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the Creative Commons Attribution 4.0 License.
| 02 Oct 2024
Assessing the lifetime of anthropogenic CO2 and its sensitivity to different carbon cycle processes
Abstract. Although it is well-established that anthropogenic CO2 emitted into the atmosphere will persist for a long time, the duration of the anthropogenic climate perturbation will depend on how rapidly the excess CO2 is removed from the climate system by different biogeochemical processes. The uncertainty around the long-term climate evolution is therefore not only linked to the future of anthropogenic CO2 emissions, but to our insufficient understanding of the long-term carbon cycle. Here, we use the fast Earth system model CLIMBER-X, which features a comprehensive carbon cycle, to examine the lifetime of anthropogenic CO2 and its effects on the long-term evolution of atmospheric CO2 concentration. This is done through an ensemble of 100,000 year long simulations, each driven by idealized CO2 emission pulses. Our findings indicate that, depending on the magnitude of the emission, 75 % of anthropogenic CO2 is removed within 197–1,820 years after emissions end. Approximately 4.3 % of anthropogenic CO2 will remain beyond 100 kyr. We find that the uptake of carbon by land, which has only been marginally considered in previous studies, has a significant long-term effect, storing approximately 4–13 % of anthropogenic carbon by the end of the simulation. For the first time, we have quantified the effect of dynamically changing methane concentrations on the long-term carbon cycle, showing that its effects are likely negligible over long timescales. The timescale of carbon removal via silicate weathering is also reassessed here, providing an estimate (80–105 kyrs) that is significantly shorter than some previous studies due to higher climate sensitivity, stronger weathering feedbacks, and the use of a spatially explicit weathering scheme, leading to a faster removal of anthropogenic CO2 in the long-term. Our study highlights the importance of adding model complexity to the global carbon cycle in Earth system models, as to accurately represent the long-term future evolution of atmospheric CO2.
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RC1: 'Comment on egusphere-2024-2976', Anonymous Referee #1, 28 Oct 2024
In their paper 'Assessing the lifetime of anthropogenic CO2 and its sensitivity to different carbon cycle processes' Kaufhold et al. apply the CLIMBER-X Earth system model of intermediate complexity to investigate the atmospheric lifetime and the removal processes of cumulative CO2 emission in a set of 100 kyr experiments.
They include a variety of sensitivity experiments to further investigate the role of the landbiosphere and weathering feedbacks in removing atmospheric CO2 perturbations.Overall, the paper is well written and provides interesting results and nicely addresses the question of the landbiosphere and weathering feedbacks for the removal of atmospheric CO2 perturbations and provides and a wealth of figures. The spatially explicit weathering scheme of CLIMBER-X is an important addition to the investigation! In summary, the study is well suited for publication in Biogeosciences.
I have three more general aspects and a few minor comments the authors may address during revision.
1) Presentation of the C-perturbation
In the paper, the authors alternate between presenting the atmospheric C-perturbation (or the perturbation of other reservoirs) in absolute values (i.e. ppm CO2 or PgC) and as fraction of the maximum CO2 perturbation. For comparison with other studies and to address non-linearities it would, in my eyes, be much easier to show results as fractions of the CO2 perturbation rather than as absolute values (for example also in Fig. 1). The fact that the atmospheric CO2 perturbation (in ppm) is larger for larger cumulative emissions is not surprising and it could be interesting to investigate the non-linearities in more detail instead.
An alternative could also be to normalize the results by the response to a certain pulse size to highlight the non-linearities (e.g. Fig. 2).
2) how emissions are prescribed
The way emissions are prescribed in this study (as Gaussian function) complicates the comparison with studies featuring a pulse-like release of carbon (as often done) to a certain degree.
This leads in this study, for example, in the case of small total cumulative emissions, to a large fraction of the atmospheric CO2 perturbation already having been removed before reaching the maximum atm. CO2 perturbation and also to less timescales required when fitting the response as a sum of exponentials (section 3.3) as compared to studies with an instantaneous pulse-like emission.
While this is acknowledged in the text, I think it should be made more clear, especially for the discussion of the timescales in sections 3.2-3.4.
Further, it might be interesting to add one additional emission pathway sensitivity experiment, where all the carbon is emitted in the first timestep, as done in many of the studies discussed in the paper. In light of how fast the CLIMBER-X model is (10'000 years per day), this might be doable.
3) Length of the paper
While the paper does a very nice job in thoroughly describing processes and visualizing a lot in figures, I found it quite lengthy to read. Maybe during revisions this could be kept in mind. For example, in my opinion, sections 3.2-3.4 could be merged and shortened with a focus on the novelties of this study (timescale of the silicate weathering feedback).Minor comments:
- p. 5, l. 105: why is the conservation of phosphate and silicate enforced and how is it done?
- p. 6, l. 127-130: please provide values for the parameters
- p. 7, l. 149ff: looking forward to the interactive ice-sheet simulations!
- p. 7, l. 155: 'pulse' might be a misleading term, maybe replace with 'idealized CO2 emission histories'?
- section 3.1.3: very nice!
- Fig. 13: maybe clarify in the figure caption as well, why the smaller cumulative emissions lower fractions removed (-> more already taken up by other reservoirs before reaching the max. CO2 perturbation)
- section 4: I really liked this section!
- Fig. 16: check caption text, not fully clear
- p. 32, l. 590: 'effect' -> 'affect'?
- p. 35, l. 646ff: move this part to the other statements about silicate weathering beforeCitation: https://doi.org/10.5194/egusphere-2024-2976-RC1 -
CC1: 'Comment on egusphere-2024-2976', Paul Pukite, 28 Oct 2024
Why is it that the predominating mechanism behind the fat-tails of CO2 persistence in the atmosphere is never mentioned in the article? CO2 enters the ocean and only gradually diffuses downward, modeled as an infinite number of slabs according to conventional 1D physics. This leads to an inverse power law tail, matching to the BERN heuristic of a set of damped exponentials and a fudge factor constant level representing the rest of the tail. The paper estimates that "75% of anthropogenic CO2 is removed within 197–1,820 years after emissions end". It would be useful to explain that statistical moments such as the mean adjustment time can only be expressed as such a range because the value will actually diverge with a fat tail.
This diffusional model of CO2 sequestration is described in detail in Mathematical GeoEnergy, Pukite, Coyne, Challou (Wiley/AGU, 2019).
Citation: https://doi.org/10.5194/egusphere-2024-2976-CC1 -
RC2: 'Comment on egusphere-2024-2976', Anonymous Referee #2, 06 Nov 2024
This study assesses the lifetime of anthropogenic CO2 and its dependence on total emissions and sensitivity to different carbon cycle processes. The authors use the fast EMIC CLIMBER-X to simulate an ensemble of 100,000-year simulations. They thoroughly analyze which carbon reservoir takes how much carbon and when. Special attention is paid to the timescales of silicate weathering and this study provides a shorter estimate of this timescale compared to previous studies. Next sensitivity experiments are discussed to assess how sensitive the results are to several important processes.
The study is interesting and textually well written. I believe most of the reasoning, as well as the used methodology, are sound. I have a couple of major comments, minor comments and specific comments for the authors to address.
Major comments:
- My first main comment is that the paper is quite long and has a lot of figures. I think the paper would benefit from being a bit shorter. I will leave it to the authors to decide exactly what to change, but I have provided some suggestions below:
- Section 5 mainly focuses on the weathering processes, and I believe this is also the main novelty in this study. Section 3 is much more elaborate. Is it necessary to, for example, put 3.1.1 and 3.1.2 in the main text or could they also go into the supplementary?
- Is it possible to combine some figures? E.g. Figures 7 and 8, and/or Figures 12 and 13.
- In the introduction the authors mention the next glacial cycle. Is it necessary to include this in the introduction?
- Are all figures necessary to go into the main text or can they go into the supplementary? E.g. Figure 1?
- One thing that remains a bit unclear to me is how the time period where the emissions occur is treated in the analysis. I think it is necessary to mention this in a clear and explicit way. Is the period with emissions included in the 100,000 years? For what analyses is it included, and for what not?
- Following on this comment, I also suggest including in the figures when there are CO2 emissions and/or when there is an atmospheric CO2 maximum. I think this will make the interpretation of the figures next to the text easier.
Minor comments:
- Abstract: Is it possible to add how the range relates to the cumulative emission range?
- Line 51: this statement misses a reference.
- Line 105, 106: and through emissions.
- Line 118, 119: I suggest mentioning here explicitly what processes the weathering scheme depends on. Since the weathering plays a major role in the manuscript, I think it is important that the weathering scheme is as explicit and clear as possible which also makes it easier to compare it to previous studies using different weathering schemes.
- Line 138: where do the numbers come from? Is there a source?
- Line 185: I do not see a large response in atmospheric CO2 concentrations, whereas the response in temperature and land carbon are much clearer. I interpret this as that it is not the CO2 concentrations that cause the warming. What does cause this warming?
- 2d, e are not referenced.
- 4 is referenced before Fig. 3.
- Section 3.1: The sediments are not treated as explicitly as the other reservoirs. Is this for a reason?
- Line 214: Is it possible to give a one or two sentence summary of Kaufhold et al. (2024) here?
- Line 219: Fig. 4a is referenced, but there is not explicit treatment of soil carbon in Fig. 4a. Is the right figure referenced?
- Figure 7a and b show more or less the same thing. Is it necessary to show them both?
- Line 313: Is it possible to determine how much of the changes in weathering rates are because of changes in temperature, and how much due to changes in run off? I think this would make for a nice addition.
- Line 315-317: I do not fully understand this sentence.
- Section 4.1: I suggest mentioning the noLAND term earlier.
- Figure 16: Would it be beneficial to also construct panels for weathering?
- Line 592: Is permafrost treated in the methane model?
- Line 597: How does temperature evolve in intCH4 compared to REF as there is quite a strong increase in CH4 concentrations?
- Line 674: I think it is good to explicitly mention that ESMs do not agree on centennial timescales.
- I suggest adding a discussion on how tipping points (might) affect the estimation of the timescales. The AMOC is already mentioned a couple of times in the text, but I think it would be good to reiterate that, and other tipping points, in Section 5.
Specific comments:
- Line 61, 62: It is not obvious how non-linear translates to the exponential functions. They are linked here through the word ‘therefore’, suggesting an obvious connection. I suggest either explaining why non-linear means exponential in this case or rewriting the second sentence a bit.
- Line 109: I suggest mentioning the units of Catm (i.e. PgC).
- Line 194: ‘At peak CO2 concentrations, …’
- Line 493: the double fraction does not look so nice in the text.
- Line 532: remaining where? In the atmosphere?
- Line 566: I suggest rewriting this sentence a bit. I first thought that it meant that if a simulation has lower CO2 concentrations, it has a lower ECS.
- Line 642: ‘the presence of land’ feels a bit awkward here.
Citation: https://doi.org/10.5194/egusphere-2024-2976-RC2 - My first main comment is that the paper is quite long and has a lot of figures. I think the paper would benefit from being a bit shorter. I will leave it to the authors to decide exactly what to change, but I have provided some suggestions below:
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RC3: 'Comment on egusphere-2024-2976', Pierre Maffre, 06 Nov 2024
Kaufhold et al. manuscript addresses the question of the fate of anthropogenic CO2 and climate in the long-term future (100 thousand years). The authors used an Earth system model of intermediate complexity, which has several new implemented processes compared to previous similar studies. They clearly explain the novelties of their study, and the new findings. The manuscript is very well written, and well organized. It is well-suited for publication in Biogeosciences, with some minor revisions.
I only have one major comment, which concerns the silicate weathering sensitivity to climate.
The authors emphasize their re-estimation of the timescale of carbon removal by silicate weathering, to shorter values than previously thought. They partly attribute this finding to a stronger weathering feedback, which is compared to several estimations (Fig. 10b) and found to fall within the range, though on the upper part (doubling of weathering flux at +4°C, that is +18% per °C of warming).
Among the processes not represented in CLIMBER-X weathering model is the erosion limitation of weathering, or the "soil shielding effect", which is a different point of view of the exact same process. Soil shielding was extensively discussed in Hartmann et al. (2014) (cited in the manuscript), but wasn't yet implemented in Hartmann et al. (2009). Actually, soil shielding is not explicitly represented in any of the model presented in Fig. 10b.
I admit that there is no consensus on how this would affect the sensitivity of weathering to global climate (i.e., the weathering feedback strength), which is the point of interest here. Yet, there are several clues that it would significantly reduce the feedback strength:
Godderis et al., Geoderma, 2008 (10.1016/j.geoderma.2008.01.020) showed that the sensitivity of tropical weathering to runoff is largely overestimated (~ 5-fold) if considered similar than for temperature climates. Indeed, in the present manuscript, tropical environments dominate the weathering flux, and its response to global warming.
Maher & Chamberlain, Science, 2014 (10.1126/science.1250770), who also addressed the issue of erosion limitation, suggested a "maximal" weathering sensitivity, in actively eroding mountains, of +5% per °C of warming (which is lowest estimate presented on current Fig. 10b), and an average sensitivity of +1.2% per °C of warming.
Another weathering model taking into account erosion limitation, and that is spatially explicit, Maffre et al., Clim. Past, 2023 (10.5194/cp-19-1461-2023), suggests a global weathering sensitivity of ~ +9% per °C of warming, though it is unclear if the best fitting functional form should be exponential or linear.Given the absence of consensus on a value for weathering sensitivity, I do not consider that the present results should be revisited. Simply, I vividly recommend the authors to add more nuances on their statement about weathering timescale (which is one of their main conclusions), and to provide more discussion about weathering sensitivity, the large uncertainty that exists in the literature concerning its value, and how it should affect the weathering timescale.
Specific comments- Section 2.2 (lines 100–110): there is a missing information here about the organic carbon cycle. As far as I understand, the sediment component is run as an open system (with sediment loss through burial), and this sediment contains organic carbon generated by marine primary productivity (lines 255–256).
Therefore, and given Eq. (1), setting Fvolc to half of the global silicate weathering flux (as indicated lines 136–138) would not result in a steady-state carbon cycle, because of this additional C sink (organic carbon burial), that would result in a net ocean-to-atmosphere flux lower than the remaining term "Fvolc – Fweath".
Unless the organic carbon cycle is forced to work as a closed system (like silicate and phosphate, lines 106–107), and all buried organic carbon is put back into the atmosphere? - Lines 125–126: I do not understand why "carbonate sedimentary rock" should be different than "carbonate", in term of weathering (Eq. 2). Moreover, why not indicating the equations for "carbonate sedimentary rocks" weathering and loess weathering?
- Lines 132–136: I think that orbital forcings could be mentioned here, among the "external forces" (line 132) excluded in the study, although it may be redundant with line 145.
- Lines 145–146: It is not completely clear here whether the fixed orbital forcings concern only the spin-up run, or all simulations (including the spin-up).
- Lines 169–170: I think it would be useful here just to indicate that climate sensitivity is altered by rescaling the pCO2 seen by the radiative code as a function of the actual pCO2, and then refer to Appendix A.
- Lines 185–186: This statement, "temperatures temporarily stabilize instead of decreasing due to the release of soil carbon into the atmosphere" seems erroneous. Temperature does stabilize during between 150yr and 1000yr in the 5000 PgC scenario (Fig. 2b), but pCO2 declines just as in the other scenarios (Fig. 2a). So how could it be an effect of the "release of soil carbon into the atmosphere"? It rather seems that there is a decoupling of CO2 and temperature, that is likely due to oceanic dynamics. Indeed, there is a small bump of global temperature at 700yr (without any pCO2 change), which coincides with abrupt AMOC recovery (Fig. 7e).
- Lines 215–221: this non-monotonous behavior is interesting. Has it been already suggested, or is it a new finding of current study?
- Line 222: This statement, "In our simulations, the land is a net carbon sink for the entire 100 kyr" also seems erroneous. From Fig. 3a, it appears that land becomes a (slight) net source of carbon at 200kyr in all simulations. Besides, I don't think that "land carbon" is defined anywhere in the manuscript. Is it simply "soil + vegetation" carbon?
- Line 364: The mention of "noLAND" comes quite abruptly here, given that the sensitivity experiments are only discussed in a later section (4). Could you remind "experiment with land carbon disabled", and refer to Table 1?
- Fig 13: It is difficult to visualize the trends of Ai and τi versus cumulative emission (trends that are discussed in the current section). I suggest adding two small panels in the figure, plotting Ai vs E and τi versus E.
- Lines 371–377: It might be useful to indicate here that Ai do not sum at 1 because the IRF does not start at 1, and that the initial value (= the sum of Ai) depends on the cumulative emission scenario.
- Lines 565–572: It seems that there is a positive feedback here: warmer temperature (for a same pCO2) generates higher pCO2, because of the warming-induced soil carbon release. It would be useful to indicate that it is a positive feedback.
- Line 594: Is methane lost by converting it into CO2? Granted that 2200 ppb of methane should not generates more than 2 ppm of CO2, with is much less than the pCO2 anomaly reported in Fig. 15d.
- Lines 644–645: Would it really influence the ATMOSPHERIC lifetime of CO2? It seems to me that the longer weathering timescale is the just a delay because of carbon storage in land before it is stored through weathering, instead of being directly stored though weathering, and that this sink transfer has no consequence regarding carbon in the atmosphere.
Technical corrections:- There are several occurrences where it should be more accurate to talk about weathering "flux", than weathering "rate", which rather refers to a specific flux (in mol/m2/yr): line 138, line 281, caption of Fig. 9, line 291, line 301...
- Line 130: It seems that "run-off" should be spelled "runoff", to be consistent with the other occurrences of that word in the manuscript.
- Caption of Fig. 6: The mean net annual NPP is in (a–c), not (a–b).
- Fig. 11: A mere suggestion: it feels more "natural" to use a colorscale with “wetter” colors (e.g., blue) for precipitation increase and “dryer” colors (e.g., red) for precipitation decrease.
- Line 526: I believe that "begin" should here be a singular, "begins".
- Line 638: Shouldn't "variation" be a plural here?
- There are a few inconsistencies between US and British spelling. I noticed the use of "behavior" and "behaviour" in the text. Please check.
- Many DOIs link have duplicated "https://doi.org/https://doi.org/" in the reference list. Pleas check.
Citation: https://doi.org/10.5194/egusphere-2024-2976-RC3 - Section 2.2 (lines 100–110): there is a missing information here about the organic carbon cycle. As far as I understand, the sediment component is run as an open system (with sediment loss through burial), and this sediment contains organic carbon generated by marine primary productivity (lines 255–256).
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