| 03 Dec 2025
Unexpected land-surface warming following a low-to-moderate forcing hypothetical nuclear war
Abstract. Nuclear conflicts could ignite intense urban fires that inject considerable amounts of black carbon (BC) into the upper atmosphere, with the potential to disrupt global climate. While uncertainties in the total BC injection remain large, relatively few modeling studies and limited model diversity have explored the climatic response to low-to-moderate BC injections, leaving key aspects of their climate impact poorly understood. Here, we investigate the climate response to a set of low-to-moderate forcing scenarios (12 to 24 Tg BC) – roughly one-tenth to one-fifth the strength of the standard high-end cases – using the Canadian Earth System Model version 5. Consistent with previous work, we find prolonged global reductions in surface temperature and precipitation, driven by decreased downwelling shortwave radiation at the surface and increased atmospheric stability. Unexpectedly, however, a transient surface warming develops in the first boreal summer following a boreal-winter injection, linked to reduced net longwave and turbulent fluxes. Precipitation remains suppressed because of enhanced stability. The transient warming is most pronounced for the lowest forcing cases, indicating a nonlinear response across the forcing range. These results underscore the need for broader multi-model assessments and systematic exploration across a wider range of scenarios, given their potential for complex, societally relevant outcomes.
This paper needs major revisions. It reports unusual responses to black carbon injections, but does not diagnose the climate response to explain why. It seems that your model does not loft the BC into the stratosphere nor rain it out, and so it persists somehow in the upper troposphere, heating it and probably evaporating clouds. Is this physically realistic? Please show the vertical distribution of the BC and clouds over the regions that experience surface warming in the first JJA. Without looking at the distribution of clouds, precipitation, and BC, how can we figure out why there is warming. Is it downward emission of longwave from the BC cloud? How is this possible if there are water clouds? The paper calls for future work on that, but this paper needs that analysis. Otherwise, the strange result is not explained.
Why does the paper choose the BC injections that they did? They call for model intercomparison studies, but do not use the same injection scenarios as other studies. It seems they are not aware of Toon et al. (2019) which examined scenarios with the same range as this paper, 5 Tg, 15 Tg, 27 Tg, and 37 Tg. This paper should have used the same ones so they could be compared.
Toon, Owen B., Charles G. Bardeen, Alan Robock, Lili Xia, Hans Kristensen, Matthew McKinzie, R. J. Peterson, Cheryl Harrison, Nicole S. Lovenduski, and Richard P. Turco, 2019: Rapid expansion of nuclear arsenals by Pakistan and India portends regional and global catastrophe. Science Advances, 5, eaay5478, doi:10.1126/sciadv.aay5478.
They also should be aware of Oman et al. (2006). They found summer warming over Africa and Asia after a volcanic eruption, and the explanation was a weaker summer monsoon, which produced fewer clouds. This paper needs to show the distribution and anomalies of clouds and precipitation in the first JJA, and diagnose whether the response they found was due to a dynamic response, or simply an evaporation of the clouds due to upper troposphere heating.
Oman, Luke, Alan Robock, Georgiy L. Stenchikov, and Thorvaldur Thordarson, 2006: High-latitude eruptions cast shadow over the African monsoon and the flow of the Nile. Geophys. Res. Lett., 33, L18711, doi:10.1029/2006GL027665.
The BC lifetime in this simulation is much shorter than in previous work. This has to be explained, with figures showing the BC distribution vertically. What was not washed out immediately should have been transported to the upper stratosphere and lasted for years. Previous work had e-fording lifetimes of 5-7 years. Why is it so short here?
The terminology of “low-moderate” injections, “standard high-end cases,” and “relatively weak” injection needs to be changed. There is no standard case, and all incidents of nuclear war, including 5 Tg injections would be horrific, and would not be low or moderate.
The caption for Fig. 4 says sea ice is shown by orange lines, but I don’t see any.
There are 23 additional comments on the attached annotated manuscript which also should be addressed.
Review by Alan Robock