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That's an important point. I was only thinking in terms of the average temperature, but you may be right that the tropical temperatures would remain above freezing. That might provide a safe haven for life even in times of greenhouse gas loss. So I guess my question is, which overall climate is a typical "ice age" closer to-- what we have now, or what we'd have in the absence of greenhouse gases?
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The earth is an average of 14 degrees above zero. A drastic reduction in CO2 would cool the earth by about 3 degrees. However, expanding ice sheets would reduce the earth's temperature further by increasing albedo. Just by how much I don't know. Reductions in albedo due to decreased cloud cover also have to be figured in. But with current levels of solar insolation the earth should not snowball up.
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Then the entire issue of plate tectonics, raised in this thread, may be a red herring-- if tectonics are not the source of our greenhouse gases. If they are, one gets right back to the stability issue that I'm talking about.
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Well there are two kinds of volcanoes on earth. Ones that result from plate tentonics, and ones that don't. The ones found at plate margins release lots of water vapour because the magma has mixed with wet seafloor. Carbon from the seafloor is also released. These volcanos aren't the source of our carbon, but recycle carbon from the seafloor. Volcanoes that aren't found at plate margins, such as the Hawaii ones don't have magma that is mixed with subducted seafloor. They release less gas and tend to be less explody. But they still release some gas, as can be clearly seen if you watch a documentary on Hawaiian eruptions.