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© Copyright 1998, Jim Loy
The people who simulate global weather on computers sometimes encounter a disturbing event. This event is a runaway winter. This begins with an amazingly cold winter. The temperature gets so low that much of the earth is covered by snow. The snow reflects the sun's light back into space. So, the sun's light does not heat the earth very much, even when summer comes around. Subsequent winters become colder and colder, as the snow cover spreads farther and farther into the tropics. Eventually, the entire earth is covered with ice, worse than any ice age.
Runaway winter is very very very rare. So it is not likely to happen anytime soon. But when it happens, it is apparently irreversible. As rare as it is, it should have happened in the past. But if it did, it should still be with us. And it is not. There is no evidence that it has ever happened.
So, something is wrong with the theory. Something happens to prevent runaway winter. People are working on it.
Would a nuclear winter (years long winter caused by a massive thermonuclear war) cause a runaway winter. Maybe. But my impression is that it is not likely.