the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A comparison of the climate and carbon cycle effects of carbon removal by Afforestation and an equivalent reduction in Fossil fuel emissions
- Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore-560012, India
- Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore-560012, India
Abstract. Afforestation and reduction of fossil fuel emissions are two major components of climate mitigation policies. However, their effects on the earth’s climate are different because reduction of fossil fuel emissions directly alters the biogeochemical cycle of the climate system, while afforestation causes biophysical changes in addition to changes in the biogeochemical cycle. In this paper, we compare the climate and carbon cycle consequences of carbon removal by afforestation and an equivalent fossil fuel emission reduction using simulations from an intermediate complexity Earth system model. Our simulations show that the climate is cooler by 0.36 °C, 0.47 °C, and 0.42 °C in the long term (2471–2500) in the case of reduced fossil fuel emissions compared to the case with afforestation when the emissions follow the SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios, respectively. Though afforestation results in a lower atmospheric CO2, the cooling from the reduced atmospheric CO2 is partly offset by the warming from surface albedo decrease associated with the regrowth of forests. Since this warming effect from surface albedo decrease is nearly absent in the reduced fossil fuel emission case, the climate is relatively cooler, even though the atmospheric CO2 levels are similar to the afforestation case. Thus, in terms of climate benefits, reducing fossil fuel emissions is relatively more beneficial than afforestation for the same amount of carbon removed from the atmosphere. Nevertheless, fossil fuel emission reduction and afforestation efforts should be pursued simultaneously as both lead to a decrease in global mean warming and reduced ocean acidification.
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Koramanghat Unnikrishnan Jayakrishnan and Govindasamy Bala
Status: final response (author comments only)
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RC1: 'Comment on bg-2022-227', Anonymous Referee #1, 20 Dec 2022
Review of the manuscript
“A comparison of the climate and carbon cycle effects of carbon removal by Afforestation and an equivalent reduction in Fossil fuel emissions”
by
Jayakrishnan & Bala
for
Biogeosciences
General remarks
In this study, the climate effects of two different methods to reduce atmospheric CO2 concentrations, namely afforestation and reduced CO2 emissions, are analyzed. For this purpose, long-term climate simulations with an Earth System Model of intermediate complexity are performed. In a first step, reference simulations are performed for three different climate change scenarios. Subsequently, the three simulations are repeated, but with an afforested land mass, where forests replace agricultural areas wherever forest can naturally grow. In a last experiment, the different amounts of CO2 that would haven been removed by these afforestation runs are finally implemented in the three scenario runs, but without afforestation.
Results show that CO2 reductions caused by reduced CO2 emissions lead to a stronger temperature reduction than CO2 reductions caused by afforestation, because of the lower surface albedo of forests and the resulting increase in absorbed solar radiation. In addition, the results of the study indicate that the climate benefit of afforestation depends on the background climate and is less pronounced in a warmer climate.
The manuscript is timely, clearly structured and its topic fits to the scope of Biogeosciences. However, there are some issues which need to be addressed by the authors before the manuscript is ready for publication.
Major Comments:
- The whole manuscript is written like a research paper and some aspects are succinctly formulated. In some cases, presented results are tried to be shortly explained in only one sentence, but without success (see point 2 and minor comments). More detailed explanations would therefore be good for the manuscript and increase its comprehensibility. For instance, the additional text in the supplement should be integrated in the manuscript. Some of the supplement figures (e.g. S6, S7, S16, S17) should also be included.
- The explanation for the dependency of the cooling effect of reduced CO2 concentrations on the climate change scenario is too short. I understand that, in a scenario with a generally high CO2 concentration, even a larger removal of CO2 has a smaller cooling effect, since already for such a reduced CO2 concentration some kind of saturation effect takes place. However, the CO2 cooling effect is comparable in SSP2 and SSP3, although the CO2 concentrations are quite high in SSP3. Is there a critical point in the CO2 concentrations which is not yet reached in SSP3? In addition, the albedo warming effect is in SSP3 almost as strong as in SSP5, although the CO2 cooling effect is in SSP3 much stronger. This might be related to biogeophysical feedbacks (e.g. changes in cloud cover etc.), which take place in SSP5 but not in SSP3. It would be nice if the authors would discuss this.
- In order to be able to understand all these processes, more information about the model are required and how the complex interactions between the land surface and the atmosphere are considered in the model. By the way, what is the spatial resolution of the model?
- The dependency of a positive climate effect of afforestation on the background climate is from my point of view the second important message of this study, beside the counteracting albedo effect. This finding is new, at least for me. Therefore, this point should be stronger emphasized and its reasons should be discussed in more detail, and not only be mentioned in one sentence (line 293).
Minor Comments:
Line 185: mean global land surface albedo
Line 229: the warming, caused by the albedo effect of forests!?
Line 244: Please replace the comma with a dot after “(Table S1)”
Line 254: Please explain this buffering effect in more detail? Isn’t this reduced ocean carbon uptake a result of the increased temperatures?!
Line 289: warming effect caused by a reduced albedo?!
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RC2: 'Comment on bg-2022-227', Anonymous Referee #2, 05 Jan 2023
The manuscript entitled 'A comparison of the climate and carbon cycle effects of carbon removal by afforestation and an equivalent reduction in fossil fuel emissions' investigates the method of atmospheric carbon dioxide reduction in a climate change context. More specifically, the authors use a simple Earth system model to investigate the impact on the climate system of reducing atmospheric CO2 by fossil fuel emission reduction or by afforestation (increase C uptake by vegetation). The authors pose an interesting and timely question as to whether the carbon dioxide reduction method matters, but the manuscript has in its current form several shortcomings, that need to be addressed before further publication.
Overall, the manuscript would benefit from thorough proofreading, as many paragraphs are not clear and concise. Below follows major and minor comments and suggestions to improve the manuscript.
Major comments:
Biophysical feedbacks: Throughout the manuscript, the authors state how afforestation affects biophysical feedbacks. Yet, they only consider one aspect of biophysics, namely albedo. The authors completely fail to mention the cooling effect of evapotranspiration. This is a major shortcoming of the current study and needs to be addressed. I am not familiar with the UVic ESM, but since it includes an energy-moisture model for the atmosphere, I assume that evapotranspiration can be inferred. This needs to be included in the analysis.
Discussion of results is lacking: The authors are completely uncritical towards their results and do not relate or put them in context to other similar studies. This is a major shortcoming of the manuscript and needs to be included.
The impact on the ocean: The authors have chosen to look at how carbon content in the ocean and pH are affected by afforestation and a reduction in fossil fuel emissions. This part is not well linked to the rest of the manuscript. The two variables they a focusing on are tightly linked to the atmospheric CO2, and as this is essentially the same in both simulations (AFFOREST and REDUCED_FF) they arrive at the same results. It is by no means a surprise and it follows logically that when the atmospheric CO2 is lower than in the reference simulation, the carbon content of the ocean is lower, and the pH is higher. Maybe some more interesting variables to investigate could be sea surface temperatures or variables related to heat transport in the ocean. The analysis of the impact on the ocean needs to be a better couple to the rest of the manuscript.
Introduction: The introduction is not well linked to the rest of the manuscript. Some suggestions as to how to improve the introduction follow below:
- L28-L42: the facts described here are all well-established. This could be shortened to free more space to describe how the different methods of reducing atmospheric CO2 would affect the climate system.
- L66- L71 focus on deforestation, while the focus of the manuscript is on afforestation, why afforestation should also be the focus of the introduction. Thus, it would make more sense to rewrite this with a focus on afforestation, and likely also new references should be included.
- Several of the studies highlighted in the introduction are not well described. For example, in L48-L50 Jayakrishnan et al., 2022 find that fossil fuel emissions and deforestation affect the climate system fundamentally differently, but it is not stated wherein this difference lay. Please include such information. In L51—L53 Simmons and Matthews, 2016, find that it is important to include biophysics, but it is not stated why it is important. Please explain why.
- L71-L72 is a very bold statement. A vast number of studies are indeed looking at the biophysical effects of changes in vegetation cover (Zeng et al., 2017, Luyssaert et al., 2018, Alkama et al., 2022 and many more). Therefore, you cannot state that biophysical effects are often neglected.
- The link to the ocean (which is analysed in section 3.3) is not introduced in the introduction, thus this analysis seems very decoupled from the rest of the manuscript.
Method and evaluation of the modelling setup: The method could benefit from some more details in particular concerning the vegetation dynamic. I would go more into detail as to how the dynamic vegetation works in terms of competition between the PFTs (it seems rather static for the non-afforestation cases). Also, the present-day fraction of forest is only 0.2 which is low. It would be good with a validation of the model against present-day forest extend or LAI. Moreover, I would move the description of VEGBURN from the result section to the methodology section, and I would even include some of the descriptions from the supplement as this is better described. In addition, you also need to describe how the ocean is spun-up.
Figures and tables: I would suggest moving fig. S6 to the methodology section in the manuscript. I would recommend moving table S2 to the results section in the manuscript, as it very nicely summarises the results. Moreover, I would combine figures 1, 2, 3 and 4 into one figure, and I would combine figures 6 and 7 into one figure. In addition, I find it confusing all the references to the figures in the supplement, thus you might want to consider combining or moving them to the main manuscript and whether all these figures are needed. In addition, the figure captions contain a lot of text describing the results which are also included in the main text. I suggest you delete this, as it is already contained in the main text.
Minor comments:
L13-L14 state that ‘fossil fuel emissions directly alter the biogeochemical cycle of the climate system’ but it also affects the physics of the troposphere via its impact on radiation and the energy budget. Thus, please rephrase this sentence.
L34 change ‘fossil fuel use’ to ‘fossil fuel emissions’
L35-36 please delete ‘in the recent decade’ and rephrase to ‘During the period 2010-19, CO2 emissions from fossil fuel use and land use and land cover changes were 9.6±0.5 PgC yr-1 and 1.6 ± 0.7 PgC yr–1, respectively’
L42 please rephrase to ‘two major strategies considered for mitigating climate change are’
L130 move to L111. e.g. : i) prescribed fossil fuel emission simulation with fixed agricultural land (FIXED_AGR) corresponding to the year 2005, which corresponds to the reference simulation
L137 please change ‘land carbon’ to ‘land carbon stock’, also throughout the manuscript.
L139 please add globally to ‘approximately 0.2 to 0.4 globally’
L160-L163 please rephrase these sentences as it is very hard to understand.
L172 please change ‘over land’ to ‘in land’
L175-L176 additional already implies the amount is larger, rephrase the sentence.
L176-L179 Please rephrase for clarity
L184-L185 please add global ‘the global land surface albedo’
L186-187 please explain why the global albedo decreases initially.
L188 please change ‘less’ to ‘lower’
L189 is the 0.011 globally?
L194-196 this could be deleted, as this is more fit for the conclusion
L205-206 please rephrase to: Initially, atmospheric CO2 increases until around the cessation of fossil fuel emissions in the year 2250 whereafter the cessation of emissions around 2250, atmospheric CO2 decreases slightly until …
L207-L208 I do not understand the last part of the sentence ‘because of further carbon uptake by the ocean (Sect. 3.3) in all nine simulations’. Please elaborate.
L214 I think you need to add reduced_ff to the sentence thus ‘relative to AFFOREST and REDUCED_FF simulations’
L222 Please explain why the cooling impact decreases with the warmer SSP scenarios.
L225 please rephrase ‘this offsetting is almost full’
L229 please add ‘the warming effect from decreases in surface albedo due to’
L236-L239 this could be deleted, as this is more fit for the conclusion
L262 There is no Figure 22. Is this figure S2?
L271-L273 this could be deleted, as this is more fit for the conclusion
L298 ‘grow trees artificially’, what do you be by that? By using irrigation?
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RC3: 'Comment on bg-2022-227', Anonymous Referee #3, 05 Jan 2023
general comments
The manuscript compares the climate effects of a world with high land based mitigation (afforestation) and one the same carbon removed for 3 different SSP scenarios. They use the University of Victoria Earth System Climate Model and make the comparison for a variety of variables related to the carbon cycle, surface energy cycle and ocean. In general the research is novel and is executed in a thorough way and answers interesting questions. However, at this point the manuscript somewhat lacks a critical discussion and doesn’t relate nor compares the outcomes of this study to previous literature as much as it should. I think the manuscript can improve greatly by including for example a discussion section which can elevate the results and compare them to previous literature.
specific comments
- The comparison between the afforestation simulation (AFFOREST) and the reduced emission simulation (REDUCED_FF) is interesting, the way I see it in the REDUCED_FF simulation you account for the carbon effects of afforestation but neglect the biogeophysiscal effects, thus the difference between both simulations allows to quantify the biogeophysical effects of afforestation. It would be interesting to compare these effects in temperature to previous studies both observational (Duveiller et al., 2018; Alkama and Cescatti, 2016) but also modelling studies with more complex Earth System Models that quantify the biogeophysiscal effects (a.o. Boysen et al., 2020; Winckler et al., 2019; Portmann et al.,2022). This way you could assess how well the UVIC model represents biogeophysical effects or whether there are potentially biases (a comparison can only be qualitative of course as the amounts of afforestation differ across studies).
- In the analysis of the temperature response you mainly focus on albedo as an explaining variable, this is warranted as the effects are mostly cooling however previous research as mentioned in my comment above found that afforestation might also cause a local cooling effect due to changes in turbulent heat fluxes. It might be interesting to check some of the other energy balance components in order to understand whether these effects are absent in the model or whether the temperature response is simply dominated by the global warming as a consequence of the albedo effects.
- The description of the model is too limited and should be elaborated as at this point it is not clear what important processes for afforestation are resolved and which not. You should elaborate this description (or add a more detailed section in the supplements) with a larger focus on the land surface scheme (e.g. list of PFT’s).
- Line 186-188: you explain that the albedo increases initially in all simulations, but you never explain why this happens or what process is behind it. I assume it is the remaining natural vegetation reaching their climax as the model employs a dynamic vegetation model but it would be good to clarify that here.
- Line 238: this is still the section regarding temperature effects it is a bit strange that in the summary of this section you mention ocean acidification, I would remove this.
- The results have clear narrative and important conclusions which are clear and well founded, however the article would benefit from including more similar literature in a discussion chapter in order to facilitate the understanding and critical review of results which can also help draw out future research suggestions. At this point in time I lack this greatly as there is a lot of work out there on the effects of afforestation on climate and carbon (see eg Pongratz et al. 2022), it would also be good to check if some studies have assessed the ocean effects of afforestation (I am not aware of any literature regarding this so I cannot suggest any). This comparison to literature can help highlight strengths and limitations from the approach used in this study.
technical corrections
- This is in part my opinion but I don’t think you need lat lon labels in a global spatial plot, just the ticks suffice for the readability of the plot and in general it just gets cluttered more by adding all those labels, however this is just an opnion so if you want to stick with the plots you have now thats fine too.
- Line 213 : Sect. 3.1 ii)) one bracket should be removed after ii
- Line 262: Figure22 should be Figure S22
References:
- Pongratz, Julia, et al. "Land use effects on climate: current state, recent Progress, and emerging topics." Current Climate Change Reports(2021): 1-22.
- Duveiller, Gregory, Josh Hooker, and Alessandro Cescatti. "The mark of vegetation change on Earth’s surface energy balance." Nature communications1 (2018): 1-12.
- Alkama, Ramdane, and Alessandro Cescatti. "Biophysical climate impacts of recent changes in global forest cover." Science6273 (2016): 600-604.
- Boysen, Lena R., et al. "Global climate response to idealized deforestation in CMIP6 models." Biogeosciences22 (2020): 5615-5638.
- Winckler, Johannes, et al. "Nonlocal effects dominate the global mean surface temperature response to the biogeophysical effects of deforestation." Geophysical Research Letters2 (2019): 745-755.
- Portmann R, Beyerle U, Davin E, Fischer EM, De Hertog S, Schemm S. Global forestation and deforestation affect remote climate via adjusted atmosphere and ocean circulation. Nature Communications. 2022 Oct 4;13(1):1-1.
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RC4: 'Comment on bg-2022-227', Anonymous Referee #4, 05 Jan 2023
“A comparison of the climate and carbon cycle effects of carbon removal by Afforestation and an equivalent reduction in Fossil fuel emissions”
Jayakrishnan & Bala in Biogeosciences
This study addresses the question of the climate impact of different carbon mitigation strategies. The authors raise the question of whether a similar decrease in atmospheric carbon resulting either from afforestation or from a cut in anthropogenic emissions have the same effect on land surface temperature and ocean carbon content.
To tackle this question, they set up two extreme modeling experiments under 3 future climate scenarios (SSP2, SSP3, SSP5): 1/ all agricultural land is abandoned and left for forest to grow (AFFOREST) or 2/ the same amount of carbon that would be captured by such a large-scale afforestation is cut from the anthropogenic emissions (REDUCED_FF). Both extremes are compared to a baseline simulation where there is no land use change (FIXED_AGR).
The authors conclude that the reduction of atmospheric carbon from a net reduction in anthropogenic emissions has a larger impact on atmospheric cooling compared to the afforestation strategy. This results is explained by the biophysical feedbacks of albedo. The study concludes that reducing fossil fuel emissions is more effective than afforestation in mitigating climate change.
Despite a timely topic, some aspects of the paper leave me doubtful about the relevance of the experimental setup and the conclusions.
Experimental setup
The scenarios are presented as ” idealized” (L75). Indeed abandoning all agricultural land has no other reality than the brutal mass extinction of humanity, which then would be followed by the total cut of FF emissions. The unrealism of the scenarios is a strong limit to the study’s conclusions that should therefore be toned down as they should not be translated into policy recommendations (L23, L306).
No order or magnitude is given for the carbon emissions that are cut (L284). Making it difficult to get an idea of the strength of the scenario. Give some reference values, for example global carbon stored on land.
It is not clear to me what level of feedbacks is included in the model between emissions and climate? Are SSP climate trajectories and AFFOREST and REDUCED_FF mitigation scenarios consistent, and if not, does it matter? In the scenario REDUCED_FF, fossil emissions are cut significantly with respect to FIXED_AGR and AFFOREST scenarios. Then climate in which the trees grow, say for the SSP8.5 climate trajectory should not be the same. Is this accounted for?
Model description
The description of an Earth System Model cannot be done in 10 lines. The readers need to know which processes are included and how they are modeled to be able to understand critically the simulation outputs. Some of the questions that need to be addressed are:
What types of vegetation types are included? How is the model parameterized? In the afforestation scenario, what vegetation type takes over the abandoned agricultural land? How do simulated biomass, carbon fluxes and stocks, water fluxes compare to observations (for example the value given on L145 needs to be compared to literature)?
Another information needed in the model description is the sensitivity of the model to drought-induced mortality or other disturbances? Climate change scenarios as applied here for 500 years come with intense and frequent droughts, fires or storms that might impede the growth, and hence the sink simulated from fertilization effect, especially in the afforestation scenario. Are these processes included? If not, the time horizon could be reduced and the effect of these assumptions should be discussed.
Results
About the explanation of the different global climate effect between AFFOREST and REDUCED_FF, the authors argue that it is the result of albedo changes only. This is overly simplistic. As an example, many studies (for example Li et al., 2015 Nature comm. or Bonan 2008) show that growth of tropical forests have a cooling effect due to water fluxes as opposed to boreal forests that would have a warming effect due to the albedo decrease. The current study’s results in the tropics are the opposite of these (L235 afforestation in the tropics leads to warming) and no explanation is given to this apparent contradiction. More generally, the water-based energy exchanges are not discussed at all even though they are a key part of the climate system. This is a key lack of the study.
Also in Figure 5 the difference in AFFOREST result between the 3 climate trajectories are not explained when it is an important and surprising result (L235 and L293).
Format
The choice of figures needs to be rethought.
- Too many figures from SM are cited showing their relevance to the authors’ demonstration, then they should be included. Also it is hard to follow because there are so many figures. Some should be merged.
- The only displayed figures in the article show differences to the reference scenario and hide the inherent dynamics of the scenarios. These dynamics are however essential to understand the processes so the figures currently shown in SM showing the 3 scenarios are the ones that should be in the paper.
Koramanghat Unnikrishnan Jayakrishnan and Govindasamy Bala
Koramanghat Unnikrishnan Jayakrishnan and Govindasamy Bala
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