the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Assessing Responses and Impacts of Solar climate intervention on the Earth system with stratospheric aerosol injection (ARISE-SAI)
- 1Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder CO
- 2Sibley School for Mechanical and Aerospace Engineering, Cornell University, Ithaca NY
- 1Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder CO
- 2Sibley School for Mechanical and Aerospace Engineering, Cornell University, Ithaca NY
Abstract. Solar climate intervention using stratospheric aerosol injection is a proposed method of reducing global mean temperatures to reduce some of the consequences of climate change. A detailed assessment of responses and impacts of such an intervention is needed with multiple global models to support societal decisions regarding the use of these approaches to help address climate change. We present here a new modeling protocol and a 10-member ensemble of simulations using one of the most comprehensive Earth system models, aimed at simulating a plausible deployment of stratospheric aerosol injection and reproducibility of simulations using other Earth system models to enable community assessment of responses of the Earth system to solar climate intervention. The Assessing Responses and Impacts of Solar climate intervention on the Earth system with stratospheric aerosol injection (ARISE-SAI) simulations utilize a moderate emission scenario, introduce stratospheric aerosol injection at ~ 21 km in year 2035, and keep global mean surface air temperature near 1.5 °C above the pre-industrial value (ARISE-SAI-1.5). We present here the detailed set-up, aerosol injection strategy, and mean surface climate changes in these simulations so they can be reproduced in other global models.
Jadwiga Richter et al.
Status: final response (author comments only)
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CC1: 'Comment on egusphere-2022-125', Richard Rosen, 26 Apr 2022
My first concern about this paper is that it does not discuss seasonal impacts on temperature and precipitation as a function of latitude at all. This must be done. For example, we know that the annual average temperature impacts of climate change on annual average temperature occur most in the high northern latitudes, in places like Alaska. Furthermore, in theory, having more CO2 in the atmosphere over such regions clearly has a huge impact in reducing radiative cooling in winter, thus increasing surface temperatures substantially. Yet, having sulfate particles over Alaska in winter won't have much impact in reducing temperatures since the periods of sunlight are so short. Furthermore, having most of the sulfate particulates much farther south, as shown in Figure 2, would seem to imply that incoming summer radiation will not be reflected very much in the far north were it is needed to be reflected during the long daytimes of summer to cool the air. Similar seasonal assymetries are probably important for precipitation impacts of climate change even though these would be much harder to model accurately. The seasonal assymetries with regard to surface temperature seem to derive much more simply from the physics of CO2 concentrations and the density of sulfate particles in the air. Thus, concluding that "on average" over the year or over many years solar geoengineering can mitigate climate change is not very helpful when trying to analyze the impact of climate change on human society and the ecology. Seasonal and time of day (day vs. night) differences in impact on temperature and precipitation are very important to consider.
My second concern is that I do not quickly see any discussion of how the impact of sulfate particles on the reflectivity of solar radiation is modelled at different wavelengths, and at different times of the day. Also, I do not see any discussion of the impact of continually falling particles have on air quality, human beings breathing the air, and on ecology and agriculture.
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CC2: 'Reply on CC1', Richard Rosen, 20 May 2022
It is unfortunate that the authors of this paper have not yet responded to my earlier comments. It is hard to have a community discussion, as the EGU intends for papers submitted to it without such a response. But I will supplement my earlier comments here.
1. It is clear from simple physics that if enough sulfate particles are introduced into the atmoshere that the average temperature at the surface of the globe will cool sufficiently to offset the amount of average global heating due to the increase in greenhouse gases the world is emitting. Again, this could clearly be achieved on average across the entire surface area of the globe. But even the IPCC acknowledges that the distributional effects of such cooling on temperature and its impacts on precipitation would not offset the impacts of climate change at the regional/local scale. One does not need to sophisticated computer model of the climate system to understand why this would be true, and I gave some examples in my earlier comments on this paper of the physics behind this issue.
2. Different climate models give fairly different results, thus the authors need to justify why their paper should focus on only one such model. Why should we believe that the model focused on will yield more accurate regional impacts for sulfate geoengineering and baseline cases than other models in the future?
3. Since large amounts of sulfate particles injected into the atmosphere are likely to diminish the strength of sunlight hitting the surface of the earth, the authors should discuss the impact of this likely effect on agricultural productivity for different food crops, since different crops may have different degrees of sensitivity to the dimming of sunlight during their growing seasons. The same should be discussed for forest growth and health.
4. Again, to the extent that sulfate geoengineering will likely yield different precipitation patterns across the globe relative to normal rainfall patterns, and relative to the rainfall patterns that will be produced by climate change over the next decades, this issue should be explored. However, it is well known that it is likely to be the case that computer-based climate models have a harder time accurately computing the impact of climate change on precipitation then on temperature, and this is likely to even more true for their ability to compute the changes in rainfall patterns due to sulfate geoengineering. This issue needs to be more fully explored in a revised paper.
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AC3: 'Reply on CC2', Jadwiga Richter, 24 May 2022
Thank you for your comments. Our responses are in italics.
It is unfortunate that the authors of this paper have not yet responded to my earlier comments. It is hard to have a community discussion, as the EGU intends for papers submitted to it without such a response. But I will supplement my earlier comments here.
- It is clear from simple physics that if enough sulfate particles are introduced into the atmoshere that the average temperature at the surface of the globe will cool sufficiently to offset the amount of average global heating due to the increase in greenhouse gases the world is emitting. Again, this could clearly be achieved on average across the entire surface area of the globe. But even the IPCC acknowledges that the distributional effects of such cooling on temperature and its impacts on precipitation would not offset the impacts of climate change at the regional/local scale. One does not need to sophisticated computer model of the climate system to understand why this would be true, and I gave some examples in my earlier comments on this paper of the physics behind this issue.
Please see our response to your earlier comment. We are in full agreement: all impacts on seasonal and regional levels need to be evaluated, however this does not fit in one manuscript.
- Different climate models give fairly different results, thus the authors need to justify why their paper should focus on only one such model. Why should we believe that the model focused on will yield more accurate regional impacts for sulfate geoengineering and baseline cases than other models in the future?
You’re absolutely correct. Here we describe the experimental protocol so other models can repeat this set-up. Already, the UKESM model is running these simulations and those will be made available to the community as well. The comparison between various models will elucidate uncertainties in impacts of climate intervention, and help highlight inter-model differences both in light of climate change and of climate intervention strategies. For a discussion on inter-model uncertainties in previous GeoMIP simulations, see for instance Visioni et al. (2021b).Here we are describing new simulations that we consider to be more policy-relevant in terms of underlying emission scenarios used and target temperature achieved, but we are not claiming that only one model has the correct answer. Rather, as we already explained above, we are providing a blueprint for other models to repeat the same simulations. - Since large amounts of sulfate particles injected into the atmosphere are likely to diminish the strength of sunlight hitting the surface of the earth, the authors should discuss the impact of this likely effect on agricultural productivity for different food crops, since different crops may have different degrees of sensitivity to the dimming of sunlight during their growing seasons. The same should be discussed for forest growth and health.
There are several people at Rutgers University already looking at changes in crops and agricultural productivity in these simulations. These will be discussed in subsequent manuscripts. Other previous papers already discussed these issues in a generic way (i.e. Zarnetske et al., 2021; Visioni et al., 2021a) or for other, in our view less policy-relevant, simulations (i.e. Fan et al., 2021)
- Again, to the extent that sulfate geoengineering will likely yield different precipitation patterns across the globe relative to normal rainfall patterns, and relative to the rainfall patterns that will be produced by climate change over the next decades, this issue should be explored. However, it is well known that it is likely to be the case that computer-based climate models have a harder time accurately computing the impact of climate change on precipitation then on temperature, and this is likely to even more true for their ability to compute the changes in rainfall patterns due to sulfate geoengineering. This issue needs to be more fully explored in a revised paper.
The issue the commenter raised has been discussed before, for instance in Visioni et al. (2021b) for GeoMIP CMIP6 simulations (see Figure 6 and Figure 9 there), but also in other places (Irvine et al., 2020). It is true that climate models present larger uncertainties in their hydrological response to increased CO2 than they do for temperature; similarly, the drivers of hydrological changes in climate intervention simulations (Simpson et al., 2019) need to be better investigated.
References:
Fan, Y., Tjiputra, J., Muri, H. et al. Solar geoengineering can alleviate climate change pressures on crop yields. Nat Food 2, 373–381 (2021). https://doi.org/10.1038/s43016-021-00278-w
Irvine, Peter J., and David W. Keith. "Halving warming with stratospheric aerosol geoengineering moderates policy-relevant climate hazards." Environmental Research Letters 15.4 (2020): 044011.
Simpson, I. R., Tilmes, S., Richter, J. H., Kravitz, B., MacMartin, D. G., Mills, M. J., et al. (2019). The regional hydroclimate response to stratospheric sulfate geoengineering and the role of stratospheric heating. Journal of Geophysical Research: Atmospheres, 124, 12587– 12616. https://doi.org/10.1029/2019JD031093
Visioni, D., MacMartin, D. G., & Kravitz, B. (2021a). Is turning down the sun a good proxy for stratospheric sulfate geoengineering? Journal of Geophysical Research: Atmospheres, 126, e2020JD033952. https://doi.org/10.1029/2020JD033952
Visioni, D., MacMartin, D. G., Kravitz, B., Boucher, O., Jones, A., Lurton, T., Martine, M., Mills, M. J., Nabat, P., Niemeier, U., Séférian, R., and Tilmes, S.: Identifying the sources of uncertainty in climate model simulations of solar radiation modification with the G6sulfur and G6solar Geoengineering Model Intercomparison Project (GeoMIP) simulations, Atmos. Chem. Phys., 21, 10039–10063, https://doi.org/10.5194/acp-21-10039-2021, 2021b
Zarnetske, P. L., Gurevitch, J., Franklin, J., Groffman, P., Harrison, C., Hellmann, J., Hoffman, F. M., Kothari, S., Robock, A., Tilmes, S., Visioni, D., Wu, J., Xia, L., and Yang, C.-E.: Potential ecological impacts of climate intervention by reflecting sunlight to cool Earth: Ecological impacts of climate intervention, P. Natl. Acad. Sci. USA, 118, e1921854118, https://doi.org/10.1073/pnas.1921854118, 2021
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CC3: 'Reply on AC3', Richard Rosen, 25 May 2022
Dear Authors,
I am glad that you agree with so many of the points that I had raised previously about key analyses that need to be performed when studying sulfate geoengineering and its impacts on the earth system. Unfortunately, you seem to be unwilling to enhance your current proposed publication with these additional analyses which seem critical to me to help readers understand the potential risks and benefits of this approach to trying to keep our planet reasonably cool, on average. But what I am unclear about now, is what are the main new findings of your modeling work. We seem to agree that one does not need to do modeling to decide that if enough sulfate particulates are injected into the upper atmosphere that the earth's average temperature can be lowered enough to offset climate change. Anyway many other modelers have made this point. So what are the unique/new findings of your research? I suggest that you rethink the organization of your article and list all really new findings up front, and then explain how you arrived at those findings. And compare your research on each item with prior research findings to the extent that they might be similar or different. Thanks.
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CC4: 'Reply on CC3', Daniele Visioni, 25 May 2022
Dear Richard,
amongst the possible article types to be considered for peer-review publication on GMD, there is
- model experiment descriptions, including experimental details and project protocols
which is what we set out to do, and did, in our manuscript. You can find multiple cases in the previous literature in GMD where no novel results are described. For instance, see Tilmes et al. (2015) (https://gmd.copernicus.org/articles/8/43/2015/) and Kravitz et al. (2015) (https://gmd.copernicus.org/articles/8/3379/2015/) for other works in GMD describing geoengineering experiments. In both, some preliminary results from a subset of models that already performed one or some of the experiments are shown. Having an easy to reference manuscript where novel experimental protocols are described in details is useful for other modeling centers aiming to reproduce similar experiments.
We have been happy to follow up with some more analyses as proposed. However, we believe that most of these analyses, in the framework of this new experimental set-up we proposed and that is different from previous experimental set-up we discussed, should be left to future works that can perform more in depth analyses.
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CC4: 'Reply on CC3', Daniele Visioni, 25 May 2022
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AC3: 'Reply on CC2', Jadwiga Richter, 24 May 2022
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AC1: 'Reply on CC1', Jadwiga Richter, 24 May 2022
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AC2: 'Reply on AC1', Jadwiga Richter, 24 May 2022
Thank you for your comments. Our responses are in italics (this is a copy of the previous response that was completely in PDF which is not showing up on my system)
My first concern about this paper is that it does not discuss seasonal impacts on temperature and precipitation as a function of latitude at all. This must be done. For example, we know that the annual average temperature impacts of climate change on annual average temperature occur most in the high northern latitudes, in places like Alaska. Furthermore, in theory, having more CO2 in the atmosphere over such regions clearly has a huge impact in reducing radiative cooling in winter, thus increasing surface temperatures substantially. Yet, having sulfate particles over Alaska in winter won't have much impact in reducing temperatures since the periods of sunlight are so short. Furthermore, having most of the sulfate particulates much farther south, as shown in Figure 2, would seem to imply that incoming summer radiation will not be reflected very much in the far north were it is needed to be reflected during the long daytimes of summer to cool the air. Similar seasonal assymetries are probably important for precipitation impacts of climate change even though these would be much harder to model accurately. The seasonal assymetries with regard to surface temperature seem to derive much more simply from the physics of CO2 concentrations and the density of sulfate particles in the air. Thus, concluding that "on average" over the year or over many years solar geoengineering can mitigate climate change is not very helpful when trying to analyze the impact of climate change on human society and the ecology. Seasonal and time of day (day vs. night) differences in impact on temperature and precipitation are very important to consider.
We completely agree with you that the seasonal impacts on temperature and precipitation must be explored, and they have been in previous simulations (for instance, see Simpson et al., 2019, Visioni et al., 2020). The reason they are not included in this manuscript is that the purpose of this paper is to provide an overview of the simulations and experimental protocol, and we expect subsequent publications will address detailed changes, including seasonal changes. In particular, detailed calculations of extreme temperature and precipitation changes are also in progress (and will be coming in forthcoming manuscripts). Since this topic is clearly important to you, we include the plots of seasonal precipitation and temperature changes in this response for the time period (2050 - 2069) - (2020-2039) in Figures 1 and 2 below. We will wait for the editor's comments on whether they should be included in the revised manuscript.
My second concern is that I do not quickly see any discussion of how the impact of sulfate particles on the reflectivity of solar radiation is modelled at different wavelengths, and at different times of the day.
We will add some discussion of this specific topic. In particular, Earth System models treat solar radiation in discrete spectral bands, and especially in terms of model output only offer information for the overall shortwave and longwave radiation incoming (both direct and diffuse) and outgoing. Previous discussions over changes in ratio of diffuse/direct radiation can be found for instance in Visioni et al. (2021). In the visible part of the spectrum, the impact of the added sulfate burden has been discussed in Kravitz et al. (2012).
Also, I do not see any discussion of the impact of continually falling particles have on air quality, human beings breathing the air, and on ecology and agriculture.
Similarly to the first comment, we fully agree that all of these impacts need to be evaluated, however evaluating all of the above suggested impacts simply can not fit into one manuscript. The idea with this overview paper is to get the data out to the community, fully explain the rationale behind our modeling choices and offer detailed information in order for more models to be able to reproduce our results, and have subsequent manuscripts evaluating impacts on all aspects of the Earth system. To this point, our simulations include extensive output for all model components that can be used to investigate more in depth some of the points raised by the reviewer. Some of the points raised by the reviewer have been discussed in Zarnetske et al. (2021), which we will add in the revised manuscript.
References:
Kravitz, B., MacMartin, D. G., and Caldeira, K. (2012), Geoengineering: Whiter skies?, Geophys. Res. Lett., 39, L11801, doi:10.1029/2012GL051652.
Simpson, I. R., Tilmes, S., Richter, J. H., Kravitz, B., MacMartin, D. G., Mills, M. J., et al. (2019). The regional hydroclimate response to stratospheric sulfate geoengineering and the role of stratospheric heating. Journal of Geophysical Research: Atmospheres, 124, 12587– 12616. https://doi.org/10.1029/2019JD031093
Visioni, D., MacMartin, D. G., Kravitz, B., Richter, J. H., Tilmes, S., & Mills, M. J. (2020). Seasonally modulated stratospheric aerosol geoengineering alters the climate outcomes. Geophysical Research Letters, 47, e2020GL088337. https://doi.org/10.1029/2020GL088337
Visioni, D., MacMartin, D. G., & Kravitz, B. (2021a). Is turning down the sun a good proxy for stratospheric sulfate geoengineering? Journal of Geophysical Research: Atmospheres, 126, e2020JD033952. https://doi.org/10.1029/2020JD033952
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AC2: 'Reply on AC1', Jadwiga Richter, 24 May 2022
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CC2: 'Reply on CC1', Richard Rosen, 20 May 2022
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RC1: 'Comment on egusphere-2022-125', Anonymous Referee #1, 01 Jun 2022
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/egusphere-2022-125/egusphere-2022-125-RC1-supplement.pdf
- AC4: 'Reply on RC1', Jadwiga Richter, 08 Sep 2022
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RC2: 'Comment on egusphere-2022-125', Anonymous Referee #2, 22 Jul 2022
The manuscript presents a description of a new experiment to explore the effects of Stratospheric Aerosol Injection (SAI) on climate, part of a larger set of experiments (ARISE) to explore different types of solar radiation management. In particular, the manuscript presents an overview of the experimental setup and a very modest assessment of the results using the CESM2(WACCM6) model. In contrast to a previous SAI experiments (GLENS) run with the CESM1(WACCM) model, the new set of experiments uses a control experiment with a much lower radiative forcing scenario (SSP2-4.5) than the previous set (RCP8.5) thus necessitating a lower level of SO2 injection to stabilize near-surface temperature. Many have argued that the high radiative forcing scenarios like RCP8.5 are unrealistic, and the authors also make the argument that assessing SAI for a moderate radiative forcing scenario may prove to be a more applicable to our future.
The manuscript is very well written and a small selection of results are clearly presented. The few results that are presented, the effects of SAI on a number of measures related to near-surface temperature and precipitation, will be quite familiar to researchers in the field. The most intriguing result, the need for an injection of SO2 much more heavily weighted towards the Southern Hemisphere in this experiment with CESM2(WACCM6)-SSP2-4.5 in comparison with the earlier GLENS experiment with CESM1(WACCM)-RCP8.5 is discussed in a number of places but not illustrated with any results. I understand the reasons for the difference is part of on-going research, but a clearer picture of how the GLENS simulations compares with the ARISE simulations would add considerably to the paper and not compromise any on-going work to understand the reasons for the differences. Figures presenting the same information for the GLENS experiment as is given in Figure 2 (zonal mean stratospheric sulphate distribution) and Figure 3 (time series of T0, T1, and T2) would provide the reader with a much more complete idea of the differences. I would urge the authors to provide a bit more of a description of the differences between the GLENS and ARISE simulations.
The model description in Section 2.1 seems quite detailed for a paper that is ostensibly about a new geoengineering experiment, particularly in describing changes between CESM1 and CESM2. I would suggest this section could be shortened somewhat.
Other than these two suggestions I only have a small number of minor corrections that are listed below.
Minor comments:
Line 68 – I think the reference for Tilmes et al. (2020) is missing from the reference list.
Line 127 – Flagging the ‘to’ in ‘trends to not shift’
Lines 163 – 164 – From the text a bit further down it is evident that T0 is used broadly to denote the global mean near-surface air temperature (at line 175) so it should be defined here and not later.-
CC5: 'Reply on RC2', Richard Rosen, 24 Jul 2022
I object to the use of the word "experiments" when refering to computer simulations of anything. The word "experiments" should be reserved for actual physical experiments in the real world, not computer simulations where we have no idea how well the computer models might simulate the real world.
- AC5: 'Reply on RC2', Jadwiga Richter, 08 Sep 2022
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CC5: 'Reply on RC2', Richard Rosen, 24 Jul 2022
Jadwiga Richter et al.
Data sets
ARISE-SAI-1.5 Jadwiga H. Richter, Daniel Visioni, Douglas G. MacMartin, David A. Bailey https://zenodo.org/record/6473775#.YmCAdy-B3qA
CESM2(WACCM6) SSP2-4.5 Simulations Michael Mills, Jadwiga Richter, David A. Bailey, Daniel Visioni https://zenodo.org/record/6473954#.YmCAwy-B3qA
Model code and software
SO2 Injection Controller Algorithm Daniel Visioni, Doug MacMartin, Ben Kravitz https://zenodo.org/record/6471092#.YmFzli-B3qB
Scripts to produce simulations Nan Rosenbloom, Daniel Visioni, Jadwiga H. Richter https://zenodo.org/record/6474201
Jadwiga Richter et al.
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