With rising geopolitical tension and proliferation of regional nuclear powers, global society continues to face the risk of regional nuclear warfare. In a regional nuclear conflict scenario, in addition to local damage and health effects, firestorms triggered at urban centers could inject a great amount of black carbon (BC) into the upper troposphere – lower stratosphere. Using global climate models, past studies have shown that the BC, which is a highly absorbing aerosol, can heat the upper atmosphere and cool the Earth’s surface for years by increasing the absorption of solar radiation aloft. The surface cooling may lead to shortage in global food supply and famine, slowing down societal recovery.

While uncertainties in the total amount of BC injection remain large, relatively few modeling studies have explored the climate response to relatively low-to-moderate BC injection. In this study, the Canadian Earth System Model version 5 (CanESM5) is used to study the climate response to a set of low-to-moderate forcing scenarios (12 to 24 Tg BC; still large compared to PyroCbs in recent major wildfire events) – roughly one-tenth to one-fifth the strength of the typically assumed cases. Consistent with past studies, prolonged global reductions in surface temperature and precipitation are found, driven by decreased shortwave radiation at the surface and increased atmospheric stability. However, an unexpected transient surface warming develops midway through the first year, linked to reduced net longwave and turbulent fluxes. During the transient warming period, precipitation remains suppressed because of enhanced stability. In addition to this set of results, I will briefly discuss ongoing work to investigate the impact of the injection on stratospheric circulation in the simulations.