Comments of Boris Chubarenko to ESD manuscript:



Title: Oceanographic regional climate projections for the Baltic Sea until 2100

Author(s): H. E. Markus Meier et al.

MS No.: esd-2021-68

MS type: Review

Special Issue: The Baltic Earth Assessment Reports (BEAR)

General comments

The paper is very informative and well structured. It is written in clear language and wishes to explain the main aspects and details to a reader sincerely.

The paper clear overviews the results of several profoundly advanced (at its time) attempts to develop the climate projections for the Baltic Sea. These attempts were made during BACC (2008), BACC II (2015). BEAR (this study),  ECOSUPPORT, BalticApp and CLEAMSEA projects. All of them are mentioned in the text everywhere. It would be good to present the general overview scheme to help the reader quickly understand the differences between these initiatives and projects without reading the whole text.

The paper emphasizes a significant step of CLEMASEAS - including a new driver, the global sea-level rise. It appeared (according to the Conclusions) that this third driver causes “a more or less complete compensation for the projected increasing river runoff” that changes previously concluded the future drop of the average salinity. It would be better to clarify whether it is the same for all used scenarios of the global sea-level rise (0.9, 1.26 or 2.34 m water depth rise) or the extreme only?  

For such complicated issues as climate projections for the Baltic Sea, I would expect a more extended summary that includes final statements for all analyzed variables. Or at least the reference to the appropriate section of the text.

My opinion is that the idea that NAO well controls the interannual variability of the climate variables in the Baltic Sea has outlived its usefulness. Figure 25 clearly illustrates that correlation is so low that the discussion of this relationship is on the verge of physical meaning (no technically reliable instrument will not work with such characteristic correlation). The idea to find the good simple predictor for the Baltic Sea climate variability is attractable, but unfortunately, it seems, it is not realizable.

It is remarkable, and I personally very much support the massage of the paper that “BSAP would lead to a significant improvement” of the state of the Baltic Sea. And, more generally, human activity in the Baltic Sea catchment and the sea has a more substantial influence than the natural influence of global climate change. In this regard, it is not clear why the previously formulated (probably in ECOSUPPORT) strong sound statement “that climate change will worsen the situation in the Baltic if people do nothing” is not included in the number of conclusions.

Specific comments

The statement (lines 796-797) needs more clarification as it is not well understood. Maybe via the words that the existed natural negative south-north gradient will be partly compensated.

The sentence (lines 832-835) is too long. Better to split into several more simple sentences to more clearly present the idea. 

The list of abbreviations would be helpful.

If possible (in addition to Table 1 and Table3), the table with the list and the main characteristics of RCM (RCSM) and Baltic Sea ecosystem models would be handy to understand the progress. 

Technical corrections

Lines 774-776: It seems that the word ‘model is absent after ‘physical-biochemical’.

Figure 25: Please, insert the legend explanations in the figure caption.

Table 3: Please, introduce the column titles to link with the Table caption (via numbers, for example). The acronym of scenarios (BSAP, REF and BAU) have to be explained in the caption.

Table 4: Please, introduce the sign ‘+’ (as in the other tables) to indicate the positive changes. There is enough space in the table to have explicit column titles; for example, December-January-February (DJF), not only DJF.

Table 6 and 7: the last raw ‘BAU/WORST’: Why is only one number given for these two scenarios? Better explain in the caption.

General comments

The presented manuscript analyzes and compares the results of two ensembles of scenario simulations (performed in projects ECOSUPPORT and BalticAPP / CLIMSEA ) for the Baltic Sea including marine biogeochemistry performed using different Regional Climate System Models (RCSM). In addition to different RCSMs, these projection ensembles differ in the Earth System Model (ESM) forcing, the Baltic Sea ecosystem model, the greenhouse gas (GHG) concentration scenario, the nutrient input scenario, the sea level rise (SLR) scenario and the simulation period.

Despite the uncertainties related to global and regional climate and impact models, as well as the unknown pathways of GHG and nutrient emissions, which make it very difficult to compare these two ensembles of projections, the authors obtained a number of important new results. Among them, I note: 1) global mean SLR results in a more or less complete compensation for the Baltic Sea salinity reduction for the projected increasing river runoff, 2) the global SLR can be identified as a new driver that has a strong impact on bottom oxygen concentration, 3) most noticeable are the differences in projected biogeochemical variables between ECOSUPPORT and BalticAPP / CLIMSEA ensembles. The article is undoubtedly important and interesting for ESD readers not only with the presented results, but also with a discussion of knowledge gaps and prospects for further research. The main, but, incidentally, a small drawback of the manuscript is not always a sufficiently detailed explanation of the results obtained, which is discussed below in specific comments.

Specific comments

Abstract

In the abstract there is no description of the differences in setups in the two ensembles of projections (ECOSUPPORT and BalticAPP / CLIMSEA simulations). It would be nice to list them in accordance with Table 3.

Main text

L.114-115

“An overview was given by  (Schrum et al., 2016)…”

Reference is missing from the bibliography

L.193-208, 866-874 (Fig.3)

Nutrient input scenarios in BalticAPP and  CLIMSEA simulations are described using Fig. 3. The latter is of very poor quality and does not allow you to see the differences between the blue, green, orange and red curves (Fig.3, lower panels) corresponding different scenarios. The choice of colors appears to be unfortunate. In addition, plotting all the curves on one graph creates the impression of complete chaos, nothing more (although, perhaps, the authors wanted to show just that).

The ECOSUPPORT nutrient input scenarios (Gustafsson et al., 2011; their Fig. 3.1) are not displayed in the Fig. 3. At the same time, they are important (especially for the historical period) in order to better understand the differences in the projections of biogeochemical variables in the two сomparable ensembles, and it would be good to add them as a separate panel in Fig.3.

L.460-463

 The good agreement between simulated annual mean surface phytoplankton concentrations and reanalysis data everywhere (with the exception of coastal regions) is somewhat surprising. As far as the author of the review is aware, the deviations of the simulated chlorophyll concentration from the individual observations can be very large. Perhaps the agreement is due to the small number of observations and their corresponding small contribution to the results of the reanalysis?

L. 641-658

“3.2.6 Oxygen concentration and hypoxic area

Bottom oxygen concentration"

This subsection discusses the differences between ECOSUPPORT and BalticAPP ensembles in bottom oxygen concentration. Discussion of differences ends with the phrase:

“These results are explained by the historical nutrient input reductions and the slow response of the Baltic Sea”. (L.658)

This unsatisfactory, too general explanation makes the reader think, and due to what differences in historical nutrient input this situation could have happened, and he turns to Fig. 3 and does not find there the necessary information about historical nutrient input in ECOSUPPORT (see above remark about Fig. 3). Further, an inquisitive, but already somewhat irritated reader continues reading and discovers a detailed explanation of the differences between ECOSUPPORT and BalticAPP ensembles in the biogeochemical cycling (for some reason, in the Knowledge gaps section) in lines 751-763.

  In my opinion, it would be better not to test the patience of the reader and move this explanation to section 3.2.6. It would also be good to expand this explanation by indicating the difference in the initial conditions for biogeochemical variables between ECOSUPPORT and BalticAPP ensembles and how this difference affected the final results.

L. 1046-1050 (Table 3)

Why is there no simulation for ESM IPSL-CM5A-MR under RCP8.5 scenario in the list of simulations in the project BalticAPP?

Technical corrections

L.480

“ …driven by reconstructed atmospheric and hydrological data with in in situ observations of nitrogen”

Change by

 “ …driven by reconstructed atmospheric and hydrological data with in situ observations of nitrogen”

 L. 774-776

A word "models" after “physical-biogeochemical” is missed