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.

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.

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. 

1. 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.
2. 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)

1. 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

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