The fourth tipping point is Greenland's glaciers, which hold enough water to cause sea levels to rise by more than twenty feet. It takes a while for that much ice to melt, of course. Currently, the Intergovernmental Panel on Climate Change projections say it will take on the order of a thou- sand years. Scientists currently don't have a good handle on how such a big hunk of ice melts. For plenty of reasons it could happen much more quickly—recent observations suggest that the melting has not only exceeded what models predict, but has also begun to accelerate. A marked retreat of ice in coastal areas has led to an infusion of ocean water, which is relatively warm and promotes melting.
All this leads Lenton to conclude that the Greenland ice sheets could make a transition to an alternate state in three hundred years, rather than a thousand or more. Such a quick melting of Greenland would have a knock-on effect on the ocean currents that run up the Atlantic, bringing warmth to northern Europe and Scandinavia, the Atlantic thermohaline circulation. A sudden change in this current could plunge much of Europe back into an ice age. Scientists were getting nervous about this possibility a few years ago, until further research suggested that any switch in current is a long way off—perhaps a thousand years off. Lenton argues that an accelerated melting of Greenland would throw more freshwater on the northern Atlantic than these reassuring calculations have taken into account. "The canary in the coal mine is the Arctic losing its summer sea-ice cover," says Lenton. "I am really worried about the Greenland ice sheet. It's already losing mass and shrinking."
If Greenland flipped into a completely ice-free state, it would cause massive rises in sea level—on the order of six or seven meters. Even if this took three hundred years to happen, "it would be an absolute disaster," says Lenton, "a real game changer." At such a rate of sea-level rise, it would be- come more and more difficult to protect coastlines. Low-lying areas would have to be abandoned. That includes cities such as New York, Los Angeles, San Francisco, London, Tokyo, and Hong Kong, not to mention the entire state of Florida and vast swaths of Indochina.
Tipping point number six—the west Antarctic ice sheet—is even scarier. It has enough ice on it to raise sea levels by about eighty meters. The ice is melting, but slowly—most worst-case scenarios give the ice centuries to melt. But there are some niggling doubts about whether the West Antarctic Ice Sheet could calve into the sea more quickly than expected, as the glaciers contract. If that happened, it would push sea levels up by five meters in as short a time as a century. Most experts consider this unlikely, but if it did happen, Lenton thinks the sheet could flip in as little time as three hundred years—three times faster than most models predict.
Water and ice aren't the only worries. The Amazon rain forest, the seventh of Lenton's tipping points, is also in jeopardy. Rain forests are always pretty wet, but they have dry seasons, and those dry seasons turn out to be a limiting factor on the survival of flora and fauna. As loggers reduce the number of trees that produce moisture to feed the gathering rains, the drier the dry seasons get, and the longer they last. Lately dry seasons in the Amazon have gotten more severe and have put a crimp on the survival of many of the trees that form the forest canopy, which is the backbone of the rain-forest ecosystem. As the dry season continues to lengthen, the flora draw more and more water from the soil, which eventually begins to dry out. The trees get stressed and begin to die. There's more fodder on the forest floor for wildfires. This is not hypothetical; it's already begun to happen. We saw this during the estimated twelve thousand wildfires that occurred in the Amazon during the drought of 2010. As the forest loses more and more trees, it loses its ability to feed the weather patterns with warm, moist air.