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.