Gaia: Complex systems prediction for time to adapt to climate shocks

Hunt, Allen G.; Sahimi, Muhammad; Faybishenko, Boris; Egli, Markus; Kabala, Zbigniew J.; Ghanbarian, Behzad; Yu, Fang

doi:https://doi.org/10.5194/esd-2023-21

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https://doi.org/10.5194/esd-2023-21

Preprints

27 Sep 2023

| 27 Sep 2023

Status: this discussion paper is a preprint. It has been under review for the journal Earth System Dynamics (ESD). The manuscript was not accepted for further review after discussion.

Gaia: Complex systems prediction for time to adapt to climate shocks

Allen G. Hunt, Muhammad Sahimi, Boris Faybishenko, Markus Egli, Zbigniew J. Kabala, Behzad Ghanbarian, and Fang Yu

Abstract. Earth’s climate has undergone significant fluctuations in the geologic past. We focus on the glacial episodes that followed the major waves of invasion of land plants. Twice in Earth’s history the impacts of land plant innovations on the atmosphere through increased CO₂ drawdown have precipitated sufficient cooling to produce ice ages. Each time, however, adaptation of soil ecosystems eventually helped re-establish apparent steady-state conditions, i.e., new equilibrium temperature and atmospheric CO₂ content, ending each of the glacial episodes. In each case, the time interval between the initial innovation and the emergence from the glacial episode was approximately 60 Myr. The consistency of the time scale of the response invites an explanation in terms of a universal rate of dispersal of genetic information that encapsulates the biogeochemical cycle of cellulose production and decay. In this paper, we postulate that the long time for adaptation is a consequence of the time required for the spread of an entire clade though the soil to continental scale. Although 60 Myr appears to be a long time, it is very short compared to the time required for diffusion to transport even molecules like HCO_3⁻ or sugar through the soil over a continental distance of 5,000 km, which is between 10¹⁴ and 10¹⁶ years for solutes with such soil diffusion constants in the range 10⁻¹¹ m²s⁻¹. Horizontal solute transport through heterogeneous media by advection might be considered as a dispersal mechanism, but is also known to require enormous time scales (ca. 150 Myr for 500 m). We also seek a relevant mechanism in the known scaling of plant and fungal growth rates as a function of time, which can facilitate as well the movement of bacteria, and predicted these rates theoretically on the basis of the universal optimal 2D paths tortuosity from percolation. Comparison with actual data pairs (7,000) for plant and fungal growth rates were used to verify these predictions over 13 orders of magnitude of time, from about one minute to 100 kyr; extrapolation over less than three additional orders of time yields a continental-scale transport time of 80 Myr. We now interpret this prediction in terms of Margulis’ understanding of emergent behavior of coupled (soil and plant) ecosystems responding to climate shocks induced by plant innovations.

Received: 05 Jul 2023 – Discussion started: 27 Sep 2023

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Allen G. Hunt, Muhammad Sahimi, Boris Faybishenko, Markus Egli, Zbigniew J. Kabala, Behzad Ghanbarian, and Fang Yu

Status: closed

RC1:
'Comment on esd-2023-21', Stefano Manzoni, 02 Oct 2023

Hunt and co-authors propose a new hypothesis to explain the duration of glaciations during the period 500-350 million years ago, based on the characteristic velocity of root and hyphal spread in soil. They argue that roots and hyphae (and associated bacteria) would spread over continental scales, thus transporting genetic information (and evolutionary adaptations), with a characteristic time scale comparable to those typical of the glaciation durations in that period. This would indicate that these components of the ecosystem cooperate to stabilize climate even if plants by themselves would tend to destabilize it due to their efficient uptake of atmospheric CO2 (which in turn cools climate). Cooperation would work because fungi and bacteria would co-evolve with plants to efficiently decompose organic matter, returning CO2 to the atmosphere. I hope I understood the proposed rationale correctly, because it is not described very clearly.
This hypothesis is tested using a relation between root or hyphae extension as a function of time derived from 2D percolation in porous media. The idea is that roots and hyphae extend in soil in an ‘optimal’ way, so that the distance travelled scales as a power law of time with a predictable exponent from percolation theory. This relation is consistent with the rate of growth of plants in optimal growing conditions, which in turn is proportional to the rate of root growth, lending some support to the theory. However, this theory was developed at individual plant scale, extending at most to clonal plants or hyphal networks spreading over ~100s of meters (or few km for some fungal networks). Here the authors extend this concept to the continental scale, resulting in time scales ~60 million years.
My main concern is at a conceptual level—evolutionary innovations do not need to be transported through the soil because they can spread orders of magnitude faster by other media. Spores and seeds of plants, and spore (or entire cells) of microorganisms can be transported by wind or animals, or via eroded soil in surface water bodies. Underground dispersal might be used, but over short distances, and it seems a bit far fetched to hypothesize that it is a relevant mechanism over continental scales. One could even argue that it would take a single river or a mountain chain to stop entirely this mode of dispersal across a continent. Anything stopping roots and hyphae would drastically hamper transport underground. Moreover, from an evolutionary perspective, solutions that are costly are outcompeted—and growing roots and hyphae is a costly way to colonize new land.
My other general comment is that it is not clear which adaptations would be transported. I would present evidence of co-evolution of say lignin-based wood and ligninolytic enzymes to develop arguments of coordination between plants and saprotrophs. But even with strong evidence of such a co-evolution, I am not sure it would be possible to support the proposed hypothesis given the other (faster, metabolically less costly) dispersal modes available to producers and decomposers.

Other comments
L91: I would not agree with this strong statement (see my comment above)
L129: just a detail—aren’t data and observations the same thing?
L135: how do human improvement of plant growing conditions fit in this work? We are dealing with deep past when conditions were probably far from optimal in many areas of the world
L141-142: argument is not clear
L184: “subaerial stems” meaning roots and hyphae?
L192: plant xylem is not a random medium, so I am not sure percolation theory can be applied as in soil
L203-213: I cannot follow this argument—how are the time scales of horizontal transport and weathering connected? My understanding was that the proposed hypothesis was not about weathering, but about producers and decomposers finding a ‘balance’ to recycle CO2 and keep the Earth warm
L229-234: also here I find it difficult to follow the presented arguments—what is the connection with the proposed hypothesis?
L238: gross primary productivity varies with increasing atmospheric CO2 in the range 1-6 gC/m^2/y/ppm (https://doi.org/10.1073/pnas.2115627119). Thus, a 25% increase in CO2 (100 ppm) leads to an increase ~100-600 gC/m^2/y, which is much more than the 2% figure reported. Additional explanations are needed here, including recent estimates of productivity sensitivity to CO2
L238-239: productivity is related to transpiration rate (only a fraction of evapotranspiration), but when changing temperature, the relation might break down as warmer conditions (for given water vapor content) trigger stomatal closure and decrease photosynthetic rates

Citation: https://doi.org/10.5194/esd-2023-21-RC1
- CC1: 'Reply on RC1', Allen G. Hunt, 08 Oct 2023
  
  At short time scales under modern conditions, diffusion of seeds by, e.g., birds, is so rapid that such arguments as suggested by Dr. Manzoni, when applied to single species over short length scales, are undoubtedly valid. A common model of seed dispersal is diffusion and a cursory investigation of reported model parameters (Allen et al. 1991) reveals that relevant diffusion coefficients are on the order of, e.g., 25m²/yr. But, application of such arguments to the distant past may be based on biases from the present. We suggest several caveats should be considered that, when taken together, can change the relative importance of soil-based and atmospheric pathways. First, it should be noted that advection-based mechanisms, such as the growth through the soil mechanism that we proposed, gain in speed relative to diffusion when the spatial scale increases. From 5 meters to a continental scale of 5,000 km represents a 6 order of magnitude increase in length scales. Although this increase might not produce enough slowing of diffusion for domination of our root-growth related mechanism today, Paleozoic conditions were very different. For example, the extracted parameter referenced above included individual time steps controlled by the advection from flying birds over a period of time equal to that required for seeds to pass through the bird gut. The transport path of seeds through birds did not exist until the Mesozoic. Second, the overall conditions for plant germination and growth were very different in the era before the plants modified both soil and atmosphere (Kleidon, 2002) to their advantage. Third, it is necessary to transport the entire clade, rather than an isolated species. Fourth, we can appeal to the dates already mentioned in the manuscript to suggest that plant dispersal in the Paleozoic was not orders of magnitude faster than the relaxation to homeostasis. After the initial invasion of the land by plants, it took at least 12 million years before initiation of an ice age (from 500 Ma to 488 Ma), and after the second wave of innovation starting at 420Ma, 48 million years (until 372 Ma). These two results support an inference that attaining homeostasis requires time scales between 5 times as long and 5/4 as long as the full exploitation of land-accessible resources by plants, which is not unexpected in view of the increased number of species involved. Nevertheless, if diffusion were not a noticeably faster mechanism, at least over smaller length scales, one would not expect that interruptions by rivers could be overcome by a faster mechanism of dispersal operating in series with the slower “through the soil” mechanism postulated here. Under these circumstances, we do still suggest that the close correspondence of a parameter-free prediction with the observed time scale warrants serious consideration as a viable hypothesis.
  Allen, L. J. S., Allen, E. J., Kunst, C. R. G., & Sosebee, R. E. (1991). A diffusion model for dispersal of Opuntia imbricata (cholla) on rangelanThe Journal of Ecology, 1123-1135. https://doi.org/10.2307/2261103.
  Kleidon, A.: Testing the effect of life on Earth’s functioning: how Gaian is the Earth system?, Climatic Change, 52(4), 383–389, https://doi.org/10.1023/A:1014213811518, 2002.
  
  Citation: https://doi.org/10.5194/esd-2023-21-CC1
- AC1: 'Reply on RC1', Allen G. Hunt, 12 Feb 2024
  
  The comment was uploaded in the form of a supplement: https://esd.copernicus.org/preprints/esd-2023-21/esd-2023-21-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/esd-2023-21-AC1
RC2:
'Comment on esd-2023-21', Anonymous Referee #2, 19 Dec 2023

Building on the idea of the Earth’s biosphere as a self-organizing global organism, the authors present an “interesting coincidence” between the time it took ecosystems and he Earth to re-equilibrate following ice ages (60Myrs) and a timescale needed for transport over continental scale predicted from percolation theory (80Myrs). There is no doubt that such a similarity is intriguing, even though a bit speculative. It can trigger an interesting debate on whether the Earth is truly able to self-organize as if it was a single organism, for which case scaling relationships originating from such spatiotemporal correlations may exist.
The only concern I have here and would like to authors to elaborate more on this, is whether predictions from percolation theory, which may be suitable in heterogeneous porous media, like soils, can be extrapolated to spatial scales larger than scales at which transport in soils is dominant. In other words, transport in soils may be explained by percolation theory. Why would this apply to continental scale? Applying the same scaling relationship over this broad range of scales implies that the underlying spatial correlations are given by the same dominant processes. But transport processes in soils are different from transport processes over continental scales, so shouldn’t at least the exponent of the scaling change? For example, even between organisms, we can use one single exponent to predict metabolic rates based on body mass. However, for larger organisms where vascular transport is important you find shifts in the scaling exponents. This is because there is a new process at play that affects transport, hence correlation within the organism leading to an organismal metabolic rate.
Another complication when we go to continental scale is the impact of climate and dominant winds based on location. I am not sure how this plays a role when considering the whole globe over timescales of Myrs, but commenting on this aspect is also warranted.
To conclude, I may not agree at the moment on the specific relationships applied over this scale, but this is a debate to have, and I commend the authors on pursuing such interesting questions.

Citation: https://doi.org/10.5194/esd-2023-21-RC2
- CC2: 'Reply on RC2', Allen G. Hunt, 20 Dec 2023
  
  We would like to thank the referee for his/her thoughtful comments.
  The referee has two concerns. One is that climate and dominant wind (directions and speed) could be important. We agree that further clarification is important here. The second concern is the relevant scaling relationship could change form with increasing scales. The referee is concerned that percolation theory for transport through soils might not even be valid at global or continental scales, or that an alternative scaling relationship may become more important. We think that these two concerns are actually interrelated and would like to respond as follows.
  Climate impacts the kind of soil transport that we have proposed through the water fluxes, which modulate the scaling law we invoked. Thus, our prediction would underestimate the time scale at which self-organization could occur, if the climate over too much of a continent were too arid, in which case it could lead to a significant overestimate and reduce the impact of the convergence of predicted and deduced time scales. Wind speed or direction has no impact on our scaling relationship, but it could, in principle, have an impact on competing scaling relationships, such as one based on diffusion, increasing the step lengths and/or correlating the step directions. We have pointed out in response to the first referee that the form of diffusion relations for, e.g., seed dispersal, is such that they rapidly become insignificant relative to advective-based relationships, if the number of individual steps encompassing the largest scales is large. Here, the conclusion would remain the same, unless wind speeds had been orders of magnitude larger than what are today observed, but it might be possible that the condition applied to the typical number correlated steps is violated in the case of positive correlations of the individual steps. Thus, it would be possible that if the same wind direction would be relevant at most times and most areas on a continent our argument could fail.
  The second concern is that the relevant scaling relationship could change with increasing spatial scales. Scaling laws of percolation theory with relevance to the Earth and Planetary Science have, in principle, no limits in scale (see last paragraph). The one we proposed has been directly verified at the scale of an island, i.e., 10 km, around which a monogenetic individual plant grew in 100,000 years. At a very early point in our research on this subject, we discussed the possibility that 3D scaling exponents of percolation theory should apply; which we thought would be the case as long as the root lateral spread of a plant is much less than the soil depth. In fact, the first author initially proposed this. However, no evidence in the data (well over 50 references, including a database with 6,000 individuals) was found to support such a cross-over. As of this date, we have still found no evidence to suggest the relevance of any cross-over to other dimensionalities or processes at scales from 10 μm to 10 km, even though our initial hypothesis was that there should be at least one such cross-over. Thus, we propose that this particular concern (though not supported by data known from investigations to date) as well as the others addressed above, could nevertheless be grounds for future investigations, in line with the referee's comment that this beginning could stimulate useful discussion.
  Finally, regarding the applicability of percolation scaling laws to the largest (global) length scale, we note that A.A. Saberi (Phys. Rev. Lett. 110,178501, 2013) invoked percolation scaling laws to explain topographical data for both Earth and Moon. He also used the same concepts (Astrophysical Journal Letters 876, l25, 2020) to provide evidence, based on data, for the existence of an ancient sea level on Mars, a hotly debated issue. Thus, there is no question that percolation scaling laws are applicable even at the largest length scales.
  
  Citation: https://doi.org/10.5194/esd-2023-21-CC2
- AC2: 'Reply on RC2', Allen G. Hunt, 12 Feb 2024
  
  We would like to thank the referee for his/her thoughtful comments as well as note that there exists some overlap with the comments of the first referee, Dr. Manzoni. Thus our response does not overly emphasize the similarities.
  The referee has two concerns. One is that climate and dominant wind (directions and speed) could be important. We agree that further clarification is important here. The second concern is the relevant scaling relationship could change form with increasing scales. The referee is concerned that percolation theory for transport through soils might not even be valid at global or continental scales, or that an alternative scaling relationship may become more important. We think that these two concerns are actually interrelated and would like to respond as follows.
  Climate impacts the kind of soil transport that we have proposed through the water fluxes, which modulate the scaling law we invoked. Thus, our prediction would underestimate the time scale at which self-organization could occur, if the climate over too much of a continent were too arid, in which case it could lead to a significant overestimate and reduce the impact of the convergence of predicted and deduced time scales. Wind speed or direction has no impact on our scaling relationship, but it could, in principle, have an impact on competing scaling relationships, such as one based on diffusion, increasing the step lengths and/or correlating the step directions. We have pointed out in response to the first referee that the form of diffusion relations for, e.g., seed dispersal, is such that they rapidly become insignificant relative to advective-based relationships, if the number of individual steps encompassing the largest scales is large. Here, the conclusion would remain the same, unless wind speeds had been orders of magnitude larger than what are today observed, but it might be possible that the condition applied to the typical number correlated steps is violated in the case of positive correlations of the individual steps. Thus, it would be possible that if the same wind direction would be relevant at most times and most areas on a continent our argument could fail.
  The second concern is that the relevant scaling relationship could change with increasing spatial scales. Scaling laws of percolation theory with relevance to the Earth and Planetary Science have, in principle, no limits in scale (see last paragraph). The one we proposed has been directly verified at the scale of an island, i.e., 10 km, around which a monogenetic individual plant grew in 100,000 years. At a very early point in our research on this subject, we discussed the possibility that 3D scaling exponents of percolation theory should apply; which we thought would be the case as long as the root lateral spread of a plant is much less than the soil depth. In fact, the first author initially proposed this. However, no evidence in the data (well over 50 references, including a database with 6,000 individuals) was found to support such a cross-over. As of this date, we have still found no evidence to suggest the relevance of any cross-over to other dimensionalities or processes at scales from 10 μm to 10 km, even though our initial hypothesis was that there should be at least one such cross-over. Thus, we propose that this particular concern (though not supported by data known from investigations to date) as well as the others addressed above, could nevertheless be grounds for future investigations, in line with the referee's comment that this beginning could stimulate useful discussion.
  Finally, regarding the applicability of percolation scaling laws to the largest (global) length scale, we note that A.A. Saberi (Phys. Rev. Lett. 110,178501, 2013) invoked percolation scaling laws to explain topographical data for both Earth and Moon. He also used the same concepts (Astrophysical Journal Letters 876, l25, 2020) to provide evidence, based on data, for the existence of an ancient sea level on Mars, a hotly debated issue. Thus, there is no question that percolation scaling laws are applicable even at the largest length scales.
  
  Citation: https://doi.org/10.5194/esd-2023-21-AC2
EC1:
'Comment on esd-2023-21', Axel Kleidon, 09 Feb 2024

Dear authors,
In the meantime, I have received a third review, which was sent to me in a rush, so it is quite short, but includes valuable points of critique that would need serious thought. I paste the review below.
Axel Kleidon (Editor)
====
I found that very hard to understand. They need to

1 - clearly articulate what the Gaia Hypothesis/Theory planetary homeostasis is. They need to provide a clear definition. Their review/articulation and potential understanding of Gaia is I think potentially limited. For mechanisms see

Lenton, T.M., Daines, S.J., Dyke, J.G., Nicholson, A.E., Wilkinson, D.M. and Williams, H.T., 2018. Selection for Gaia across multiple scales. Trends in Ecology & Evolution, 33(8), pp.633-645.
Lenton, T.M., Dutreuil, S. and Latour, B., 2020. Life on Earth is hard to spot. The Anthropocene Review, 7(3), pp.248-272.

Etc… it’s not hard to find...

2 - they then need to clearly articulate how global surface temperature controls evolve & change *without/prior* to their proposed mechanism.

3 - they then need to clearly explain and describe what their proposed mechanism is. A figure/diagram or two would help…

4 - then they can get into their methods and results.
====

Citation: https://doi.org/10.5194/esd-2023-21-EC1
- AC3: 'Reply on EC1', Allen G. Hunt, 12 Feb 2024
  
  As authors, we are addressing important time scales with known rates in checking inferences. This is akin to addressing rates of evolutionary drift to determine what time scale is implied by certain changes in genetics. In this context, I would like to address the Stommel diagram provided by the author in Figure 1 in the first publication. Understanding what is possible at a given time scale, together with comparisons of rates is, of course, important to an overall understanding of history, as we agree. However, it is our opinion that the Stommel diagram does not accomplish this.
  The bubble for weathering covers time scales of 30kyr to 100Myr for length scales from about 20km to 100km
  These numbers make absolutely no sense for describing the evolution of the vertical direction of the weathered layer. But they make no sense in the horizontal directions either (soil development across landscapes). The author would be correct in assuming that there is no particular rate associated with the horizontal dimension (the more or less horizontal orientation of the bubble), but the horizontal dimension is bounded only by the size of a continent (not by the tens of kilometer scale) and weathering is going on all the time. Of course, field weathering is limited by transport only on the scale of months to 100’s of million years; for exposures that are even younger (landslides, treethrow, glacial retreat, mining tailings, etc.) weathering can be limited by reaction kinetics for time scales up to a month, or in extreme cases, a couple of decades. If a vertical spatial scale were meant, the diagonal extent of the bubble on a logarithmic graph would have a slope of about 1/2, going from about 1cm at one year to about 100m at around 100 Myr. At smaller time scales, the slope would be 1, consistent with a weathering rate limited by reaction kinetics. For experimental confirmation see the paper of White and Brantley (2003) “The effect of time on the weathering of silicate minerals: Why do weathering rates differ in the laboratory and in the field?” For actual theory compared verified by predictions, see our papers listed at the end, but particularly the articles Yu and Hunt, 2017 (Earth and Space Chemistry), Egli et al. 2018, and the article Hunt et al. (2021).
  
  The bubble for soil erosion also extends horizontally rather than diagonally.
  
  The horizontal orientation of the bubble means that there is no scale dependence of erosion times. Again, this particular criticism does not apply if the author is considering horizontal dimensions. But locating the bubble at length scales between 100m and 10km makes no sense in that case, as, again the scale limit would be in the thousands of kilometers (continental), unless the author is talking about some particular horizontal organization (a catchment? an industrial development, a farm?). Soil erosion is occurring everywhere at all times. Natural soil erosion rates range from less than 1m / Myr to above 2000m/Myr, consistent with a vertically oriented bubble covering in excess of 3 orders of magnitude, if it is wished to explain rate variability, or if it is wished to describe the scale dependence, a diagonal bubble with slope 1 and a vertical variability of three orders of magnitude. The fact that the soil depth tends to increase more slowly than a slope of 1 (about ½) means that areas with a larger erosion rate must have a smaller soil depth, in order for there to be an equivalence between the two and a steady-state system. Is the author interested in any details or predictions?
  
  The bubble for tectonics is also horizontal with limits on the time frames considered.
  
  Plate tectonics is a process associated with horizontal scales that increase linearly in time. If the author is trying to describe horizontal evolution, as may be the case in the other processes mentioned above, this bubble should not be oriented horizontally but diagonally with a constant slope. Other interpretations are equally difficult to understand: the author states that plate tectonics as a mechanism for global change has not been in existence until the last billion years or so; then why does the bubble extend from about 10Myr to nearly 100Gyr? Should it not extend back to time scales when effects are observed in the changes of stream networks, i.e., 10kyr – 100kyr? Or in earthquake intervals that can cause coastal forests to be drowned or exhumed, i.e., 500yr?
  
  Or let’s look at the bubble for glaciation. This is again horizontal, implying the existence of no rates of significance. The horizontal scale extends from about 10kyr to 100Myr. The vertical scale extends from about 500km to about 5000km. This corresponds to the extents of ice sheets on major islands to continents. The time scale of 10kyr is a good estimate of the time it takes to melt. The time it takes to form a continental glacier is on the order of 100kyr, and the time span of the Pleistocene is on the order of a couple of million years, whereas Paleozoic glaciations lasted about 60 Myr. Thus, three separate processes are included in the same bubble; glacier melting, glacier formation, glaciation episodes (which may be oscillating between glaciation and interglacials). This bubble is relatively accurate, but encompasses such a wide range of implications for biota, that particular evolutionary connections can be causal in either direction, or simply amplifications of orbital cycles. A Stommel diagram must be carefully defined and accurately drawn, in order to assist in association of the processes of principal relevance to observed events.
  
  The author also cites the presumed natural weathering thermostat. However, such a thermostat does not exist. Since reaction kinetics is irrelevant to silicate weathering rates for surfaces that have been exposed for longer than a few months, there is no silicate weathering thermostat. There is an indirect connection, however, between silicate weathering and temperatures, as long as warmer climates produce an accelerated water cycle. Such an association, however was not present during the Permian extinction event (associated with the mega eruptions of the Siberian traps) and the build-up of atmospheric carbon could not be removed through weathering due to the aridity of large sections of the interconnected landmass of Pangaea Hunt, A. G., and M. Sahimi, 2017, The Spaces in Between, Editor’s Vox, Eos https://eos.org/editors-vox/the-spaces-in-between. Lovelock’s supposition that bacteria were responsible for the establishment of homeostasis was verified, Ball, P. James Lovelock reflects on Gaia’s legacy. Nature (2014). doi:10.1038/nature.2014.15017.
  
  I think that citation of the particular paper chosen by the referee is not particularly helpful. The mechanisms for evolution and the associated time scales are not justified carefully; if they are not less sloppily formulated than the processes that I can define carefully, then their relevance to the speculations presented by the author is, at best, unclear.
  
  The mechanism described in our paper is quite clear and in complete accord with the understanding of Lynn Margulis (Margulis, L. Symbiotic Planet: A New Look At Evolution. Houston: Basic Book. (1999)) regarding interacting ecosystems: plants in exclusion, and the entire soil ecosystem for example. It is necessary for the microbiota to be able to decompose lignin and other constituents of woody plants in order for the carbon cycle to revert to an equilibrium at higher temperatures. We have shown in other places (and cited the sources) that a greedy plant model (in accord with Odum’s 1959 hypothesis that maximum plant productivity guides plant evolution) yields the first accurate predictions of the water balance (see, for example the citation in Eos, https://eos.org/editor-highlights/how-much-terrestrial-precipitation-is-used-by-vegetation) regarding our original publication in AGU Advances, and is thus compatible with the idea that invasion of the land by plants would generate a disequilibrium in the carbon cycle, with a rapidly dropping temperature, that could lead to an ice age (which definitely occurred). The increase in temperature through re-establishment of homeostasis would then lead also to an additional increase in plant productivity (strengthening adaptation for maximum productivity) for multiple reasons, e.g., new exposure of bare ground, higher temperatures, decreased aridity, higher rates of decomposition and nutrient cycling, etc. The problem is, however, that development of the appropriate genetic combination to be able to decompose wood in one geographical microhabitat does not obviously lead to the dispersal of both the producers and the consumers throughout a continent. We have provided a potential means to do that, through the scaling rates of growth of plant root systems with their associated communities of fungi (synergistic and pathogenic) and bacteria through the soils. The argument that the community as a whole must be transported from one place to another is strengthened by a recent research result, https://www.eurekalert.org/news-releases/1033091, “This study challenges traditional views that predominantly focus on competitive interactions in ecological networks by showing that facilitative networks are more common in soil ecosystems than previously thought. This paradigm shift stems from recognizing that soil processes, such as decomposition of organic matter, necessitate cooperation among various species.” The original publication is here. https://www.pnas.org/doi/10.1073/pnas.2308769121.
  
  Hunt, A. G., and B. Ghanbarian, 2016, Percolation theory for solute transport in porous media: Geochemistry, geomorphology, and carbon cycling, Water Resources Research, 52: 7444-7459
  Yu, F., B. Faybishenko, A. G. Hunt, and B. Ghanbarian, 2017, A simple model of the variability of topsoil depths Water 9 (7) 460 doi:10.3390/w9070460 .
  Yu, F., and A. G. Hunt, 2017, An examination of the steady-state assumption in certain soil production models with application to landscape evolution, Earth Surface Processes and Landforms. DOI: 10.1002/esp.4209.
  Yu., F. and A. G. Hunt, 2017, Predicting soil formation on the basis of transport-limited chemical weathering, Geomorphology, https://doi.org/10.1016/j.geomorph.2017.10.027.
  Yu, F., A. G. Hunt, M. Egli, and G. Raab, 2019, Comparison and contrast in soil depth evolution for steady-state and stochastic erosion processes: Possible implications for landslide prediction, Geochemistry, Geophysics, Geosystems, https://doi.org/10.1029/2018GC008125.
  Yu, F. and A. G. Hunt, 2017, Damköhler number input to transport-limited chemical weathering and soil production calculations, Earth and Space Chemistry. DOI: 10.1021/acsearthspacechem.6b00007.
  Egli,M., A. G. Hunt, D. Dahms, G. Raab, C. Derungs, S. Raimondi, F Yu, 2018, Prediction of soil formation as a function of age using the percolation theory approach, invited by Frontiers in Environmental Sciences, 28: https://doi.org/10.3389/fenvs.2018.00108
  Hunt, A G., B. A. Faybishenko, and B. Ghanbarian, 2021, Non-linear hydrologic organization, Non-linear Processes in Geophysics, 28: 599-614. https://doi.org/10.5194/npg-28-599-2021.
  
  Citation: https://doi.org/10.5194/esd-2023-21-AC3

Status: closed

RC1:
'Comment on esd-2023-21', Stefano Manzoni, 02 Oct 2023

Hunt and co-authors propose a new hypothesis to explain the duration of glaciations during the period 500-350 million years ago, based on the characteristic velocity of root and hyphal spread in soil. They argue that roots and hyphae (and associated bacteria) would spread over continental scales, thus transporting genetic information (and evolutionary adaptations), with a characteristic time scale comparable to those typical of the glaciation durations in that period. This would indicate that these components of the ecosystem cooperate to stabilize climate even if plants by themselves would tend to destabilize it due to their efficient uptake of atmospheric CO2 (which in turn cools climate). Cooperation would work because fungi and bacteria would co-evolve with plants to efficiently decompose organic matter, returning CO2 to the atmosphere. I hope I understood the proposed rationale correctly, because it is not described very clearly.
This hypothesis is tested using a relation between root or hyphae extension as a function of time derived from 2D percolation in porous media. The idea is that roots and hyphae extend in soil in an ‘optimal’ way, so that the distance travelled scales as a power law of time with a predictable exponent from percolation theory. This relation is consistent with the rate of growth of plants in optimal growing conditions, which in turn is proportional to the rate of root growth, lending some support to the theory. However, this theory was developed at individual plant scale, extending at most to clonal plants or hyphal networks spreading over ~100s of meters (or few km for some fungal networks). Here the authors extend this concept to the continental scale, resulting in time scales ~60 million years.
My main concern is at a conceptual level—evolutionary innovations do not need to be transported through the soil because they can spread orders of magnitude faster by other media. Spores and seeds of plants, and spore (or entire cells) of microorganisms can be transported by wind or animals, or via eroded soil in surface water bodies. Underground dispersal might be used, but over short distances, and it seems a bit far fetched to hypothesize that it is a relevant mechanism over continental scales. One could even argue that it would take a single river or a mountain chain to stop entirely this mode of dispersal across a continent. Anything stopping roots and hyphae would drastically hamper transport underground. Moreover, from an evolutionary perspective, solutions that are costly are outcompeted—and growing roots and hyphae is a costly way to colonize new land.
My other general comment is that it is not clear which adaptations would be transported. I would present evidence of co-evolution of say lignin-based wood and ligninolytic enzymes to develop arguments of coordination between plants and saprotrophs. But even with strong evidence of such a co-evolution, I am not sure it would be possible to support the proposed hypothesis given the other (faster, metabolically less costly) dispersal modes available to producers and decomposers.

Other comments
L91: I would not agree with this strong statement (see my comment above)
L129: just a detail—aren’t data and observations the same thing?
L135: how do human improvement of plant growing conditions fit in this work? We are dealing with deep past when conditions were probably far from optimal in many areas of the world
L141-142: argument is not clear
L184: “subaerial stems” meaning roots and hyphae?
L192: plant xylem is not a random medium, so I am not sure percolation theory can be applied as in soil
L203-213: I cannot follow this argument—how are the time scales of horizontal transport and weathering connected? My understanding was that the proposed hypothesis was not about weathering, but about producers and decomposers finding a ‘balance’ to recycle CO2 and keep the Earth warm
L229-234: also here I find it difficult to follow the presented arguments—what is the connection with the proposed hypothesis?
L238: gross primary productivity varies with increasing atmospheric CO2 in the range 1-6 gC/m^2/y/ppm (https://doi.org/10.1073/pnas.2115627119). Thus, a 25% increase in CO2 (100 ppm) leads to an increase ~100-600 gC/m^2/y, which is much more than the 2% figure reported. Additional explanations are needed here, including recent estimates of productivity sensitivity to CO2
L238-239: productivity is related to transpiration rate (only a fraction of evapotranspiration), but when changing temperature, the relation might break down as warmer conditions (for given water vapor content) trigger stomatal closure and decrease photosynthetic rates

Citation: https://doi.org/10.5194/esd-2023-21-RC1
- CC1: 'Reply on RC1', Allen G. Hunt, 08 Oct 2023
  
  At short time scales under modern conditions, diffusion of seeds by, e.g., birds, is so rapid that such arguments as suggested by Dr. Manzoni, when applied to single species over short length scales, are undoubtedly valid. A common model of seed dispersal is diffusion and a cursory investigation of reported model parameters (Allen et al. 1991) reveals that relevant diffusion coefficients are on the order of, e.g., 25m²/yr. But, application of such arguments to the distant past may be based on biases from the present. We suggest several caveats should be considered that, when taken together, can change the relative importance of soil-based and atmospheric pathways. First, it should be noted that advection-based mechanisms, such as the growth through the soil mechanism that we proposed, gain in speed relative to diffusion when the spatial scale increases. From 5 meters to a continental scale of 5,000 km represents a 6 order of magnitude increase in length scales. Although this increase might not produce enough slowing of diffusion for domination of our root-growth related mechanism today, Paleozoic conditions were very different. For example, the extracted parameter referenced above included individual time steps controlled by the advection from flying birds over a period of time equal to that required for seeds to pass through the bird gut. The transport path of seeds through birds did not exist until the Mesozoic. Second, the overall conditions for plant germination and growth were very different in the era before the plants modified both soil and atmosphere (Kleidon, 2002) to their advantage. Third, it is necessary to transport the entire clade, rather than an isolated species. Fourth, we can appeal to the dates already mentioned in the manuscript to suggest that plant dispersal in the Paleozoic was not orders of magnitude faster than the relaxation to homeostasis. After the initial invasion of the land by plants, it took at least 12 million years before initiation of an ice age (from 500 Ma to 488 Ma), and after the second wave of innovation starting at 420Ma, 48 million years (until 372 Ma). These two results support an inference that attaining homeostasis requires time scales between 5 times as long and 5/4 as long as the full exploitation of land-accessible resources by plants, which is not unexpected in view of the increased number of species involved. Nevertheless, if diffusion were not a noticeably faster mechanism, at least over smaller length scales, one would not expect that interruptions by rivers could be overcome by a faster mechanism of dispersal operating in series with the slower “through the soil” mechanism postulated here. Under these circumstances, we do still suggest that the close correspondence of a parameter-free prediction with the observed time scale warrants serious consideration as a viable hypothesis.
  Allen, L. J. S., Allen, E. J., Kunst, C. R. G., & Sosebee, R. E. (1991). A diffusion model for dispersal of Opuntia imbricata (cholla) on rangelanThe Journal of Ecology, 1123-1135. https://doi.org/10.2307/2261103.
  Kleidon, A.: Testing the effect of life on Earth’s functioning: how Gaian is the Earth system?, Climatic Change, 52(4), 383–389, https://doi.org/10.1023/A:1014213811518, 2002.
  
  Citation: https://doi.org/10.5194/esd-2023-21-CC1
- AC1: 'Reply on RC1', Allen G. Hunt, 12 Feb 2024
  
  The comment was uploaded in the form of a supplement: https://esd.copernicus.org/preprints/esd-2023-21/esd-2023-21-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/esd-2023-21-AC1
RC2:
'Comment on esd-2023-21', Anonymous Referee #2, 19 Dec 2023

Building on the idea of the Earth’s biosphere as a self-organizing global organism, the authors present an “interesting coincidence” between the time it took ecosystems and he Earth to re-equilibrate following ice ages (60Myrs) and a timescale needed for transport over continental scale predicted from percolation theory (80Myrs). There is no doubt that such a similarity is intriguing, even though a bit speculative. It can trigger an interesting debate on whether the Earth is truly able to self-organize as if it was a single organism, for which case scaling relationships originating from such spatiotemporal correlations may exist.
The only concern I have here and would like to authors to elaborate more on this, is whether predictions from percolation theory, which may be suitable in heterogeneous porous media, like soils, can be extrapolated to spatial scales larger than scales at which transport in soils is dominant. In other words, transport in soils may be explained by percolation theory. Why would this apply to continental scale? Applying the same scaling relationship over this broad range of scales implies that the underlying spatial correlations are given by the same dominant processes. But transport processes in soils are different from transport processes over continental scales, so shouldn’t at least the exponent of the scaling change? For example, even between organisms, we can use one single exponent to predict metabolic rates based on body mass. However, for larger organisms where vascular transport is important you find shifts in the scaling exponents. This is because there is a new process at play that affects transport, hence correlation within the organism leading to an organismal metabolic rate.
Another complication when we go to continental scale is the impact of climate and dominant winds based on location. I am not sure how this plays a role when considering the whole globe over timescales of Myrs, but commenting on this aspect is also warranted.
To conclude, I may not agree at the moment on the specific relationships applied over this scale, but this is a debate to have, and I commend the authors on pursuing such interesting questions.

Citation: https://doi.org/10.5194/esd-2023-21-RC2
- CC2: 'Reply on RC2', Allen G. Hunt, 20 Dec 2023
  
  We would like to thank the referee for his/her thoughtful comments.
  The referee has two concerns. One is that climate and dominant wind (directions and speed) could be important. We agree that further clarification is important here. The second concern is the relevant scaling relationship could change form with increasing scales. The referee is concerned that percolation theory for transport through soils might not even be valid at global or continental scales, or that an alternative scaling relationship may become more important. We think that these two concerns are actually interrelated and would like to respond as follows.
  Climate impacts the kind of soil transport that we have proposed through the water fluxes, which modulate the scaling law we invoked. Thus, our prediction would underestimate the time scale at which self-organization could occur, if the climate over too much of a continent were too arid, in which case it could lead to a significant overestimate and reduce the impact of the convergence of predicted and deduced time scales. Wind speed or direction has no impact on our scaling relationship, but it could, in principle, have an impact on competing scaling relationships, such as one based on diffusion, increasing the step lengths and/or correlating the step directions. We have pointed out in response to the first referee that the form of diffusion relations for, e.g., seed dispersal, is such that they rapidly become insignificant relative to advective-based relationships, if the number of individual steps encompassing the largest scales is large. Here, the conclusion would remain the same, unless wind speeds had been orders of magnitude larger than what are today observed, but it might be possible that the condition applied to the typical number correlated steps is violated in the case of positive correlations of the individual steps. Thus, it would be possible that if the same wind direction would be relevant at most times and most areas on a continent our argument could fail.
  The second concern is that the relevant scaling relationship could change with increasing spatial scales. Scaling laws of percolation theory with relevance to the Earth and Planetary Science have, in principle, no limits in scale (see last paragraph). The one we proposed has been directly verified at the scale of an island, i.e., 10 km, around which a monogenetic individual plant grew in 100,000 years. At a very early point in our research on this subject, we discussed the possibility that 3D scaling exponents of percolation theory should apply; which we thought would be the case as long as the root lateral spread of a plant is much less than the soil depth. In fact, the first author initially proposed this. However, no evidence in the data (well over 50 references, including a database with 6,000 individuals) was found to support such a cross-over. As of this date, we have still found no evidence to suggest the relevance of any cross-over to other dimensionalities or processes at scales from 10 μm to 10 km, even though our initial hypothesis was that there should be at least one such cross-over. Thus, we propose that this particular concern (though not supported by data known from investigations to date) as well as the others addressed above, could nevertheless be grounds for future investigations, in line with the referee's comment that this beginning could stimulate useful discussion.
  Finally, regarding the applicability of percolation scaling laws to the largest (global) length scale, we note that A.A. Saberi (Phys. Rev. Lett. 110,178501, 2013) invoked percolation scaling laws to explain topographical data for both Earth and Moon. He also used the same concepts (Astrophysical Journal Letters 876, l25, 2020) to provide evidence, based on data, for the existence of an ancient sea level on Mars, a hotly debated issue. Thus, there is no question that percolation scaling laws are applicable even at the largest length scales.
  
  Citation: https://doi.org/10.5194/esd-2023-21-CC2
- AC2: 'Reply on RC2', Allen G. Hunt, 12 Feb 2024
  
  We would like to thank the referee for his/her thoughtful comments as well as note that there exists some overlap with the comments of the first referee, Dr. Manzoni. Thus our response does not overly emphasize the similarities.
  The referee has two concerns. One is that climate and dominant wind (directions and speed) could be important. We agree that further clarification is important here. The second concern is the relevant scaling relationship could change form with increasing scales. The referee is concerned that percolation theory for transport through soils might not even be valid at global or continental scales, or that an alternative scaling relationship may become more important. We think that these two concerns are actually interrelated and would like to respond as follows.
  Climate impacts the kind of soil transport that we have proposed through the water fluxes, which modulate the scaling law we invoked. Thus, our prediction would underestimate the time scale at which self-organization could occur, if the climate over too much of a continent were too arid, in which case it could lead to a significant overestimate and reduce the impact of the convergence of predicted and deduced time scales. Wind speed or direction has no impact on our scaling relationship, but it could, in principle, have an impact on competing scaling relationships, such as one based on diffusion, increasing the step lengths and/or correlating the step directions. We have pointed out in response to the first referee that the form of diffusion relations for, e.g., seed dispersal, is such that they rapidly become insignificant relative to advective-based relationships, if the number of individual steps encompassing the largest scales is large. Here, the conclusion would remain the same, unless wind speeds had been orders of magnitude larger than what are today observed, but it might be possible that the condition applied to the typical number correlated steps is violated in the case of positive correlations of the individual steps. Thus, it would be possible that if the same wind direction would be relevant at most times and most areas on a continent our argument could fail.
  The second concern is that the relevant scaling relationship could change with increasing spatial scales. Scaling laws of percolation theory with relevance to the Earth and Planetary Science have, in principle, no limits in scale (see last paragraph). The one we proposed has been directly verified at the scale of an island, i.e., 10 km, around which a monogenetic individual plant grew in 100,000 years. At a very early point in our research on this subject, we discussed the possibility that 3D scaling exponents of percolation theory should apply; which we thought would be the case as long as the root lateral spread of a plant is much less than the soil depth. In fact, the first author initially proposed this. However, no evidence in the data (well over 50 references, including a database with 6,000 individuals) was found to support such a cross-over. As of this date, we have still found no evidence to suggest the relevance of any cross-over to other dimensionalities or processes at scales from 10 μm to 10 km, even though our initial hypothesis was that there should be at least one such cross-over. Thus, we propose that this particular concern (though not supported by data known from investigations to date) as well as the others addressed above, could nevertheless be grounds for future investigations, in line with the referee's comment that this beginning could stimulate useful discussion.
  Finally, regarding the applicability of percolation scaling laws to the largest (global) length scale, we note that A.A. Saberi (Phys. Rev. Lett. 110,178501, 2013) invoked percolation scaling laws to explain topographical data for both Earth and Moon. He also used the same concepts (Astrophysical Journal Letters 876, l25, 2020) to provide evidence, based on data, for the existence of an ancient sea level on Mars, a hotly debated issue. Thus, there is no question that percolation scaling laws are applicable even at the largest length scales.
  
  Citation: https://doi.org/10.5194/esd-2023-21-AC2
EC1:
'Comment on esd-2023-21', Axel Kleidon, 09 Feb 2024

Dear authors,
In the meantime, I have received a third review, which was sent to me in a rush, so it is quite short, but includes valuable points of critique that would need serious thought. I paste the review below.
Axel Kleidon (Editor)
====
I found that very hard to understand. They need to

1 - clearly articulate what the Gaia Hypothesis/Theory planetary homeostasis is. They need to provide a clear definition. Their review/articulation and potential understanding of Gaia is I think potentially limited. For mechanisms see

Lenton, T.M., Daines, S.J., Dyke, J.G., Nicholson, A.E., Wilkinson, D.M. and Williams, H.T., 2018. Selection for Gaia across multiple scales. Trends in Ecology & Evolution, 33(8), pp.633-645.
Lenton, T.M., Dutreuil, S. and Latour, B., 2020. Life on Earth is hard to spot. The Anthropocene Review, 7(3), pp.248-272.

Etc… it’s not hard to find...

2 - they then need to clearly articulate how global surface temperature controls evolve & change *without/prior* to their proposed mechanism.

3 - they then need to clearly explain and describe what their proposed mechanism is. A figure/diagram or two would help…

4 - then they can get into their methods and results.
====

Citation: https://doi.org/10.5194/esd-2023-21-EC1
- AC3: 'Reply on EC1', Allen G. Hunt, 12 Feb 2024
  
  As authors, we are addressing important time scales with known rates in checking inferences. This is akin to addressing rates of evolutionary drift to determine what time scale is implied by certain changes in genetics. In this context, I would like to address the Stommel diagram provided by the author in Figure 1 in the first publication. Understanding what is possible at a given time scale, together with comparisons of rates is, of course, important to an overall understanding of history, as we agree. However, it is our opinion that the Stommel diagram does not accomplish this.
  The bubble for weathering covers time scales of 30kyr to 100Myr for length scales from about 20km to 100km
  These numbers make absolutely no sense for describing the evolution of the vertical direction of the weathered layer. But they make no sense in the horizontal directions either (soil development across landscapes). The author would be correct in assuming that there is no particular rate associated with the horizontal dimension (the more or less horizontal orientation of the bubble), but the horizontal dimension is bounded only by the size of a continent (not by the tens of kilometer scale) and weathering is going on all the time. Of course, field weathering is limited by transport only on the scale of months to 100’s of million years; for exposures that are even younger (landslides, treethrow, glacial retreat, mining tailings, etc.) weathering can be limited by reaction kinetics for time scales up to a month, or in extreme cases, a couple of decades. If a vertical spatial scale were meant, the diagonal extent of the bubble on a logarithmic graph would have a slope of about 1/2, going from about 1cm at one year to about 100m at around 100 Myr. At smaller time scales, the slope would be 1, consistent with a weathering rate limited by reaction kinetics. For experimental confirmation see the paper of White and Brantley (2003) “The effect of time on the weathering of silicate minerals: Why do weathering rates differ in the laboratory and in the field?” For actual theory compared verified by predictions, see our papers listed at the end, but particularly the articles Yu and Hunt, 2017 (Earth and Space Chemistry), Egli et al. 2018, and the article Hunt et al. (2021).
  
  The bubble for soil erosion also extends horizontally rather than diagonally.
  
  The horizontal orientation of the bubble means that there is no scale dependence of erosion times. Again, this particular criticism does not apply if the author is considering horizontal dimensions. But locating the bubble at length scales between 100m and 10km makes no sense in that case, as, again the scale limit would be in the thousands of kilometers (continental), unless the author is talking about some particular horizontal organization (a catchment? an industrial development, a farm?). Soil erosion is occurring everywhere at all times. Natural soil erosion rates range from less than 1m / Myr to above 2000m/Myr, consistent with a vertically oriented bubble covering in excess of 3 orders of magnitude, if it is wished to explain rate variability, or if it is wished to describe the scale dependence, a diagonal bubble with slope 1 and a vertical variability of three orders of magnitude. The fact that the soil depth tends to increase more slowly than a slope of 1 (about ½) means that areas with a larger erosion rate must have a smaller soil depth, in order for there to be an equivalence between the two and a steady-state system. Is the author interested in any details or predictions?
  
  The bubble for tectonics is also horizontal with limits on the time frames considered.
  
  Plate tectonics is a process associated with horizontal scales that increase linearly in time. If the author is trying to describe horizontal evolution, as may be the case in the other processes mentioned above, this bubble should not be oriented horizontally but diagonally with a constant slope. Other interpretations are equally difficult to understand: the author states that plate tectonics as a mechanism for global change has not been in existence until the last billion years or so; then why does the bubble extend from about 10Myr to nearly 100Gyr? Should it not extend back to time scales when effects are observed in the changes of stream networks, i.e., 10kyr – 100kyr? Or in earthquake intervals that can cause coastal forests to be drowned or exhumed, i.e., 500yr?
  
  Or let’s look at the bubble for glaciation. This is again horizontal, implying the existence of no rates of significance. The horizontal scale extends from about 10kyr to 100Myr. The vertical scale extends from about 500km to about 5000km. This corresponds to the extents of ice sheets on major islands to continents. The time scale of 10kyr is a good estimate of the time it takes to melt. The time it takes to form a continental glacier is on the order of 100kyr, and the time span of the Pleistocene is on the order of a couple of million years, whereas Paleozoic glaciations lasted about 60 Myr. Thus, three separate processes are included in the same bubble; glacier melting, glacier formation, glaciation episodes (which may be oscillating between glaciation and interglacials). This bubble is relatively accurate, but encompasses such a wide range of implications for biota, that particular evolutionary connections can be causal in either direction, or simply amplifications of orbital cycles. A Stommel diagram must be carefully defined and accurately drawn, in order to assist in association of the processes of principal relevance to observed events.
  
  The author also cites the presumed natural weathering thermostat. However, such a thermostat does not exist. Since reaction kinetics is irrelevant to silicate weathering rates for surfaces that have been exposed for longer than a few months, there is no silicate weathering thermostat. There is an indirect connection, however, between silicate weathering and temperatures, as long as warmer climates produce an accelerated water cycle. Such an association, however was not present during the Permian extinction event (associated with the mega eruptions of the Siberian traps) and the build-up of atmospheric carbon could not be removed through weathering due to the aridity of large sections of the interconnected landmass of Pangaea Hunt, A. G., and M. Sahimi, 2017, The Spaces in Between, Editor’s Vox, Eos https://eos.org/editors-vox/the-spaces-in-between. Lovelock’s supposition that bacteria were responsible for the establishment of homeostasis was verified, Ball, P. James Lovelock reflects on Gaia’s legacy. Nature (2014). doi:10.1038/nature.2014.15017.
  
  I think that citation of the particular paper chosen by the referee is not particularly helpful. The mechanisms for evolution and the associated time scales are not justified carefully; if they are not less sloppily formulated than the processes that I can define carefully, then their relevance to the speculations presented by the author is, at best, unclear.
  
  The mechanism described in our paper is quite clear and in complete accord with the understanding of Lynn Margulis (Margulis, L. Symbiotic Planet: A New Look At Evolution. Houston: Basic Book. (1999)) regarding interacting ecosystems: plants in exclusion, and the entire soil ecosystem for example. It is necessary for the microbiota to be able to decompose lignin and other constituents of woody plants in order for the carbon cycle to revert to an equilibrium at higher temperatures. We have shown in other places (and cited the sources) that a greedy plant model (in accord with Odum’s 1959 hypothesis that maximum plant productivity guides plant evolution) yields the first accurate predictions of the water balance (see, for example the citation in Eos, https://eos.org/editor-highlights/how-much-terrestrial-precipitation-is-used-by-vegetation) regarding our original publication in AGU Advances, and is thus compatible with the idea that invasion of the land by plants would generate a disequilibrium in the carbon cycle, with a rapidly dropping temperature, that could lead to an ice age (which definitely occurred). The increase in temperature through re-establishment of homeostasis would then lead also to an additional increase in plant productivity (strengthening adaptation for maximum productivity) for multiple reasons, e.g., new exposure of bare ground, higher temperatures, decreased aridity, higher rates of decomposition and nutrient cycling, etc. The problem is, however, that development of the appropriate genetic combination to be able to decompose wood in one geographical microhabitat does not obviously lead to the dispersal of both the producers and the consumers throughout a continent. We have provided a potential means to do that, through the scaling rates of growth of plant root systems with their associated communities of fungi (synergistic and pathogenic) and bacteria through the soils. The argument that the community as a whole must be transported from one place to another is strengthened by a recent research result, https://www.eurekalert.org/news-releases/1033091, “This study challenges traditional views that predominantly focus on competitive interactions in ecological networks by showing that facilitative networks are more common in soil ecosystems than previously thought. This paradigm shift stems from recognizing that soil processes, such as decomposition of organic matter, necessitate cooperation among various species.” The original publication is here. https://www.pnas.org/doi/10.1073/pnas.2308769121.
  
  Hunt, A. G., and B. Ghanbarian, 2016, Percolation theory for solute transport in porous media: Geochemistry, geomorphology, and carbon cycling, Water Resources Research, 52: 7444-7459
  Yu, F., B. Faybishenko, A. G. Hunt, and B. Ghanbarian, 2017, A simple model of the variability of topsoil depths Water 9 (7) 460 doi:10.3390/w9070460 .
  Yu, F., and A. G. Hunt, 2017, An examination of the steady-state assumption in certain soil production models with application to landscape evolution, Earth Surface Processes and Landforms. DOI: 10.1002/esp.4209.
  Yu., F. and A. G. Hunt, 2017, Predicting soil formation on the basis of transport-limited chemical weathering, Geomorphology, https://doi.org/10.1016/j.geomorph.2017.10.027.
  Yu, F., A. G. Hunt, M. Egli, and G. Raab, 2019, Comparison and contrast in soil depth evolution for steady-state and stochastic erosion processes: Possible implications for landslide prediction, Geochemistry, Geophysics, Geosystems, https://doi.org/10.1029/2018GC008125.
  Yu, F. and A. G. Hunt, 2017, Damköhler number input to transport-limited chemical weathering and soil production calculations, Earth and Space Chemistry. DOI: 10.1021/acsearthspacechem.6b00007.
  Egli,M., A. G. Hunt, D. Dahms, G. Raab, C. Derungs, S. Raimondi, F Yu, 2018, Prediction of soil formation as a function of age using the percolation theory approach, invited by Frontiers in Environmental Sciences, 28: https://doi.org/10.3389/fenvs.2018.00108
  Hunt, A G., B. A. Faybishenko, and B. Ghanbarian, 2021, Non-linear hydrologic organization, Non-linear Processes in Geophysics, 28: 599-614. https://doi.org/10.5194/npg-28-599-2021.
  
  Citation: https://doi.org/10.5194/esd-2023-21-AC3

Allen G. Hunt, Muhammad Sahimi, Boris Faybishenko, Markus Egli, Zbigniew J. Kabala, Behzad Ghanbarian, and Fang Yu

Data sets

Biological and physical transport processes: Gaia A. G. Hunt, M. Sahimi, B. Faybishenko, M. Egli, Z. J. Kabala, B. Ghanbarian, and F. Yu http://www.hydroshare.org/resource/eef6b82c0eaa48e78b68b752da2b4ba5

Allen G. Hunt, Muhammad Sahimi, Boris Faybishenko, Markus Egli, Zbigniew J. Kabala, Behzad Ghanbarian, and Fang Yu

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Short summary

Relative stability of Earth’s climate system is considered an emergent property of coupled ecosystems. We apply a spatio-temporal scaling relation for root growth to couple bacterial/fungal/vegetational response to climate crises triggered by land plant invasion and predict an absolute time scale to reach homeostasis. The predicted time is 33 % larger than required for the biosphere to emerge from associated Paleozoic ice ages. We propose a basis for understanding the biosphere and critical zone.


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