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Second on the tipping points list came the melting of Greenland’s vast ice sheet. Thick enough to raise the global oceans by seven meters if it melted entirely, the stability of Greenland matters hugely to faraway nations like Bangladesh and the Maldives, which face partial or total inundation (in the case of the latter) if it melts because of global warming. So where does the tipping point lie that might doom the Greenland ice cap to eventual destruction? Between just 1 and 2 degrees above today’s temperatures, the experts concluded, meaning that a 350 ppm trajectory is once again the least we will need to achieve to protect it. Here too the process could become self-reinforcing. The center of Greenland is extremely cold because the thickness of the ice sheet means that it extends into high altitude: Greenland’s Summit Camp is located 3,200 meters above sea level. But as global warming nibbles away at the edges of this enormous ice body, more of it comes into the lower altitude zone, exposing the ice to higher temperatures and increasing the melt rate. Although eliminating a whole continent’s worth of ice will take time, the process could be completed in as little as three centuries, dramatically changing the coastal geography of the planet. Once again, this is a tipping point that humanity would be wise not to trigger.

Greenland is not the only vulnerable polar ice sheet, of course. Third on the list came the West Antarctic Ice Sheet, again of serious concern because—like Greenland—its loss could trigger multi-meter rates of sea-level rise. The West Antarctic also could be subject to a positive feedback process once a serious melt got under way, not just because of the change in altitude but because most of the ice sheet is actually grounded well below today’s sea level. As warming waters penetrate underneath the ice mass they could trigger a collapse that would be unstoppable, and would eventually raise global sea levels by another 5 meters. Here we may be on slightly safer ground, as the experts conclude that a global warming of 3–5°C will likely be necessary to lead to complete collapse. So the 350 ppm boundary would appear to be well within the safety margin according to the models.

As with the Arctic sea ice, however, the real world may prove the models of Greenland and the West Antarctic to be overly conservative. The most recent satellite data from the GRACE (Gravity Recovery and Climate Experiment) mission shows a doubling in ice mass lost from both Greenland and Antarctica over the last decade32—despite a thickening of Greenland’s higher interior where warmer winds have increased snowfall rates. Until recently the massive East Antarctic ice sheet was probably stable, but it too began losing ice in coastal areas after about 2006.33 In total the Earth’s great ice sheets are now shedding a few hundred billion tonnes of ice annually, and sea levels are rising by slightly more than 3 mm per year as a result—nearly double the rate for most of the twentieth century.34 A rise in sea levels by 2100 of somewhere between 60 cm and 1.6 meters is now in the cards,35 substantially more than was suggested just a few years ago by the IPCC.36

A more familiar tipping point was examined next, one that has even been made into a dramatic Hollywood film. In The Day After Tomorrow, a sudden ice age is seen flooding and then freezing New York (why is it always New York?) after global warming destabilizes the circulation of the Atlantic Ocean. Although the flash-freezing depicted in the movie is thermodynamically impossible, the scenario of a collapsing Atlantic current is not complete science fiction. All the models examined by the expert group led by Tim Lenton showed a tipping point in the North Atlantic where warmer, fresher waters could shut down the circulation pattern that brings comparatively balmy temperatures to the eastern U.S. and high-latitude Western Europe. This shutdown would not trigger a new ice age, but temperatures in these regions could fall for