the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 May 2024
Carbon-climate feedback higher when assuming Michaelis-Menten kinetics of respiration
Abstract. Earth system models simplify complex terrestrial respiration processes assuming a first-order chemical reaction or assuming a Michaelis-Menten kinetics. The epistemic uncertainty related to the respective mathematical representations is unclear. Using a simplified model of biogeochemical feedbacks to climate, we show that the terrestrial carbon-climate feedback is more than 35 % higher, and hence the remaining carbon budget to keep global warming below 2 °C is 89–158 Pg C higher, when assuming Michaelis-Menten kinetics instead of first-order kinetics, but these differences depend on the underlying emission scenario. These results show the importance of an increased understanding of the mathematical model structure of respiration processes in Earth System Models for more reliably projecting future carbon dynamics and climate, related feedback mechanisms, and hence to estimate a valid remaining anthropogenic carbon budget.
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RC1: 'Comment on egusphere-2024-1504', William Wieder, 26 Jul 2024
Beer uses a reduced complexity model to look at different formulations of a land ecosystem respiration term that uses first order vs. Michaelis-Menten kinetics (FOK and MMK, respectively). The paper reports a stronger land C uptake with the Michaelis-Menten parameterization. The manuscript is a nice, albeit unsurprising, illustration of structural uncertainty in land models and how they impact the magnitude of the terrestrial C sink and potential carbon-cycle climate sensitivities.
I like the work, but feel some additional information and clarification is needed to make the findings more straight forward to understand / interpret. Mainly. I’m a little fuzzy about the use of the term carbon-climate feedback here, which seems analogous to the gamma term in the C4MIP literature (e.g. Arora et al 2020; typically expressed as PgC per degree Celsius). With this convention, gamma_land is typically negative (from the atmosphere perspective), reflecting less land carbon storage under warmer conditions.
Conceptually, this looks similar to the left side of Fig 1, which focuses on positive feedbacks between temperature, ecosystem respiration and atmospheric CO2 burden. The rest of the results, however, don’t share this perspective, which makes the conclusion (and title) confusing. I’ll try to walk though some sources of this confusion and offer suggestions on how to clarify:
- Results (Fig 3) shows higher land C uptake and lower atmospheric burden of CO2 with the MMK. To me this implies a reduction in strength of the temperature-respiration feedback with MMK, which allows for more land C uptake. Is this accurate?
- The “terrestrial carbon-climate feedback”, Table 2, it’s unclear if this is basically the size of the cumulative land sink, which is positive (i.e.; from the land perspective) and expressed as Pg C, (not Pg C/degC). Or if this is the difference in atmospheric CO2 accumulation from the “feedback on”, (Q10=2) vs. “feedbacks off” (Q10=1). If it’s the later, this suggests a larger atmospheric CO2 burden from warming with the MMK approach, which is difficult to square with the results in Fig 3. Maybe the heading for this figure can be clarified?
- Finally the “feedback factor” seems to be some kind of a ratio (see additional question below), maybe comparing the runs with Q10=2 (“feedbacks on”) vs. Q10=1 (feedbacks off)? this may allow diagnoses of the inferred temperatures sensitivity of FOK vs. MMK respiration schemes, but it’s not really clear what this metric is communicating, or where the results from the “feedbacks off” simulations come into play here?
The simplest solution here may be to remove the use of “carbon-climate feedbacks” here, unless results can be presented in a way that similar to the C4MIP conventions (commonly a reduction in land C uptake per deg warming). If taking this approach, the manuscript can focus on land C sink or land C uptake from the title and throughout the manuscript to be more consistent with results presented. If taking this approach Fig 1 may not be necessary. Clarification on the ‘feedback factor’ (Table 3, Fig 5) would also be necessary. This may not be accurate, however, if Table 2 is actually showing the difference in atmospheric CO2 burden from Q10=2 vs. Q10=1 experiments.
Minor and technical questions
Abstract (and elsewhere?) for a single author paper use “I” not “we”.
I’m not really clear how the “carbon climate feedbacks” were calculated (Table 2), or what the carbon-climate feedback factor represents (Table 3 and Fig 5)? It’s the ratio of temporal changes in the land C stocks (end of 21st century pools / end of 19th century pools) Line 134? This doesn’t add up when if I do the math on numbers reported in Tables 1 & 2, please clarify in methods. Maybe it’s the ratio of the temporal changes in the runs with Q10=2 (feedbacks on) vs. Q10=1 (feedbacks off)? Some addition text in the method and results would help clarify these results.
Citation: https://doi.org/10.5194/egusphere-2024-1504-RC1 -
AC2: 'Reply on RC1', Christian Beer, 10 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1504/egusphere-2024-1504-AC2-supplement.pdf
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AC1: 'Reply on RC2', Christian Beer, 10 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1504/egusphere-2024-1504-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2024-1504', Anonymous Referee #2, 08 Aug 2024
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AC1: 'Reply on RC2', Christian Beer, 10 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1504/egusphere-2024-1504-AC1-supplement.pdf
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AC2: 'Reply on RC1', Christian Beer, 10 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1504/egusphere-2024-1504-AC2-supplement.pdf
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AC1: 'Reply on RC2', Christian Beer, 10 Sep 2024
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