might lower Earth’s average surface temperature by 7°C: a decade-long mini ice age caused by a U.S.-Russia nuclear war.
This is colder than the 5°C cooling we endured 20,000 years ago during the last ice age. The good news is that, according to state-of-the-art climate models by Alan Robock at Rutgers University, a nuclear mini ice age would be rather brief, with about half of the cooling gone after a decade. The bad news is that this more than long enough for most people on Earth to starve to death if farming collapses. Robock’s all-out-war scenario shows cooling by about 20°C (36°F) in much of the core farming regions of the U.S., Europe, Russia and China (by 35°C in parts of Russia) for the first two summers.
The science behind nuclear climate change is rather simple. Smoke from small fires doesn’t rise as high as the highest rain clouds, so rain washes the smoke away before too long. In contrast, massive firestorms from burning nuked cities can rise into the upper stratosphere, many times higher than commercial jet planes fly. There are no clouds that high and for this reason, the firestorm smoke never gets rained out. Moreover, this smoke absorbs sunlight and heats up, allowing it to get lofted to even higher altitudes where it might stay for approximately a decade, soon spreading around the globe to cover both the U.S. and Russia even if only one of the two got nuked. Since much of the solar heat absorbed by the smoke gets radiated back into space instead of warming the ground, nuclear winter ensues if there’s enough smoke.