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but only over several thousand years, and only if the released methane is not dissolved or oxidized first in the ocean before it has time to escape into the atmosphere.44 This is a “slow tipping point,” Archer concludes: It takes a long time for warming to penetrate the oceans, even longer for this to melt and release hydrates, and longer still for this methane to warm the atmosphere and the oceans further in a positive feedback loop. Happily, this is a tipping point we have still not crossed—“We have not yet activated strong climate feedbacks from permafrost and CH4 [methane] hydrates,” reported a team of scientists in 2009.45 In the case of methane hydrates, respecting the climate boundary is not necessarily about protecting ourselves or even our children, but the stability of the Earth system over the very long term—for this tipping point, while slow to activate, would be essentially irreversible once crossed.

350: PAST EVIDENCE

If current observations of accelerating climate change and worries about tipping points in the future make two very good reasons why 350 ppm is the right place for a climate change planetary boundary, even stronger evidence comes from the Earth’s more distant climatic past. Climate models projections such as those published by the IPCC tend to project nice smooth—albeit upward-pointing—curves of likely future temperature trends. But a glance back in time, courtesy of ice-core records drilled in Greenland and Antarctica, shows that gentle, slow changes are far from being the norm in the Earth’s past. Instead, these records of past climate—which now reach back almost a million years—show climatic swings of extraordinary and terrifying abruptness. One extremely sudden warming took place in Greenland 11,700 years ago; it involved a temperature rise of 10 degrees Celsius within just three years.46 Rapid shifts are observed elsewhere too: 12,679 years ago, according to sediments recovered from a lake in western Germany, the European climate saw a sudden transition to more stormy conditions between one year and the next.47 The lesson is clear. Abrupt climate change is not the exception in the past, it is the norm. As the veteran oceanographer Wally Broecker says: “The climate is an angry beast, and we are poking it with a stick.”

Although current CO2 levels are higher than they have been for a million years, if we look even further back into the geological past there are episodes when both carbon dioxide and temperatures were far above where they are now. But rather than suggesting we have nothing to worry about, they further strengthen the evidence for counting 350 ppm as the crucial planetary boundary. For example, during the Pliocene epoch, about 3 million years ago, sea levels were 25 meters higher than today because the major ice sheets were much smaller than now due to a warmer climate. The CO2 concentration then? About 360 ppm—a line we crossed in 1995.48

The Earth was completely ice-free—and sea levels 80 meters or more higher—until about 33 million years ago, early in the geological epoch called the Oligocene. After having been at 1000 ppm or higher throughout the Cretaceous, Eocene, and Paleocene, this was the moment when CO2 levels dropped past a crucial threshold allowing continental-scale ice sheets to form on Antarctica for the first time in perhaps a hundred million years.49 This CO2 level was 750 ppm, a level expected to be crossed again in about 2075 if carbon emissions continue to rise unabated. For the following 31 million years, only Antarctica held substantial ice sheets—until, late in the Pliocene, the more recent ice-age cycles began. There was another CO2 threshold at play here, one that allowed Northern Hemisphere ice sheets (such as the current one on Greenland) to form for the first time. That level was 280 ppm, which we crossed right at the start of the Anthropocene at the turn of the nineteenth century. Were Greenland to be ice-free at the moment, in other words, CO2 levels are already too high for an ice sheet to form. Once again, 350 ppm seems to be the minimum necessary