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The scientists who published these calculations always added a caveat. In the Antarctic record, atmospheric CO2 levels over the past 750,000 years had cycled between about 180 and 280 parts per million. The level by 2012 had climbed almost to 400 and kept climbing. (The other main greenhouse gas, methane, was soaring even farther above any level seen in the long ice record.) Greenhouse warming and other human influences seemed strong enough to overwhelm any natural trend. One scientist, paleoclimatologist William Ruddiman, even argued that the rise of human agriculture had already produced enough greenhouse gases to counteract the gradual cooling that should have come during the past several thousand years; every previous cycle had begun a steady cooling soon after its peak, rather than leveling off as ours had done.(53a) As emissions climbed exponentially, we might not only cancel the next ice age but launch our planet into an altogether new climate regime.

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Our current situation was altogether different. The warming was not started by a small shift of sunlight, as in previous epochs. Our addition of gases to the atmosphere was initiating the process, with the temperature rise lagging a few decades behind the rise of gas levels. Emissions were surging far more swiftly than anything in the Pleistocene record. Already by the 1980s the levels of greenhouse gases had climbed far higher than anything seen for many millions of years.

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