the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jan 2025
Dynamic vegetation reveals unavoidable climate feedbacks and their dependence on climate mean state
Abstract. We investigate seasonal vegetation feedbacks considering mid-Holocene and pre-industrial simulations with the IPSL climate models for which dynamic vegetation is switch on. We consider four different settings for the land surface model designed to improve the representation of boreal forest. They combine different choices for bare soil evaporation, photosynthesis and associated parameters, and tree mortality. Whatever the model set up, the major seasonal differences expected between the mid Holocene and preindustrial climates remain similar, and consistent with the mid Holocene greening of the Sahara and northward shift of the northern limit of forest in the northern hemisphere. However, the way vegetation-climate interactions trigger unavoidable radiative surface albedo and water vapor feedbacks depend on the model content. Cascading feedbacks involve local snow-vegetation interactions, as well as remote water vapor and long wave radiative feedbacks in the tropics, which are needed to fulfill the global energy conservation constraint of the climate system. We show that the parameterization of bare soil evaporation is a key factor that control tree growth in mid and high latitudes. Photosynthesis parameterization appears to be critical in controlling the functioning of vegetation and vegetation-climate interactions. It affects the seasonal evolution of the vegetation and leaf area index, as well as their effect on radiative feedbacks and the sensitivity of the vegetation feedback to the climate mean state. This sensitivity needs to be considered when developing and tuning climate models.
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RC1: 'Comment on egusphere-2024-4075', Anonymous Referee #1, 21 Feb 2025
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Comments on the manuscript
Dynamic vegetation reveals unavoidable climate feedbacks and their dependence on climate mean state
by Braconnot, Viory, Marti
Pascale Braconnot and coauthors present a study on vegetation-climate feedbacks. They use the new IPSLCM6 with four different parameter sets with respect to the representation of photosynthesis, bare soil evaporation and two parameters defining vegetation competition and distribution. They thoroughly analyze the effect of changing the parameter sets on the simulated global climate and vegetation patterns for preindustrial and for mid-Holocene climate. I am convinced that the specific results of their study strongly depend on the climate system model used and on the specific choice of the parameter sets. Other climate system models and other variation of other parameters would presumably yield other results. And that should explicitly be stated. However, I am also convinced that modeling groups can learn from the paper when addressing the general issue of tuning and exploring the sensitivity of vegetation-climate feedbacks on the choice of vegetation and soil parameters. Thus, I recommend publication of the study with minor corrections, but I strongly suggest skipping the term “unavoidable” in the title and in the text. This term is not explained in the text. It just sounds alarmistic, as if the model would have a choice to “avoid” any negative consequences of model tuning. Quite the contrary, model tuning is done to improve the model performance.
When reading the paper, I see that the new IPSLCM6 yields a much greener mid-Holocene Sahara than the former IPSLCM5 did. That is an exciting result. The authors highlight this achievement in one sentence (line 278/9) and a half-sentence (line 284/285). It would deserve more appreciation in the conclusions. Perhaps, a figure with a zoom on African biomes, using the biomization tool by Dallmeyer et al (2019), for example, which has already been applied to ORCIDEE PFTs, would be useful for a better comparison with other ESM simulations. But I leave this to the authors to decide. I do not like reviewers who suggest completely re-writing a paper or writing a new story. Hence, I only would like to encourage the authors considering an additional study with a perhaps even more interesting focus on the effects resulting from the upgrade from IPSLCM5 to IPSLCM6 versus changing the parameterization of bare soil and vegetation.
The sensitivity study is clearly written. Perhaps a more formal analysis using the Alpert-Stein factor separation would yield a better understanding of feedbacks and synergies between feedback. But, again, I do not insist on doing a new analysis which would require 24 simulations. It would be sufficient to mention that the present study does not differentiate between the pure contributions triggered by a new parameterization and the possible synergies emerging from combining new parameterizations.
Finally, I suggest skipping trivial common places like the very last paragraph (lines 630 – 634). It is completely true that dynamical vegetation is an important factor which should be considered in ESMs. But this study is not the first one to point to the importance of dynamic vegetation. We (including the authors) have convincingly addressed this topic by numerous studies over the last roughly 30 years.
Minor comments:
Line 116 and following: It would be useful to learn something about the interaction with the C-cycle. Into which carbon pools of the plants and the soil is the carbon gain by photosynthesis fed? Or does this issue do not play any role here?
Line 242, Fig. 3: The abbreviations in the title lines (dtas, dpr) are not defined in the caption. Why not put a \Delta T_s or \Delta P_r in the title lines and in the caption?
Line 257, Fig. 4: The new parameterizations increase the simulated annual mean precipitation in WA, but still, the simulated precip amounts to only a factor of 0.4 of the reconstructed precip. How is the aggregation of data points and comparison with grid box results done? Using any area-mean? (Would be sensible to only consider grid boxes for which reconstructions are available.)
Line 283 ff: “It results from vegetation feedbacks amplified by synergy with ocean feedbacks …” surely, it does. But without differentiation between feedback and synergy, it remains a trivial statement and could be skipped – in contrast to the second half of the sentence which likely is the real reason and would deserve more attention.
Line 287, Fig. 5: The labels on the colorbars are partly hidden behind the colorbars. Please shift. The global maps, specifically for the differences in lai, are too small to see any details outside the tropics. Please enlarge the figures to the size of the other global maps in the other figures.
Line 304: It would be helpful to note that alpha_p is the surface albedo. Commonly, one would symbolize the planetary albedo with the subscript ‘p’.
Eq.(3) and other places in the text: Sometimes the subscripts appear as subscripts, sometimes as an extension of the variable, for example as in SWsi vs. SW_{si} or LWsup vs. LW_{sup}. Please harmonize.
Line 318: gases instead of gazes
Line 348: What are pft 7 and 8? It would help reading, if the names of the PFTs are mentioned here or in a table.
Line 454 and other places: Sahara Sahel or Sahel Sahara sounds a bit cumbersome, because the Sahara and the Sahel (region) are pretty different regions.
Line 457: …, so that the magnitude …
Line 509: “The suite of mid-Holocene … allow us to dig into the complexity of the Earth’s climate system.” That is a rather generic and bold statement as this study just touches a small subset and very specific aspects of the global climate system.
Line 510: “We insist on the fact …” I do not understand, why you have to ‘insist’ on the fact, instead of highlighting the fact.
Line 519 ff: I do not quite understand the meaning of this sentence. Perhaps it is just the wording ‘associate to’ … The word ‘fulfil’ should be ‘fulfill’.
Line 527: “We show that dynamical vegetation reveals how ….” I am not sure how dynamical vegetation can reveal anything. The analysis of the climate-vegetation interaction can certainly do, but, again, only with respect to the processes considered, not with respect of the entire complexity of all biospheric processes.
Line 537: Which “step changes between the model version … is (shouldn’t is ‘are’) different from …?
Line 542: Which “model content” … lead(s) to different vegetation cover …?
Line 562 ff. Indeed, the statement that “simulated vegetation is an integrator …” is “trivial”. Perhaps a more modest statement would be sensible. This study is not the very first one to highlight the importance of vegetation dynamics.
Line 565/567. I agree that one cannot infer vegetation feedbacks from studies in which vegetation patterns are kept fixed. In this sense, the titles of early studies (e.g. Kutzbach et al. Nature 1996) are misleading. These studies analyzed impacts rather than feedbacks.
Line 596: This would require (instead of requires)
Line 600: … because land use (not land used) is not
Citation: https://doi.org/10.5194/egusphere-2024-4075-RC1
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