detailed below to the review, we have dedicated our attention wherever possible to the suggestion that we take the opportunity to provide an interpretation of our findings. To this end, we have emphasized in our responses where we plan to anchor the descriptions of our large ensemble behavior to mechanisms and model behaviors identified in previous studies.
Our detailed responses are below. For purposes of clarity, we have put the reviewer comments/questions in italicized text, and responded in plain text.
SPECIFIC COMMENTS
ll 33-37: I find a bit limiting the notion of fluctuations as “characterized by spectral variance peaks superimposed upon a broad noise background”, as I think it does not entail the possibility that modes of spatio-temporal variability are in fact influenced by the “noise background” itself. Especially when one deals with processes that have clearly non-Gaussian PDFs, as in the case of this analysis, it is worth mentioning that at least multiplicative noise processes (and externally-driven changes therein) can alter the modes of variability through nonlinear interaction (e.g. Majda et al, 2009; Sardeshmukh and Sura, 2009; Sardeshmukh and Penland, 2015).
We thank the reviewer for raising this point. Indeed the role of multiplicative noise in affecting variability on a multitude of timescales is well established in the literature. The references suggested by the reviewer will be included in the revised draft, in addition to references to Levine and Jin (2010; JAS) and Jin et al. (2020, Simple ENSO Models in AGU monograph on El Niño in a changing climate). To our knowledge the Mueller paper is the first that describes the influence of multiplicative noise on second- and third-order cumulants and spectra in the context of the stochastic climate model. The other papers touch on the highlight the role of multiplicative noise in generating ENSO characteristics.
We will further revise the text by adding the following sentence: “The spectrum of observed regional-to-global climate fluctuations exhibits spectral variance peaks and a broad noise background (Hasselmann, 1976; Franzke et al., 2020). Spectral peaks can emerge from a range of mechanisms, including astronomical forcings and internal climate instabilities, such as for ENSO. Moreover, these distinct features can be further influenced by climate processes acting on different timescales. Examples for this type of nonlinear “timescale interaction” are multiplicative (state-dependent) noise (Mueller, 1987; Majda et al., 2009; Sardeshmukh and Sura, 2009; Sardeshmukh and Penland, 2015; Jin et al., 2007; Levine et al., 2010; An et al., 2020; Jin et al., 2020) and combination model dynamics (Stuecker et al., 2015).
ll 62-64: while I find that mentioning Milinski et al. (2020) objective algorithm for the detection of the required LE size is appropriate, I think that with the algorithm being model-dependent, it should be acknowledge that their conclusions do not a priori apply here. Possibly a sampling over the pre-industrial simulation, using it to test the internal variability associated with ENSO, would hint at the number of members that is actually required (even though one would have to assume that the same holds when the SSP3-7.0 forcing is applied).
The point of the reviewer regarding the model-dependence of the Milinski et al. (2020) study is well-taken, and this question is the subject of an independent study with CESM2 led by one of our coauthors, Tamas Bodai. It is indeed not possible to project the necessary ensemble size in a model based on findings from another model configuration, and there are other issues that complicate the idea of prescribing a general rule ensemble sizes required for an experiment. As for the generic idea of an ensemble-size dependence o f detectability and the accuracy of identifying forced changes, the following figure considers variance changes for Niño3 SST following the suggestion of the reviewer:
In the figure, rho denotes the detection rate, calculated with a bootstrap mean over 1e3 bootstrap samples, and F refers to the F-test for the slope, concerning whether it is significantly non-zero at the 95% significance level. HAC refers to a technique that considers heterscedasticity and auto-correlation – which are themselves in fact not detectable here, hence the agreement between rho_F and rho_F,HAC . Thus the 20th century changes in the model are already detectable with 20 ensemble members.
The study of Milinski (2020) is not concerned with detectability, but rather with the accuracy of large changes, and this is addressed by the yellow and purple lines in the figure. alpha is the temporal slope of the ensemble standard deviation, and the purple line indicates the best estimate from the 100 ensemble members. q97.5 and are the 97.5th quantile (corresponding to the upper (u) bound of the 95% confidence interval) and the standard deviation over the bootstrap samples, respectively, with the relative errors/”variance” plotted.
ll. 106-107: the choice of the section of the pre-industrial run, where the model drift is particularly small, should be better justified. The internal variability of the model might be influenced by the presence (or absence) of such bias, and it would be relevant to assess how relevant this impact is.
In the revised manuscript, we will provide a clearer description of model drift over the span of the initialization dates from the pre-industrial control run. In the CESM2 presentation paper of Danabagoglu et al. (2020), Fig 6. Showed the TOA energy imbalance for the pre-industrial run over years 1-1200, revealing minimal drift by year 1000. Additional extensive diagnostics by NCAR scientists identified minimal drift for the AMOC, and for upper ocean heat content over the North Atlantic and Southern Ocean over years 1000-1300.
ll 108-116: I am a bit puzzled by the choice of the initialization dates for the ensemble. 80 members are initialized with 4 initial dates (sampled according to the phase of the AMOC; maximum AMOC, minimum AMOC, ascending AMOC, descending AMOC), then slightly perturbing these initial conditions (20 members per date); for the additional 20 members, initial dates separated by 10 years were chosen. I find hardly justifiable that the members are to be considered as independent, and identically distributed, and that as such, conclusions can be drawn about ensemble mean moments of the distribution. I acknowledge that, as the authors state at ll. 122-123, “further quantitative exploration of the specific duration over which initial condition memory is retained is the subject of a separate ongoing study”, but I see two issues in this choice of the initial dates: 1. Members chosen according to AMOC phase are not uncorrelated by construction; 2. When it comes to the internal variability of the ocean, it is quite unlikely that 10 years are a sufficient decorrelation time.
We were not sufficiently clear in our submitted manuscript about issues surrounding the initialization strategy, through a mix of macro-and micro-perturbations. It was not our intention to argue that the initialization procedure for CESM2-LE produces members that can be considered as independent, and we should have stated this more clearly, we apologize for the misunderstanding. Our analysis with internal variability is primarily focused on the post-1960 period, so more than a century after the 1850 initialization time for all members. In order to clear up any potential confusion, in addition to more detailed text clarifying the initialization procedure, we will provide in the revised manuscript both a quantification of the decorrelation timescale for the AMOC, as well as a timeseries figure showing the evolution of AMOC transport over the late 19th century for the 4 sets of micro-perturbation runs, illustrating the timescale over which initial condition memory is lost.
This two-panel figure shows the evolution of AMOC for (top panel) individual ensemble members over 1850-2000, and (bottom panel) ensemble means for the four micro-perturbation groupings as well as the ensemble mean of the macro-perturbations, all considered at 26.5°N. By the year 1900, the bottom panel indicates convergence in the ensemble mean groupings.
This is consistent with the autocorrelation for the (detrended) AMOC calculated using years 401-2000 from the pre-industrial control run (piControl), as shown for both 26N and 45N in the above figure. This is consistent with our interpretation that the analyses in Fig. 2 and Fig. 4 occur well beyond the time when initial condition memory is important.
ll 130-136: I do not think that enough evidence is here provided that the two ensembles with different biomass burning can be assumed as being (or not being) part of the same population. An assessment through statistical tests (e.g. Mann-Whitney?) would here support such an argument.
We fully agree with the reviewer that during the period of biomass burning perturbations (1990-2020, effectively) the full suite of 100 members should not be assumed to be part of the same population, but rather considered as two sets of 50 members. This is what motivated our Supplementary Fig. 2 in the submitted draft. There are two new manuscripts under development led by ICCP scientists (coauthors on this study) that deal explicitly with the impacts of biomass burning on the climate state. For the surface temperature, sea ice, and precipitation the response is only significant over the 1990-2020 interval of the biomass burning perturbation itself. This is the reason why we chose the intervals 1960-1989 and 2070-2099 for our emphasis on changes in variance, as the first of these is prior to the biomass burning perturbation, and the second is 50+ years after the perturbation.
Upon further reflection, we also recognize that we have not been sufficiently clear in the main text about which members are grouped with the CMIP6 and SMBB representation of biomass burning. To that end we will include as Supplementary two schematic figures that have already been prepared for our online description of the model runs:
https://www.cesm.ucar.edu/projects/community-projects/LENS2/
namely the two figures shown immediately above, in our Supplementary Materials to facilitate understanding of the model output organization. Additionally, we will mention more clearly in the revised text that the code base for the SMBB simulations incorporate corrections to minor bugs that were present in the first 50 ensemble members. This pertains to the SO2, SO4, and SOAG emission datasets. For SO2 and SO4, the “shipping” and “agriculture+solvents+wate” components of forcing were inadvertently switched during the historical-to-projection transition, or more specifically in 2015. The bug with SO2 partitioning does not impact the results given that its components are summed up before use. On the other hand, the issue with SO4 datasets can impact the model state evolution because the particles sizes and numbers differ for the SO4 components. The SOAG emissions are calculated from several hydrocarbons, and they were not recalculated after an earlier bug correction in covering units of the lumped species for the biomass burning emissions. This issue was corrected, and diagnostics indicate that there are not any significant changes in the model solutions from these particular corrections with aerosols. As a related point, we will also state more explicitly that a bug corrections were introduced for the 50 SMBB simulations that correct for sporadic large CO2 uptake over land that occurred for the CMIP6 runs due to a negative flux of carbon, occurring at crop harvest time in a single time step. Although these large negative carbon flux component terms in autotrophic respiration are necessary for maintaining carbon balance, the large CO2 spikes are not desirable. To avoid these spike, the associated CO2 flux that occurred over a single time step for “dribbled” to the atmosphere over a time scale of approximately ½ year for the SMBB simulations. Our evaluations indicate that these bug fixes for carbon did not result in any climate-changing impacts for these modifications.
ll 175: out of curiosity, I was wondering why the authors chose to take into account the maximum transport at 40°N, instead of 26.5°N (which is often considered as an AMOC metric).
We agree with the reviewer that for consistency, the AMOC as represented in both Fig. 1 and Fig. S5 should be analyzed at 26.5°N. We will modify Fig. S5 accordingly for the revisions.
ll 201-202: as the authors refer here to variance and extremes, and their changes in future climate, it might be worth noticing that some promising results have been achieved with methods that synthesize several or all moments of the PDF, e.g. the Wasserstein distance (cfr. Ghil 2015; Robin et al., 2017; Vissio et al., 2020 for a climate model diagnostics application).
We thank the reviewer for pointing to these earlier publications, we will reference them in the revised manuscript.
ll 227: I do not have a clear idea for why the authors decided to retain the seasonal cycle in this context
We thank the reviewer for raising this point regarding the retention of the seasonal cycle in the wavelet analysis shown in Fig. 3, as we were not sufficiently clear about this in the submitted manuscript. Our reason for retaining the seasonal cycle stems from our interest in illustrating timescale interactions between ENSO and the seasonal cycle with the full power of large ensemble statistics. It is our hope that this will stimulate, as part of our presentation, further investigations of insights that are offered into frequency entrainment, among other questions that could arise. In our revised manuscript we will be clearer in making this point, and providing appropriate references in the context of explaining why we included the seasonal cycle.
The annual cycle and ENSO interact with each other in a complicated way, with the annual cycle itself being a forced mode (Xie, 1994). This interaction gives rise to combination models (Stuecker et al., 2015), frequency entrainment (Timmermann et al., 2007) and ENSO’s phase-locking and seasonal variance modulation (Stein et al., 2014; Stein et al., 2010). Not only does the annual cycle in the equatorial Pacific influence the amplitude and phase of ENSO, but also vice versa. Due to this intricate coupling between these modes of variability, we have decided to retain the seasonal cycle in this context.
This information will be explicitly conveyed in the revised text, and we appreciate the reviewer for having raised it.
ll. 246-247: this is one of a few sentences I found in the text, that justify my general comment above about the lack of interpretation. In particular, the authors mention a lead-lag relation between precipitation and SST seasonal maxima. The assessment of these relations are challenging in the context of climate models (Lembo et al., 2017), together with their interpretation (cfr. Su et al. 2005 for this this specific context) and the authors might want to discuss what these mean in terms of dynamics of the systems.
We agree with the reviewer that there is an opportunity here to reference published literature that presents mechanistic interpretations of the behavior we have highlighted for the CESM2-LE. For the specific issue raised here of maximum precipitation leading maximum temperature over the Niño3.4 region on seasonal timescales (red dots in Fig. 3c and 3d), current scientific understanding maintains that precipitation is largely driven by meridional SST gradients, and is thereby not directly tied in its phasing to local SST. In other words, moisture convergence is in part determined by non-local SST conditions. We will appropriately reference the study of Xie (1996), Xie et al. (2010), Williams and Patricola (2018), and Stuecker et al. (2020) on this topic in the revised text.
l. 276: This is in part already known. Several studies (e.g. Screen 2014, Chen et al. 2016, Haugen et al., 2018) have shown evidence of a relationship between Arctic amplification and reduced temperature variance over the mid- and high-latitudes of the Northern Hemisphere, and an interpretation of this has been given from a dynamical point of view (cfr. Sun et al., 2015; Schneider et al., 2015), involving the role of precipitation.
We thank the reviewer for suggesting that appropriate references be given for describing changes in variance in temperature over the mid-to-high latitudes of the Northern Hemisphere due to polar amplification. We will reference these studies in the revised version of the manuscript, as well as the studies of Holmes et al. (2016; J. Climate) and Screen et al. (2015; BAMS).
ll. 322-323: same as in my comment to l. 276. I am not surprised that the authors find a reduction in the NEP inter-annual variability, as this is linked to the variability of near-surface temperature. The link has been discussed in previous works (e.g. Yao et al., 2021), and I believe it should be taken into account here.
It seems that the reviewer may in fact be confused by the text in our manuscript. We specifically chose not to address interannual variability in NEP. Rather our focus was on interannual variability in phenology in the lower panel of Fig. 5, as this is the behavior that has to our knowledge not been previously described in published literature. In our revisions, we will move the interpretation of the cause of the forced trend earlier in the paragraph, where we discuss the time of emergence of the trend.
In order to clarify, we will modify text at the end of our discussion of phenology to say: “Our analysis indicates that for NEP aggregated over this region the phenological shift as a decadal trend becomes emergent relative to estimates of the natural variability already within the first decades of the 21st century, a trend that is broadly consistent with observations (Zhu et al., 2016; Myers-Smith et al., 2020). The forced changes in growing season length are mostly attributable to changes in the mean temperature (Lawrence et al., 2019; Lombardozzi et al., 2020). Internal variability in the date of the onset of the growing season decreases by 35% over the course of the simulations and decreases by 18% for the date of the end of the growing season (Fig. 5, lower panel).”
The statements pertaining to attribution (expansion of growing season length being attributed to temperature) will be moved to the beginning of the same paragraph, so that the attribution is more of an emphasis in the text.
ll. 328-330: see my comment at ll. 246-247. I think the authors should comment on this finding and on how this can be interpreted.
The paragraph pointed to is introducing the climate change impacts on the mean state and variability. The reviewer is asking us to interpret the result related to changes in variability of peak and trough NEP amplitude (mentioned in liens 328-330, changes in variability along the y-axis of Fig. 5), but we don’t believe that this is where the main story lies here. Instead, we focus on the discussion of the temporal trends and changes in the seasonal variability related to spring-green up (x-axis of Fig. 5). Specifically, we focus on changes in the mean state related to forced changes in phenology, notably the earlier initiation of the growing season in the Northern Hemisphere spring, where ecological functions may be disrupted. Under future scenarios we also see reduced variability in the first zero-crossing of NEP, which we attribute to the combined effects of warming and the timing of snowmelt. A subset of the coauthors of this manuscript are pursuing this question of drivers of phenology as an offshoot project that wil complement the presentation paper analysis.
ll. 350-351: this is not a new achievement, it has been long known (see e.g. Palmer, 1993; Corti et al., 1999) that climate change projects on modes of variabilty in several ways.
We fully agree with the reviewer that the concept of anthropogenic changes can project onto modes of variability has been known for some time, and we will reference the studies of Palmer (1993) and Corti (1999) in our revisions. In order to better clarify this point, we will also state more clearly that the Large Ensembles provide a means to explore this in a statistically sound way in full complexity climate models that was not yet possible in the 1990s.
ll. 360: I wonder if the authors are able to comment on how significant these findings obtained with CESM2-LE are, in relation with other Large ENsemble exercises. described in Maher et al. (2021).
The reviewer refers to the multi-model Large Ensemble intercomparison study of Maher et al. (2021), and by implication the growing number of studies that make use of the multi-model Large Ensemble Archive presented in the study of Deser et al. (2020). From the onset of our project with the CESM2-LE, our intention has been to have our simulations be available for such multi-model studies, and to that end we specifically chose the historical/SSP3-7.0 pathway recommended in the CMIP6 protocols (ScenarioMIP) study of O’Neill et al. (2016). We have opted here to not engage in an intercomparison exercise, as this is beyond the scope of this initial presentation of the CESM2-LE itself, but to reiterate we have made every effort to facilitate such inter-comparison studies by interested parties in the future. We have also made available (https://climatedata.ibs.re.kr/data/cesm2-lens/lens-diagnostics) the results of the Climate Variability Diagnostics Package for Large Ensembles (CVDP-LE) of Philips et al. (2020) for diagnostics over a broad suite of variables that can equivalently be run for other large ensembles, as a means to facilitate studies that seek to understand model differences. We will include a link to this with an explanation in the revised version of the manuscript.
ll. 364-367: the lack of interpretation of the findings is evident here. I don’t think that the take-home message is that the Earth system is “far more sensitive in its statistical characteristics to anthropogenic forcing than previously recognized”. There is actually a literature on assessing changes in higher order moments of several aspects of climate variability, often using Large Ensemble exercises, e.g. Swain et al., 2018, for regional precipitation, Tamarin-Brodsky et al., 2020, for NH temperature variability, among others. The authors might compare their results with others, in order to explain how the sensitivity of statistical characteristics was less recognized before. As mentioned above, some of the findings, taken one by one, are confirming, or possibly expanding, what was already somehow known from previous works. The manuscript might be significantly improved, if the authors would at least qualitatively discuss what drives, and what is the relation between e.g. changes in frequency and phasing of ENSO w.r.t. SST and precipitation, cross-ensemble SD for temperature and precipitation, change in ENSO’s remote correlation with regional mean temperatures and precipitation over some regions, just to mention a few features that might be interpreted in the light of changes occurring in the general circulation.
Yes, we agree with the reviewer that the sentence towards the conclusions of the manuscript that “the Earth system is far more sensitive in its statistical characteristics to anthropogenic forcing than previously recognized” would be better rephrased as “we have provided support with new examples and new global emphasis that the Earth system is sensitive in its statistical characteristics to anthropogenic forcing, thereby building on previous studies.”
With regard to the questions posed by the reviewer with regard to Fig. 4, namely ENSO teleconnections, we will also reference properly the AGU monograph published in 2020 entitled “El Niño Southern Oscillation in a Changing Climate”, in particular the chapter by Taschetto et al. entitled “ENSO Atmospheric Teleconnections”. This synthesis reference appropriately addresses the challenges for understanding how ENSO teleconnections can change, including the relative role of local diabatic forcing and modulations of ENSO for understanding regional responses.
TECHNICAL CORRECTIONS
ll. 9: replace "runs" with "run"
We will correct this error in the revised manuscript.
ll. 150: replace "weaking" wth "weakening"
This will also be corrected, following the suggestion of the reviewer.
ll. 209: replace "associate" with "asssociation"
This will also be corrected.
ll. 256: replace "n" with "in"
This will also be corrected.
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RC2: 'Comment on esd-2021-50', Alexis Tantet, 04 Aug 2021
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AC3: 'Reply on RC2', Keith Rodgers, 03 Sep 2021
We would like to thank the reviewer for offering a number of insightful and constructive comments and criticisms, which we address below. In the text that follows, the reviewer's comments are indicated in italicized text, and our responses are shown in plain text.
Minor Comments:
Abstract:
ll. 19: To emphasize the originality of the dataset, I would also mention the simulation period and the model resolution like at l. 333
We thank the reviewer for the suggestion, this will be clarified in the revised manuscript.
ll. 23: "Greenhouse warming will in particular alter...". I would rather write "Greenhouse warming in the model in particular alters..." and mention that model uncertainty is not considered here, as explained in the paragraph of line 356.
We will implement the rewording recommended by the reviewer.
Introduction:
ll. 33: “spectral variance peaks”. The “peaks” are also relatively broad for different reasons contrary to spikes (Dirac’s). I would write “relatively sharp peaks”.
We will make the change recommended by the reviewer.
ll 53: "variance is repeated
We will remove the redundant use of "variance".
ll. 59: Explain why the results of the CMIP6 version of CESM2 is expected to be conclusive relative to "earlier studies" (l. 46). Perhaps based on what is explained in Section 2.
A large number of improvements occurred with climate models since the CCSM4 version 1.4 used in the ensemble study of Zelle et al. (2005), where the effective atmospheric resolution there having been 3.75°x3.75°. In addition to greatly improved resolution, a number of improvements in process-resolution contribute to increased skill in representing variability across a broad range of timescales. This includes the importance of land processes, with changes in the CLM5 model documented below. For the atmosphere, a sampling of the many changes that have occurred are in boundary layer and shallow convection schemes, as well as representations of microphysics. New parameterizations in the atmospheric CAM6 model are described in the Danabasoglu et al. (2020) presentation paper for the CESM2 model.
We will add appropriate text to convey these points in the revised text.
Method:
ll. 78: Missing punctuation
The problem with the missing punctuation will be fixed.
ll. 81: Is the resolution for the POP model also 1 degree? It is important to know if it is eddy-resolving since this might also effect the atmospheric variability.
The nominal resolution of the POP model is 1-degree, with enhanced resolution in the meridional direction near the equator as well as in the Southern Ocean. We will state this more clearly in the revised text.
ll. 91 - paragraph: How are these improvements measured? To which extent does the land model (fire model and agricultural management in particular) rely on assumptions regarding human behavior in the future (for instance in resopnse to climate change)?
We thank the reviewer for pointing out that we were not sufficiently clear in the paragraph starting on line 91, in particular in describing the strengths of the land model and the benchmarking that has been done to assess skill in the model. We will reorganize the paragraph at line 91 to say: “An important advance of great value to Large Ensemble investigations is achieved through new developments incorporated into the Community Land Model Version 5 (CLM5) (Danabasoglu et al., 2020; Lawrence et al., 2019; Lombardozzi et al., 2020). This model addresses a number of well-known limitations relative to previous versions of CLM, including enhanced simulated cumulative CO2 uptake over the historical period (Bonan et al., 2019) and the seasonal cycle of net ecosystem production (NEP; Lawrence et al, 2019), which is highlighted in our analysis of phenology changes. Improvements in CLM are found across a broad range of simulated variables that have been documented through evaluation of model simulations against the International Land Model Benchmarking (ILAMBv2.1) package and other analyses (Collier et al., 2018; Danabasoglu et al. 2020; Lawrence et al., 2019; Wieder et al., 2019). Notable features also included in CLM5 are the explicit representation of agricultural management and improvements in the implementation of the prognostic fire model (Lombardozzi et al, 2020; Lie et al., 2013; Li and Lawrence, 2017). We note that land model trajectories are sensitive to the SSP-RCP scenarios that determine the spatial distribution and extent of land use and land cover change, as well as climate change scenarios (O’Neill et al., 2016). “
Results:
ll. 169: Could you explain what motivated the choise of these observables? I guess one factor is the relationship between the observables and climate-change impact, but this is not obvious.
The variables we chose for Fig. 2 reflect interest in both climate/ecosystem dynamics and societal relevance in terms of adaptation and resource management. We will state this more emphatically in our revisions.
ll. 169:
Instead of the Fourier transform of the observable, why not use an estimate (e.g. periodogram) of the power spectrum which can be directly related to the variance that you use in Figure 1 (as the integral of an adequately normalized power spectrum)? The variance is also used in Figure 3. If this is in fact what you are doing, please make it clearer.
In the caption for Fig. 2 and in l.169 we specified “amplitude spectrum”, which is the square root of the power spectrum. Please note that Fig. 1, Fig. 3e and Fig. 3f are cross-ensemble standard deviations using annual mean data (the annual mean is calculated first for each member, and then the standard deviation is taken across ensemble members for the same year). Furthermore, they are not representing “variance”. Thus, even if we use power spectral density (PDS) in Fig. 2, the integral of PSD is not equivalent to what is shown in either Fig. 1 or Fig. 3. We chose the amplitude spectrum in Fig. 2 to have a better understanding of the amplitudes of perturbations at different timescales, and to be consistent with the standard deviations shown in the other figures. Also, if PDS were used, the already strong annual cycle and its harmonic peaks in the amplitude spectrum would become too large, making other spectral behaviors relatively less variable.
ll. 169: To avoid spectral leakage, a window should be applied before the FFT, is this the case?
The point is well-taken, but the large degree of aggregation used for Fig. 2 (both across ensemble members but also spatial aggregation) alleviates the need for windowing. We will state this more clearly in our revised text.
ll. 171 and 172: Why 35 years and not 30 years (2070-2099 and 1960-1989)?
We thank the reviewer for catching this typo, we will modify "35 years" to say "30 years".
ll. 173: If the
power spectrum is computed, an alternative would be to first compute correlation functions for each member, average over the members and then do the FFT. I do not know which estimation method has the best properties, but could you explain why you made this choice?
There is a disparity between dynamical characteristics (such as power spectra) and statistical characteristics (like the natural measure) in that we can – for now – define only the latter in an instantaneous/shapshot or pullback sense. Given that there is no reference for the power spectrum with respect to which biases are defined it is not really possible to rank different possibly estimators. We went with what appears to be the most intuitive estimator: the E-mean of the temporally computed power spectra (or FFT amplitudes). A benefit of this quantity is that nonergodicity could be naturally defined considering the difference between this and the correct, conceptually sound quantity.
Figure 2: What are the units of the spectral amplitudes given the observables?
Please find this in the caption for Fig. 2 in the original submitted manuscript, where we state: “Spectra are shown as amplitude, with the units being the same as the x-axis for the PDFs.”
ll. 183: Even if I do not think that it is necessary to add confidence intervals to all panels and for all frequencies or to test the significance of the differences between spectra, could you give an estimate of what would be the width of these confidence intervals given that data that you use (in the supplementary material for instance)? This would also make this section more coherent with the part on wavelets.
We will provide a 95% confidence interval for Niño3.4 precipitation in the Supplementary Material, and provide a figure. The confidence interval is simply estimated as 1.96 multiplied by standard error of 100 spectra at each frequency. In this calculation, we first spatially average spectra at individual grid points over the Niño3.4 region for each member, and we treat the sampling size N=100 for the 100 ensemble members. We believe that this is a somewhat conservative approach. If we assume that all grid points can be sampled, the confidence interval becomes much narrower as the sampling number becomes much larger (N=100*number_of_gridpoints).
ll. 200 and ll 208: Although a scalar observable can techincally be seen as a bilinear form, I would reserve the term positive definite for non-trivial bilnear forms (e.g. represented by non-scalar matrices) and simply write "positive variables"
We thank the reviewer for making this point, and we wil use the term "positive variables" as suggested.
ll. 202: This is not true for all stochastic processes. I guess you mean for a Brownian motion?
We will modify the text from saying "stochastic processes" to say "white noise processes", we thank the reviewr for expressing the need to be clearer here.
Could you clarify what is meant by "cross-ensemble" everywhere this expression is used?
Yes, we will do this.
ll. 256: "minimum m" => "minimum in"
This is correct, we will make the change as suggested.
ll. 265: same as for ll. 251. . “cross-ensemble calculations applied for identical time records for each ensemble member” is not clear to me. The standard deviation is computed from a sample combining all members and all years for a given period (historic or future)? Based on the caption, I guess not, the standard deviation is computed over the ensemble and then time-averaged over the period. Could you clarify and explain why you made this choice and not the other?
We apologize for not being clearer, we will clarify this in the text. The inference here made by the reviewer is correct, the standard deviation was first computed across the ensemble members for the same time slice, and then time averaging was conducted. This was done out of pragmatism, as we were looking for convergence (or non-convergence) with the number of years included through the averaging procedure.
ll. 310: "and" => "an" I guess
Yes, this is correct, we will change this.
Figure 5: Do the histograms aggregate ensemble members for a single year only or is there also an aggregation of the 20 years in the interval (in which case the histograms would include the 20y-trend)?
The aggregation is done for individual years, we will clarify this in the text.
Figure 5: How are the histograms estimated? Using a grid? Which interval length(s)?
The width of binning for the histograms is 1-day, we will clarify this in the text, we thank the reviewer for pointing out that this wasn’t clear in the reviewed manuscript.
ll. 318: Since the spacing between the vertical grid lines in Fig. 5 represents an interval of 10 days, it seems to me that the shift in the onset is closer to 3 weeks or even 4 weeks than 2 weeks. Am I wrong?
We thank the reviewer for raising this question, we will replace the mis-statement of the duration of the onset shift to reflect that it is in fact three weeks. Comparing the mean over 1860-1869 and the mean over 2090-2099, the day of the first zero-crossing shifts 20.9 days and the day of the termination shifts 6.3 days.
ll. 322: How do you "measure" the interannual variability? Even if you read the histograms by eye, I guess that you have some metric representation of the spread in mind, such as the standard deviation. In fact, if you use the distance between the minima and the maxima, the interannual variaiblity appears comparable to the trend to me.
We will clarify this in the text, we used one standard deviation to measure the variabiltiy.
ll. 325: Same question as for ll. 322: Which measure do you use to obtain these percentages?
We thank the reviewer for raising this point, as we were insufficiently clear in the text. The calculations were performed year-by-year, where the transitions (zero crossing) were calculated for each member across the full set of 90 ensemble members as this progressed in time.
Discussion:
ll. 331: I would call this section “Summary and Discussion”, but that’s a detail.
We will follow the suggestion of the reviewer here, and change the title of the section.
ll. 335: English is not my mother tongue, so I may be wrong, but shouldn’t “affords” be replaced by “offers”?
Yes, we will make the appropriate change so that the text says “offers”.
References are hard to read because entries are not visually separated.
We will fix the formatting for the revised version of the manuscript to facilitate reading the references.
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AC3: 'Reply on RC2', Keith Rodgers, 03 Sep 2021
Peer review completion
Post-review adjustments
Interactive discussion
Status: closed
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CC1: 'Comment on esd-2021-50', Richard Rosen, 11 Jul 2021
It is always hard to know how to review articles like this which depend on large groups of climate models which relatively few experts in such models begin to understand. Thus rather than focus on the methodological details of the research which most readers will not understand anyway, it might make the most sense for the authors to focus on the implications of the results for policy makers and other interested readers. The first few sections should be shortened, and the end discussion should be greatly expanded so that policy makers are led through a clear discussion of what the results imply for real problems such as sustainable agriculture that the world faces.
One methodological problem I find in many articles which utilize very high forcing results for a scenario such as 7 watts per square meter like this one does is that by 2100 the temperature increases implied alone might imply the destruction of major parts of the world and human civilization. Thus, this is a very unlikely scenario to occur, especially since policy makers are now focusing on keeping the maximum average global temperature increase to between 1.5 and 2.0 degrees Cby 2050 or 2060, not 4.5 degrees C by 2100 as in this scenario.
Therefore, I think this kind of analysis would make much more sense if it were done for a scenario which achieved net zero carbon emissions by 2060 at the latest, in order to show policy makers why the world should try to achieve net zero carbon emissions even sooner given the implications of this kind of large ensemble modeling effort for the year 2060. Ignore the year 2100 in a revised paper - it is not relevant. Policy makers and the general public need to focus on analytical results for modeling efforts like this paper attempts over a much shorter time frame. The authors might even want to focus on the negative implications of this kind of frequency analysis on years even earlier, such as 2040 and 2050, if that could be done in a scientifically convincing way. If that is done in a revised paper in a convincing way it might get a large readership. It would be important to describe in detail what about the findings are really new in a revised paper. Given the too long run focus of the current paper, it is basically irrelevant for climate change mitigaion policy development, and should not be published as is.
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AC1: 'Reply on CC1', Keith Rodgers, 03 Sep 2021
We appreciate the comments offered through the public comment on our manuscript. The core suggestion of the reviewer concerns our choice of scenario (SSP3-7.0) and our choice to run our simulations through the year 2100 so as to consider variance changes at the end of the 21st century, rather than choose the near-term years 2040 or 2050. Our decisions in these matters were anchored in community-based decisions as reflected for example in the O’Neill et al. (2016) ScenarioMIP paper, that suggested SSP3-7.0 for large ensemble simulations. And more broadly, we chose to follow in most ways the CMIP6 protocols that were developed through broad community decision-making over the last 5 years. We wish in no way to denigrate or dissuade research focusing on nearer-term changes, nor does our work endorse or “choose” most likely outcomes of political decisions or put our money on the most likely scenario for future change. The O’Neill et al. (2016) study was quite specific in its recommendation that as a relatively strong scenario, SSP3-7.0 offers relatively strong forcing, with this being appropriate for studying changes of variance over the 21st century. We’re sorry for any misunderstandings in this regard. In the revised text, we will state more clearly how our model configuration was chosen within the context of broader CMIP6 efforts.
As a matter of procedure, we would encourage the reviewer to participate in the development of protocols for CMIP7, as this is where the protocols that shape studies such as ours are developed and expressed to the climate community. To reiterate, the interests and questions raised by the reviewer are clearly of value and interest for enhancing both public awareness and policy. But procedurally the most constructive way to bring such concerns to the table may not be through arguing posteriori that submitted manuscripts have illegitimate priorities for their chosen timescales (is any timescale illegitimate in climate science?), but rather in shaping community priorities through open processes.
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AC1: 'Reply on CC1', Keith Rodgers, 03 Sep 2021
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RC1: 'Comment on 'Ubiquity of human-induced changes in climate variability' by Rodgers et al.', Anonymous Referee #1, 28 Jul 2021
The manuscript features an evaluation of several key components of the climate system, focusing on statistics of their variability and how they change as a conseguence of anthropogenic climate change. The analysis makes use of the largest (among the recently performed) Large Ensemble (LE) experiment, carried out with CESM2 model. In order to assess changes in the statistics, historical and SSP3-7.0 scenarios are considered, and decadal averages are performed. The authors find that the signature of climate change is apparent, not only in the mean state change, but also in the variance, in the occurrence of extreme events, in the amplitude and frequency of certain periodic oscillations, and in some aspects of co-variability of selected quantities.
Overall, I think that the manuscript is scientifically sound, the methodology is reasonably correct in its implementation, consistently with the aim of exploiting the opportunity given by the huge CESM2-LE dataset to perform an in-depth analysis of climate variability from a global-scale point of view. Nevertheless, I think that the authors miss the chance to provide an interpretation of their findings. As a result, the manuscript is characterized by a collection of outputs loosely connected with each other. Secondarily, a few methodology aspects deserve more careful consideration, such as bringing together runs with different treatment of biomass burning fluxes, selecting the initialisation points according to the phase of the AMOC, or considering 10-year period as a sufficiently long decorrelation timestep for the sampling of initial conditions. Finally, I think that the manuscript would benefit by better referencing the available literature, particularly on the impact anthropogenic climate change has on variance and extremes.
Once these points, that are specifically addressed in the comments below, are taken into account, I think that the mansucript could be accepted for publication.
SPECIFIC COMMENTS
ll. 33-37: I find a bit limiting the notion of fluctuations as "characterized by spectral variance peaks superimposed upon a broad noise background", as I think it does not entails the possibility that modes of spatio-temporal variability are actually influenced by the "noise background" itself. Especially when one deals with processes that have clearly non-Gaussian PDFs, as in the case of this analysis, it is worth mentioning that at least that multiplicative noise processes (and externally driven changes therein) can alter the modes of variability through nonlinear interaction (e.g. Majda et al. 2009; Sardeshmukh and Sura 2009; Sardeshmukh and Penland 2015);
ll.62-64: while I find that mentioning Milinski et al. 2020 objective algorithm for the detection of the required LE size is appropriate, I think that, being the algorithm model dependent, it shall be acknowledged that their conclusions do not a priori apply here. Possibly, a sampling over the pre-industrial simulation, using it to test the internal variability associated with ENSO, would hint at the number of members that is actually required (even though one would have to assume that the same holds when the SSP3-7.0 forcing is applied).
ll. 106-107: the choice of the section of the pre-industrial run, where the model drift is particularly small, shall be better justified. The internal variability of the model might be influenced by the presence (or absence) of such bias, and it would be relevant to assess how relevant this impact is;
ll. 108-116: I am a bit puzzled by the choice of the initialisation dates for the ensemble. 80 members are initialised with 4 initial dates (sampled according to the phase of the AMOC; maximum AMOC, minimum AMOC, ascending AMOC, descending AMOC), then slightly perturbing these initial conditions (20 members per date); for the additional 20 members, initial dates separated by 10 years were chosen. I find hardly justifiable that the members are to be considered as independent and identically distributed, and that, as such, conclusions can be drawn about ensemble mean moments of the distribution. I acknowledge that, as the authors state at ll. 122-123, "further quantitative exploration of the specific duration over which initial condition memory is retained is the subject of a separate ongoing study" but I see two issues in this choice of the initial dates: 1. Members chosen according to AMOC phase are not uncorrelated by construction; 2. when it comes to the internal variability of the ocean, it is quite unlikely that 10 years are a sufficient decorrelation time;
ll. 130-136: I do not think that enough evidence is here provided that the two ensembles with different biomass burning can be assumed as being (or not being) part of the same population. An assessment through statistical tests (e.g. Mann-Whitney?) would here support such an argument;
ll. 175: out of curiosity, I was wondering why the authors chose to take into account the maximum transport at 40 N, instead of 26.5 N (which is often considered as an AMOC metrics);
ll. 201-202: as the authors refer here to variance and extremes, and their changes in future climate, it might be worth noticing that some promising results have been achieved with methods that synthesize several or all moments of the PDF, e.g the Wasserstein distance (cfr. Ghil 2015; Robin et al. 2017; Vissio et al. 2020 for a climate models diagnostics application);
l. 227: I do not have clear why the authors decided to retain the seasonal cycle in this context;
ll. 246-247: this is one of a few sentences I found in the text, that justify my general comment above about the lack of interpretation. In particular, the authors mention a lead-lag relation between precipitation and SST seasonal maxima. The assessment of these relations are challenging in the context of climate models (e.g. Lembo et al. 2017), together with their interpretation (cfr. Su et al. 2005 for this specific context) and the authors might want to discuss what these mean in terms of dynamics of the system;
l. 276: This is in part already known. Several studies (e.g. Screen 2014, Chen et al. 2015, Haugen et al. 2018) have evidenced the relation between Arctic amplification and reduced temperature variance over the mid- and high-latitudes of the Northern Hemisphere, and an interpretation of this has been given from a dynamical point of view (cfr. Sun et al. 2015; Schneider et al. 2015), involving the role of precipitation;
ll. 322-323: same as in my comment to l. 276. I am not surprised that the authors find a reduction in the NEP inter-annual variability, as this is linked to the variability of near-surface temperature. The link has been discussed in previous works (e.g. Yao et al. 2021) and I believe it shall be taken into account here;
ll. 328-330: see my comment at ll. 246-247. I think the authors shall comment on this finding and on how this can be interpreted;
ll. 350-351: this is not a new achievement. It has been long known (see, e.g. Palmer 1993; Corti et al. 1999) that climate change projects on modes of variability in several ways;
l. 360: I wonder if the authors are able to comment on how significant these findings obtained with CESM2-LE are, in relation with the other Large Ensemble exercices described in Maher et al. 2021;
l. 364-367: the lack of interpretation of the findings is here evident. I don’t think that the take-home message is that the Earth system is “far more sensitive in its statistical characteristics to anthropogenic forcing than previously recognized”. There is actually a literature on assessing changes in higher order moments of several aspects of climate variability, often using Large Ensemble exercises, e.g. Swain et al. 2018, for regional precipitation, Tamarin-Brodsky et al. 2020, for NH temperature variability, among others. The authors might compare their results with others, in order to explain how the sensitivity of statistical characteristics was less recognized before. As mentioned above, some of the findings, taken one by one, are confirming, or possibly expanding, what was already somehow kown from previous works. The manuscript might be significantly improved, if the authors would at least qualitatively discuss what drives and what is the relation between e.g. changes in frequency and phasing of ENSO wrt. SSTs and precipitations, cross-ensemble SD for temperature and precipitation, changes in ENSO’s remote correlation with regional mean temperatures and precipitation over some regions, just to mention a few features that might be interpreted in the light of changes occurring to the general circulation.
TECHNICAL CORRECTIONS
- l. 9: replace "runs" with "run";
- l. 150: replace "weaking" with "weakening";
- l. 209: replace "associate" with "association";
- l. 256: replace "n" with "in";
REFERENCES
- Chen, H. W., Zhang, F., and Alley, R. B. (2016). The Robustness of Midlatitude Weather Pattern Changes due to Arctic Sea Ice Loss, Journal of Climate, 29(21), 7831-7849
- Corti, S., Molteni, F. and Palmer, T. (1999). Signature of recent climate change in frequencies of natural atmospheric circulation regimes, Nature, 398, 799–802
- Ghil, M., and Lucarini, V. (2020). The physics of climate variability and climate change}, Rev. Mod. Phys., 92 (3), 035002, 77
- Haugen, M. A., Stein, M. L., Moyer, E. J., and Sriver, R. L. (2018). Estimating Changes in Temperature Distributions in a Large Ensemble of Climate Simulations Using Quantile Regression, Journal of Climate, 31(20), 8573-8588
- Lembo, V., Bordi, I., and Speranza, A. (2017). Annual and semiannual cycles of midlatitude near-surface temperature and tropospheric baroclinicity: reanalysis data and AOGCM simulations, Earth Syst. Dynam., 8, 295–312
- Majda, A., Franzke, C. L. E., and Crommelin, D. (2009). Normal forms for reduced stochastic climate models. Proceedings of the National Academy of Sciences of the United States of America, 106, 3649–3653
- Palmer, T. N. (1993). A nonlinear dynamical perspective on climate change, Weather, 48, 313–348
- Sardeshmukh, P. D. and Sura, P. (2009). Reconciling non-Gaussian climate statistics with linear dynamics. Journal Climate, 22(5), 1193–1207
- Sardeshmukh, P. D. and Penland, C. (2015). Understanding the distinctively skewed and heavy tailed character of atmospheric and oceanic probability distributions. Chaos, 25(3), 036410
- Schneider, T., Bischoff, T., and PÅotka, H. (2015). Physics of Changes in Synoptic Midlatitude Temperature Variability, Journal of Climate, 28(6)
- Screen, J. (2014). Arctic amplification decreases temperature variance in northern mid- to high-latitudes. Nature Clim Change 4, 577–582
- Su, H., Neelin, J. D., and Meyerson, J. E. (2005). Mechanisms for Lagged Atmospheric Response to ENSO SST Forcing, Journal of Climate, 18(20), 4195-4215
- Sun, L., Deser, C., and Tomas, R. A. (2015). Mechanisms of Stratospheric and Tropospheric Circulation Response to Projected Arctic Sea Ice Loss, Journal of Climate, 28(19), 7824-7845
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AC2: 'Reply on RC1', Keith Rodgers, 03 Sep 2021
We thank the reviewer for their careful reading of the manuscript. In our responses detailed below to the review, we have dedicated our attention wherever possible to the suggestion that we take the opportunity to provide an interpretation of our findings. To this end, we have emphasized in our responses where we plan to anchor the descriptions of our large ensemble behavior to mechanisms and model behaviors identified in previous studies.
Our detailed responses are below. For purposes of clarity, we have put the reviewer comments/questions in italicized text, and responded in plain text.
SPECIFIC COMMENTS
ll 33-37: I find a bit limiting the notion of fluctuations as “characterized by spectral variance peaks superimposed upon a broad noise background”, as I think it does not entail the possibility that modes of spatio-temporal variability are in fact influenced by the “noise background” itself. Especially when one deals with processes that have clearly non-Gaussian PDFs, as in the case of this analysis, it is worth mentioning that at least multiplicative noise processes (and externally-driven changes therein) can alter the modes of variability through nonlinear interaction (e.g. Majda et al, 2009; Sardeshmukh and Sura, 2009; Sardeshmukh and Penland, 2015).
We thank the reviewer for raising this point. Indeed the role of multiplicative noise in affecting variability on a multitude of timescales is well established in the literature. The references suggested by the reviewer will be included in the revised draft, in addition to references to Levine and Jin (2010; JAS) and Jin et al. (2020, Simple ENSO Models in AGU monograph on El Niño in a changing climate). To our knowledge the Mueller paper is the first that describes the influence of multiplicative noise on second- and third-order cumulants and spectra in the context of the stochastic climate model. The other papers touch on the highlight the role of multiplicative noise in generating ENSO characteristics.
We will further revise the text by adding the following sentence: “The spectrum of observed regional-to-global climate fluctuations exhibits spectral variance peaks and a broad noise background (Hasselmann, 1976; Franzke et al., 2020). Spectral peaks can emerge from a range of mechanisms, including astronomical forcings and internal climate instabilities, such as for ENSO. Moreover, these distinct features can be further influenced by climate processes acting on different timescales. Examples for this type of nonlinear “timescale interaction” are multiplicative (state-dependent) noise (Mueller, 1987; Majda et al., 2009; Sardeshmukh and Sura, 2009; Sardeshmukh and Penland, 2015; Jin et al., 2007; Levine et al., 2010; An et al., 2020; Jin et al., 2020) and combination model dynamics (Stuecker et al., 2015).
ll 62-64: while I find that mentioning Milinski et al. (2020) objective algorithm for the detection of the required LE size is appropriate, I think that with the algorithm being model-dependent, it should be acknowledge that their conclusions do not a priori apply here. Possibly a sampling over the pre-industrial simulation, using it to test the internal variability associated with ENSO, would hint at the number of members that is actually required (even though one would have to assume that the same holds when the SSP3-7.0 forcing is applied).
The point of the reviewer regarding the model-dependence of the Milinski et al. (2020) study is well-taken, and this question is the subject of an independent study with CESM2 led by one of our coauthors, Tamas Bodai. It is indeed not possible to project the necessary ensemble size in a model based on findings from another model configuration, and there are other issues that complicate the idea of prescribing a general rule ensemble sizes required for an experiment. As for the generic idea of an ensemble-size dependence o f detectability and the accuracy of identifying forced changes, the following figure considers variance changes for Niño3 SST following the suggestion of the reviewer:
In the figure, rho denotes the detection rate, calculated with a bootstrap mean over 1e3 bootstrap samples, and F refers to the F-test for the slope, concerning whether it is significantly non-zero at the 95% significance level. HAC refers to a technique that considers heterscedasticity and auto-correlation – which are themselves in fact not detectable here, hence the agreement between rho_F and rho_F,HAC . Thus the 20th century changes in the model are already detectable with 20 ensemble members.
The study of Milinski (2020) is not concerned with detectability, but rather with the accuracy of large changes, and this is addressed by the yellow and purple lines in the figure. alpha is the temporal slope of the ensemble standard deviation, and the purple line indicates the best estimate from the 100 ensemble members. q97.5 and are the 97.5th quantile (corresponding to the upper (u) bound of the 95% confidence interval) and the standard deviation over the bootstrap samples, respectively, with the relative errors/”variance” plotted.
ll. 106-107: the choice of the section of the pre-industrial run, where the model drift is particularly small, should be better justified. The internal variability of the model might be influenced by the presence (or absence) of such bias, and it would be relevant to assess how relevant this impact is.
In the revised manuscript, we will provide a clearer description of model drift over the span of the initialization dates from the pre-industrial control run. In the CESM2 presentation paper of Danabagoglu et al. (2020), Fig 6. Showed the TOA energy imbalance for the pre-industrial run over years 1-1200, revealing minimal drift by year 1000. Additional extensive diagnostics by NCAR scientists identified minimal drift for the AMOC, and for upper ocean heat content over the North Atlantic and Southern Ocean over years 1000-1300.
ll 108-116: I am a bit puzzled by the choice of the initialization dates for the ensemble. 80 members are initialized with 4 initial dates (sampled according to the phase of the AMOC; maximum AMOC, minimum AMOC, ascending AMOC, descending AMOC), then slightly perturbing these initial conditions (20 members per date); for the additional 20 members, initial dates separated by 10 years were chosen. I find hardly justifiable that the members are to be considered as independent, and identically distributed, and that as such, conclusions can be drawn about ensemble mean moments of the distribution. I acknowledge that, as the authors state at ll. 122-123, “further quantitative exploration of the specific duration over which initial condition memory is retained is the subject of a separate ongoing study”, but I see two issues in this choice of the initial dates: 1. Members chosen according to AMOC phase are not uncorrelated by construction; 2. When it comes to the internal variability of the ocean, it is quite unlikely that 10 years are a sufficient decorrelation time.
We were not sufficiently clear in our submitted manuscript about issues surrounding the initialization strategy, through a mix of macro-and micro-perturbations. It was not our intention to argue that the initialization procedure for CESM2-LE produces members that can be considered as independent, and we should have stated this more clearly, we apologize for the misunderstanding. Our analysis with internal variability is primarily focused on the post-1960 period, so more than a century after the 1850 initialization time for all members. In order to clear up any potential confusion, in addition to more detailed text clarifying the initialization procedure, we will provide in the revised manuscript both a quantification of the decorrelation timescale for the AMOC, as well as a timeseries figure showing the evolution of AMOC transport over the late 19th century for the 4 sets of micro-perturbation runs, illustrating the timescale over which initial condition memory is lost.
This two-panel figure shows the evolution of AMOC for (top panel) individual ensemble members over 1850-2000, and (bottom panel) ensemble means for the four micro-perturbation groupings as well as the ensemble mean of the macro-perturbations, all considered at 26.5°N. By the year 1900, the bottom panel indicates convergence in the ensemble mean groupings.
This is consistent with the autocorrelation for the (detrended) AMOC calculated using years 401-2000 from the pre-industrial control run (piControl), as shown for both 26N and 45N in the above figure. This is consistent with our interpretation that the analyses in Fig. 2 and Fig. 4 occur well beyond the time when initial condition memory is important.
ll 130-136: I do not think that enough evidence is here provided that the two ensembles with different biomass burning can be assumed as being (or not being) part of the same population. An assessment through statistical tests (e.g. Mann-Whitney?) would here support such an argument.
We fully agree with the reviewer that during the period of biomass burning perturbations (1990-2020, effectively) the full suite of 100 members should not be assumed to be part of the same population, but rather considered as two sets of 50 members. This is what motivated our Supplementary Fig. 2 in the submitted draft. There are two new manuscripts under development led by ICCP scientists (coauthors on this study) that deal explicitly with the impacts of biomass burning on the climate state. For the surface temperature, sea ice, and precipitation the response is only significant over the 1990-2020 interval of the biomass burning perturbation itself. This is the reason why we chose the intervals 1960-1989 and 2070-2099 for our emphasis on changes in variance, as the first of these is prior to the biomass burning perturbation, and the second is 50+ years after the perturbation.
Upon further reflection, we also recognize that we have not been sufficiently clear in the main text about which members are grouped with the CMIP6 and SMBB representation of biomass burning. To that end we will include as Supplementary two schematic figures that have already been prepared for our online description of the model runs:
https://www.cesm.ucar.edu/projects/community-projects/LENS2/
namely the two figures shown immediately above, in our Supplementary Materials to facilitate understanding of the model output organization. Additionally, we will mention more clearly in the revised text that the code base for the SMBB simulations incorporate corrections to minor bugs that were present in the first 50 ensemble members. This pertains to the SO2, SO4, and SOAG emission datasets. For SO2 and SO4, the “shipping” and “agriculture+solvents+wate” components of forcing were inadvertently switched during the historical-to-projection transition, or more specifically in 2015. The bug with SO2 partitioning does not impact the results given that its components are summed up before use. On the other hand, the issue with SO4 datasets can impact the model state evolution because the particles sizes and numbers differ for the SO4 components. The SOAG emissions are calculated from several hydrocarbons, and they were not recalculated after an earlier bug correction in covering units of the lumped species for the biomass burning emissions. This issue was corrected, and diagnostics indicate that there are not any significant changes in the model solutions from these particular corrections with aerosols. As a related point, we will also state more explicitly that a bug corrections were introduced for the 50 SMBB simulations that correct for sporadic large CO2 uptake over land that occurred for the CMIP6 runs due to a negative flux of carbon, occurring at crop harvest time in a single time step. Although these large negative carbon flux component terms in autotrophic respiration are necessary for maintaining carbon balance, the large CO2 spikes are not desirable. To avoid these spike, the associated CO2 flux that occurred over a single time step for “dribbled” to the atmosphere over a time scale of approximately ½ year for the SMBB simulations. Our evaluations indicate that these bug fixes for carbon did not result in any climate-changing impacts for these modifications.
ll 175: out of curiosity, I was wondering why the authors chose to take into account the maximum transport at 40°N, instead of 26.5°N (which is often considered as an AMOC metric).
We agree with the reviewer that for consistency, the AMOC as represented in both Fig. 1 and Fig. S5 should be analyzed at 26.5°N. We will modify Fig. S5 accordingly for the revisions.
ll 201-202: as the authors refer here to variance and extremes, and their changes in future climate, it might be worth noticing that some promising results have been achieved with methods that synthesize several or all moments of the PDF, e.g. the Wasserstein distance (cfr. Ghil 2015; Robin et al., 2017; Vissio et al., 2020 for a climate model diagnostics application).
We thank the reviewer for pointing to these earlier publications, we will reference them in the revised manuscript.
ll 227: I do not have a clear idea for why the authors decided to retain the seasonal cycle in this context
We thank the reviewer for raising this point regarding the retention of the seasonal cycle in the wavelet analysis shown in Fig. 3, as we were not sufficiently clear about this in the submitted manuscript. Our reason for retaining the seasonal cycle stems from our interest in illustrating timescale interactions between ENSO and the seasonal cycle with the full power of large ensemble statistics. It is our hope that this will stimulate, as part of our presentation, further investigations of insights that are offered into frequency entrainment, among other questions that could arise. In our revised manuscript we will be clearer in making this point, and providing appropriate references in the context of explaining why we included the seasonal cycle.
The annual cycle and ENSO interact with each other in a complicated way, with the annual cycle itself being a forced mode (Xie, 1994). This interaction gives rise to combination models (Stuecker et al., 2015), frequency entrainment (Timmermann et al., 2007) and ENSO’s phase-locking and seasonal variance modulation (Stein et al., 2014; Stein et al., 2010). Not only does the annual cycle in the equatorial Pacific influence the amplitude and phase of ENSO, but also vice versa. Due to this intricate coupling between these modes of variability, we have decided to retain the seasonal cycle in this context.
This information will be explicitly conveyed in the revised text, and we appreciate the reviewer for having raised it.
ll. 246-247: this is one of a few sentences I found in the text, that justify my general comment above about the lack of interpretation. In particular, the authors mention a lead-lag relation between precipitation and SST seasonal maxima. The assessment of these relations are challenging in the context of climate models (Lembo et al., 2017), together with their interpretation (cfr. Su et al. 2005 for this this specific context) and the authors might want to discuss what these mean in terms of dynamics of the systems.
We agree with the reviewer that there is an opportunity here to reference published literature that presents mechanistic interpretations of the behavior we have highlighted for the CESM2-LE. For the specific issue raised here of maximum precipitation leading maximum temperature over the Niño3.4 region on seasonal timescales (red dots in Fig. 3c and 3d), current scientific understanding maintains that precipitation is largely driven by meridional SST gradients, and is thereby not directly tied in its phasing to local SST. In other words, moisture convergence is in part determined by non-local SST conditions. We will appropriately reference the study of Xie (1996), Xie et al. (2010), Williams and Patricola (2018), and Stuecker et al. (2020) on this topic in the revised text.
l. 276: This is in part already known. Several studies (e.g. Screen 2014, Chen et al. 2016, Haugen et al., 2018) have shown evidence of a relationship between Arctic amplification and reduced temperature variance over the mid- and high-latitudes of the Northern Hemisphere, and an interpretation of this has been given from a dynamical point of view (cfr. Sun et al., 2015; Schneider et al., 2015), involving the role of precipitation.
We thank the reviewer for suggesting that appropriate references be given for describing changes in variance in temperature over the mid-to-high latitudes of the Northern Hemisphere due to polar amplification. We will reference these studies in the revised version of the manuscript, as well as the studies of Holmes et al. (2016; J. Climate) and Screen et al. (2015; BAMS).
ll. 322-323: same as in my comment to l. 276. I am not surprised that the authors find a reduction in the NEP inter-annual variability, as this is linked to the variability of near-surface temperature. The link has been discussed in previous works (e.g. Yao et al., 2021), and I believe it should be taken into account here.
It seems that the reviewer may in fact be confused by the text in our manuscript. We specifically chose not to address interannual variability in NEP. Rather our focus was on interannual variability in phenology in the lower panel of Fig. 5, as this is the behavior that has to our knowledge not been previously described in published literature. In our revisions, we will move the interpretation of the cause of the forced trend earlier in the paragraph, where we discuss the time of emergence of the trend.
In order to clarify, we will modify text at the end of our discussion of phenology to say: “Our analysis indicates that for NEP aggregated over this region the phenological shift as a decadal trend becomes emergent relative to estimates of the natural variability already within the first decades of the 21st century, a trend that is broadly consistent with observations (Zhu et al., 2016; Myers-Smith et al., 2020). The forced changes in growing season length are mostly attributable to changes in the mean temperature (Lawrence et al., 2019; Lombardozzi et al., 2020). Internal variability in the date of the onset of the growing season decreases by 35% over the course of the simulations and decreases by 18% for the date of the end of the growing season (Fig. 5, lower panel).”
The statements pertaining to attribution (expansion of growing season length being attributed to temperature) will be moved to the beginning of the same paragraph, so that the attribution is more of an emphasis in the text.
ll. 328-330: see my comment at ll. 246-247. I think the authors should comment on this finding and on how this can be interpreted.
The paragraph pointed to is introducing the climate change impacts on the mean state and variability. The reviewer is asking us to interpret the result related to changes in variability of peak and trough NEP amplitude (mentioned in liens 328-330, changes in variability along the y-axis of Fig. 5), but we don’t believe that this is where the main story lies here. Instead, we focus on the discussion of the temporal trends and changes in the seasonal variability related to spring-green up (x-axis of Fig. 5). Specifically, we focus on changes in the mean state related to forced changes in phenology, notably the earlier initiation of the growing season in the Northern Hemisphere spring, where ecological functions may be disrupted. Under future scenarios we also see reduced variability in the first zero-crossing of NEP, which we attribute to the combined effects of warming and the timing of snowmelt. A subset of the coauthors of this manuscript are pursuing this question of drivers of phenology as an offshoot project that wil complement the presentation paper analysis.
ll. 350-351: this is not a new achievement, it has been long known (see e.g. Palmer, 1993; Corti et al., 1999) that climate change projects on modes of variabilty in several ways.
We fully agree with the reviewer that the concept of anthropogenic changes can project onto modes of variability has been known for some time, and we will reference the studies of Palmer (1993) and Corti (1999) in our revisions. In order to better clarify this point, we will also state more clearly that the Large Ensembles provide a means to explore this in a statistically sound way in full complexity climate models that was not yet possible in the 1990s.
ll. 360: I wonder if the authors are able to comment on how significant these findings obtained with CESM2-LE are, in relation with other Large ENsemble exercises. described in Maher et al. (2021).
The reviewer refers to the multi-model Large Ensemble intercomparison study of Maher et al. (2021), and by implication the growing number of studies that make use of the multi-model Large Ensemble Archive presented in the study of Deser et al. (2020). From the onset of our project with the CESM2-LE, our intention has been to have our simulations be available for such multi-model studies, and to that end we specifically chose the historical/SSP3-7.0 pathway recommended in the CMIP6 protocols (ScenarioMIP) study of O’Neill et al. (2016). We have opted here to not engage in an intercomparison exercise, as this is beyond the scope of this initial presentation of the CESM2-LE itself, but to reiterate we have made every effort to facilitate such inter-comparison studies by interested parties in the future. We have also made available (https://climatedata.ibs.re.kr/data/cesm2-lens/lens-diagnostics) the results of the Climate Variability Diagnostics Package for Large Ensembles (CVDP-LE) of Philips et al. (2020) for diagnostics over a broad suite of variables that can equivalently be run for other large ensembles, as a means to facilitate studies that seek to understand model differences. We will include a link to this with an explanation in the revised version of the manuscript.
ll. 364-367: the lack of interpretation of the findings is evident here. I don’t think that the take-home message is that the Earth system is “far more sensitive in its statistical characteristics to anthropogenic forcing than previously recognized”. There is actually a literature on assessing changes in higher order moments of several aspects of climate variability, often using Large Ensemble exercises, e.g. Swain et al., 2018, for regional precipitation, Tamarin-Brodsky et al., 2020, for NH temperature variability, among others. The authors might compare their results with others, in order to explain how the sensitivity of statistical characteristics was less recognized before. As mentioned above, some of the findings, taken one by one, are confirming, or possibly expanding, what was already somehow known from previous works. The manuscript might be significantly improved, if the authors would at least qualitatively discuss what drives, and what is the relation between e.g. changes in frequency and phasing of ENSO w.r.t. SST and precipitation, cross-ensemble SD for temperature and precipitation, change in ENSO’s remote correlation with regional mean temperatures and precipitation over some regions, just to mention a few features that might be interpreted in the light of changes occurring in the general circulation.
Yes, we agree with the reviewer that the sentence towards the conclusions of the manuscript that “the Earth system is far more sensitive in its statistical characteristics to anthropogenic forcing than previously recognized” would be better rephrased as “we have provided support with new examples and new global emphasis that the Earth system is sensitive in its statistical characteristics to anthropogenic forcing, thereby building on previous studies.”
With regard to the questions posed by the reviewer with regard to Fig. 4, namely ENSO teleconnections, we will also reference properly the AGU monograph published in 2020 entitled “El Niño Southern Oscillation in a Changing Climate”, in particular the chapter by Taschetto et al. entitled “ENSO Atmospheric Teleconnections”. This synthesis reference appropriately addresses the challenges for understanding how ENSO teleconnections can change, including the relative role of local diabatic forcing and modulations of ENSO for understanding regional responses.
TECHNICAL CORRECTIONS
ll. 9: replace "runs" with "run"
We will correct this error in the revised manuscript.
ll. 150: replace "weaking" wth "weakening"
This will also be corrected, following the suggestion of the reviewer.
ll. 209: replace "associate" with "asssociation"
This will also be corrected.
ll. 256: replace "n" with "in"
This will also be corrected.
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RC2: 'Comment on esd-2021-50', Alexis Tantet, 04 Aug 2021
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AC3: 'Reply on RC2', Keith Rodgers, 03 Sep 2021
We would like to thank the reviewer for offering a number of insightful and constructive comments and criticisms, which we address below. In the text that follows, the reviewer's comments are indicated in italicized text, and our responses are shown in plain text.
Minor Comments:
Abstract:
ll. 19: To emphasize the originality of the dataset, I would also mention the simulation period and the model resolution like at l. 333
We thank the reviewer for the suggestion, this will be clarified in the revised manuscript.
ll. 23: "Greenhouse warming will in particular alter...". I would rather write "Greenhouse warming in the model in particular alters..." and mention that model uncertainty is not considered here, as explained in the paragraph of line 356.
We will implement the rewording recommended by the reviewer.
Introduction:
ll. 33: “spectral variance peaks”. The “peaks” are also relatively broad for different reasons contrary to spikes (Dirac’s). I would write “relatively sharp peaks”.
We will make the change recommended by the reviewer.
ll 53: "variance is repeated
We will remove the redundant use of "variance".
ll. 59: Explain why the results of the CMIP6 version of CESM2 is expected to be conclusive relative to "earlier studies" (l. 46). Perhaps based on what is explained in Section 2.
A large number of improvements occurred with climate models since the CCSM4 version 1.4 used in the ensemble study of Zelle et al. (2005), where the effective atmospheric resolution there having been 3.75°x3.75°. In addition to greatly improved resolution, a number of improvements in process-resolution contribute to increased skill in representing variability across a broad range of timescales. This includes the importance of land processes, with changes in the CLM5 model documented below. For the atmosphere, a sampling of the many changes that have occurred are in boundary layer and shallow convection schemes, as well as representations of microphysics. New parameterizations in the atmospheric CAM6 model are described in the Danabasoglu et al. (2020) presentation paper for the CESM2 model.
We will add appropriate text to convey these points in the revised text.
Method:
ll. 78: Missing punctuation
The problem with the missing punctuation will be fixed.
ll. 81: Is the resolution for the POP model also 1 degree? It is important to know if it is eddy-resolving since this might also effect the atmospheric variability.
The nominal resolution of the POP model is 1-degree, with enhanced resolution in the meridional direction near the equator as well as in the Southern Ocean. We will state this more clearly in the revised text.
ll. 91 - paragraph: How are these improvements measured? To which extent does the land model (fire model and agricultural management in particular) rely on assumptions regarding human behavior in the future (for instance in resopnse to climate change)?
We thank the reviewer for pointing out that we were not sufficiently clear in the paragraph starting on line 91, in particular in describing the strengths of the land model and the benchmarking that has been done to assess skill in the model. We will reorganize the paragraph at line 91 to say: “An important advance of great value to Large Ensemble investigations is achieved through new developments incorporated into the Community Land Model Version 5 (CLM5) (Danabasoglu et al., 2020; Lawrence et al., 2019; Lombardozzi et al., 2020). This model addresses a number of well-known limitations relative to previous versions of CLM, including enhanced simulated cumulative CO2 uptake over the historical period (Bonan et al., 2019) and the seasonal cycle of net ecosystem production (NEP; Lawrence et al, 2019), which is highlighted in our analysis of phenology changes. Improvements in CLM are found across a broad range of simulated variables that have been documented through evaluation of model simulations against the International Land Model Benchmarking (ILAMBv2.1) package and other analyses (Collier et al., 2018; Danabasoglu et al. 2020; Lawrence et al., 2019; Wieder et al., 2019). Notable features also included in CLM5 are the explicit representation of agricultural management and improvements in the implementation of the prognostic fire model (Lombardozzi et al, 2020; Lie et al., 2013; Li and Lawrence, 2017). We note that land model trajectories are sensitive to the SSP-RCP scenarios that determine the spatial distribution and extent of land use and land cover change, as well as climate change scenarios (O’Neill et al., 2016). “
Results:
ll. 169: Could you explain what motivated the choise of these observables? I guess one factor is the relationship between the observables and climate-change impact, but this is not obvious.
The variables we chose for Fig. 2 reflect interest in both climate/ecosystem dynamics and societal relevance in terms of adaptation and resource management. We will state this more emphatically in our revisions.
ll. 169:
Instead of the Fourier transform of the observable, why not use an estimate (e.g. periodogram) of the power spectrum which can be directly related to the variance that you use in Figure 1 (as the integral of an adequately normalized power spectrum)? The variance is also used in Figure 3. If this is in fact what you are doing, please make it clearer.
In the caption for Fig. 2 and in l.169 we specified “amplitude spectrum”, which is the square root of the power spectrum. Please note that Fig. 1, Fig. 3e and Fig. 3f are cross-ensemble standard deviations using annual mean data (the annual mean is calculated first for each member, and then the standard deviation is taken across ensemble members for the same year). Furthermore, they are not representing “variance”. Thus, even if we use power spectral density (PDS) in Fig. 2, the integral of PSD is not equivalent to what is shown in either Fig. 1 or Fig. 3. We chose the amplitude spectrum in Fig. 2 to have a better understanding of the amplitudes of perturbations at different timescales, and to be consistent with the standard deviations shown in the other figures. Also, if PDS were used, the already strong annual cycle and its harmonic peaks in the amplitude spectrum would become too large, making other spectral behaviors relatively less variable.
ll. 169: To avoid spectral leakage, a window should be applied before the FFT, is this the case?
The point is well-taken, but the large degree of aggregation used for Fig. 2 (both across ensemble members but also spatial aggregation) alleviates the need for windowing. We will state this more clearly in our revised text.
ll. 171 and 172: Why 35 years and not 30 years (2070-2099 and 1960-1989)?
We thank the reviewer for catching this typo, we will modify "35 years" to say "30 years".
ll. 173: If the
power spectrum is computed, an alternative would be to first compute correlation functions for each member, average over the members and then do the FFT. I do not know which estimation method has the best properties, but could you explain why you made this choice?
There is a disparity between dynamical characteristics (such as power spectra) and statistical characteristics (like the natural measure) in that we can – for now – define only the latter in an instantaneous/shapshot or pullback sense. Given that there is no reference for the power spectrum with respect to which biases are defined it is not really possible to rank different possibly estimators. We went with what appears to be the most intuitive estimator: the E-mean of the temporally computed power spectra (or FFT amplitudes). A benefit of this quantity is that nonergodicity could be naturally defined considering the difference between this and the correct, conceptually sound quantity.
Figure 2: What are the units of the spectral amplitudes given the observables?
Please find this in the caption for Fig. 2 in the original submitted manuscript, where we state: “Spectra are shown as amplitude, with the units being the same as the x-axis for the PDFs.”
ll. 183: Even if I do not think that it is necessary to add confidence intervals to all panels and for all frequencies or to test the significance of the differences between spectra, could you give an estimate of what would be the width of these confidence intervals given that data that you use (in the supplementary material for instance)? This would also make this section more coherent with the part on wavelets.
We will provide a 95% confidence interval for Niño3.4 precipitation in the Supplementary Material, and provide a figure. The confidence interval is simply estimated as 1.96 multiplied by standard error of 100 spectra at each frequency. In this calculation, we first spatially average spectra at individual grid points over the Niño3.4 region for each member, and we treat the sampling size N=100 for the 100 ensemble members. We believe that this is a somewhat conservative approach. If we assume that all grid points can be sampled, the confidence interval becomes much narrower as the sampling number becomes much larger (N=100*number_of_gridpoints).
ll. 200 and ll 208: Although a scalar observable can techincally be seen as a bilinear form, I would reserve the term positive definite for non-trivial bilnear forms (e.g. represented by non-scalar matrices) and simply write "positive variables"
We thank the reviewer for making this point, and we wil use the term "positive variables" as suggested.
ll. 202: This is not true for all stochastic processes. I guess you mean for a Brownian motion?
We will modify the text from saying "stochastic processes" to say "white noise processes", we thank the reviewr for expressing the need to be clearer here.
Could you clarify what is meant by "cross-ensemble" everywhere this expression is used?
Yes, we will do this.
ll. 256: "minimum m" => "minimum in"
This is correct, we will make the change as suggested.
ll. 265: same as for ll. 251. . “cross-ensemble calculations applied for identical time records for each ensemble member” is not clear to me. The standard deviation is computed from a sample combining all members and all years for a given period (historic or future)? Based on the caption, I guess not, the standard deviation is computed over the ensemble and then time-averaged over the period. Could you clarify and explain why you made this choice and not the other?
We apologize for not being clearer, we will clarify this in the text. The inference here made by the reviewer is correct, the standard deviation was first computed across the ensemble members for the same time slice, and then time averaging was conducted. This was done out of pragmatism, as we were looking for convergence (or non-convergence) with the number of years included through the averaging procedure.
ll. 310: "and" => "an" I guess
Yes, this is correct, we will change this.
Figure 5: Do the histograms aggregate ensemble members for a single year only or is there also an aggregation of the 20 years in the interval (in which case the histograms would include the 20y-trend)?
The aggregation is done for individual years, we will clarify this in the text.
Figure 5: How are the histograms estimated? Using a grid? Which interval length(s)?
The width of binning for the histograms is 1-day, we will clarify this in the text, we thank the reviewer for pointing out that this wasn’t clear in the reviewed manuscript.
ll. 318: Since the spacing between the vertical grid lines in Fig. 5 represents an interval of 10 days, it seems to me that the shift in the onset is closer to 3 weeks or even 4 weeks than 2 weeks. Am I wrong?
We thank the reviewer for raising this question, we will replace the mis-statement of the duration of the onset shift to reflect that it is in fact three weeks. Comparing the mean over 1860-1869 and the mean over 2090-2099, the day of the first zero-crossing shifts 20.9 days and the day of the termination shifts 6.3 days.
ll. 322: How do you "measure" the interannual variability? Even if you read the histograms by eye, I guess that you have some metric representation of the spread in mind, such as the standard deviation. In fact, if you use the distance between the minima and the maxima, the interannual variaiblity appears comparable to the trend to me.
We will clarify this in the text, we used one standard deviation to measure the variabiltiy.
ll. 325: Same question as for ll. 322: Which measure do you use to obtain these percentages?
We thank the reviewer for raising this point, as we were insufficiently clear in the text. The calculations were performed year-by-year, where the transitions (zero crossing) were calculated for each member across the full set of 90 ensemble members as this progressed in time.
Discussion:
ll. 331: I would call this section “Summary and Discussion”, but that’s a detail.
We will follow the suggestion of the reviewer here, and change the title of the section.
ll. 335: English is not my mother tongue, so I may be wrong, but shouldn’t “affords” be replaced by “offers”?
Yes, we will make the appropriate change so that the text says “offers”.
References are hard to read because entries are not visually separated.
We will fix the formatting for the revised version of the manuscript to facilitate reading the references.
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AC3: 'Reply on RC2', Keith Rodgers, 03 Sep 2021
Peer review completion
Post-review adjustments
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Keith B. Rodgers et al.
Keith B. Rodgers et al.
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