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Originally Posted by Ronald Brak
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.
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A feedback I would worry about is the dryness of the atmosphere. CO2 may not warm by more than 3 C at the moment, but H2O certainly warms by much more, perhaps 30 C, I don't recall exactly. But without H2O, the Earth might completely freeze, it seems to me, so if cooler means drier, you could have a runaway loss of the greenhouse effect. Clearly, that can't have happened much to our planet, but is that because the CO2 is maintained, or because it wouldn't happen even without any CO2?
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The ones found at plate margins release lots of water vapour because the magma has mixed with wet seafloor.
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That doesn't matter because I presume that is just a temporary addition to the water cycle. We care more about the creation of the oceans, and then the CO2 balance after that.