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knb comments on Great rationality posts in the OB archives - Less Wrong
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Ice ages are not cyclical, but glaciations are. We are currently in an ice age, but in an interglacial period within that ice age. We may be "overdue" for the next glaciation, but it is also possible that the last glaciation of this ice age has already occurred, and we are heading back into a warm earth era typical of the overwhelming majority of the planet's history.
Thanks for pointing out this terminological issue. I'm going to keep calling glaciations "ice ages," just because that's how they were labeled to the public in the 1970s, viz. the title of Emiliani's 1978 paper The cause of the ice ages, or, of course "The Weather Conspiracy: The Coming of the New Ice Age." :)
The paper
provides evidence that periods of glaciation begin when northern hemisphere insolation (which varies due to changes in the precession, obliquity and eccentricity of Earth's orbit) falls below a "trigger" level that depends on the amount of CO2 in the atmosphere. Archer & Ganopolski suggest that we're currently approaching a solar minimum, but we've already released enough CO2 into the atmosphere to avoid a glaciation in the next few thousand years. (If we burn all available fossil fuels, "The model predicts the end of the glacial cycles, with stability of the interglacial for at least the next half million years".)