If a nuclear war happened, firestorms induced by nuclear detonations in urban areas could inject black carbon (BC) into the upper atmosphere, disrupting global climate. We simulated the climate impacts of relatively limited-forcing scenarios using an Earth system model. In all scenarios, BC blocks sunlight, causes global cooling, and decreases precipitation for more than a decade. However, a drastic global warming emerges midway through the first year and lasts until the year's end.