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There are now multiple lines of evidence pointing to ongoing global warming, some of which show that we are altering the characteristics of the atmosphere in unanticipated ways. Air-pressure distribution is changing around the world, with rises in the subtropics and falls over the poles.4 The stratosphere has cooled as more heat is trapped by the troposphere underneath,5 while the boundary between these two higher and lower atmospheric layers has itself increased in height.6 Even the position of the tropical zones has begun to shift as the atmosphere circulates differently in response to rising heat.7

A more energetic atmosphere also means more extreme rainfall events as the levels of water vapor in a warmer atmosphere increase: This too has been observed.8 The catastrophic flooding events that hit Pakistan in August 2010 and Australia in January 2011 are exactly the kind of hydrological disasters that will be striking with deadly effect more often in a warmer world. While people in poorer countries are most vulnerable to the effects of floods, any country can be hit at any time: In the English Lake District the heavy rainfall event of November 18–20, 2009, had no precedent: Rainfall totals outstripped previous all-time records in over 150 years of measurements.9

Perhaps the clearest indicator of current danger—ground zero for global warming—is the rapid thaw of the Arctic. Few experts argue any more about whether the sea ice sheet covering the North Pole will melt completely, merely when. In recent years the Arctic ice cap has entered what Mark Serreze, a climatologist at the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado, calls a “death spiral.”10 The extent of Arctic ice is plummeting, and what remains is thinner and more vulnerable to melt than before. In terms of volume, less than half the ice cap of the pre-1980 era remains; more than 40 percent of the volume of multiyear ice (the thicker stuff that lasts through the summer) has disappeared since only 2005.11 Even the wintertime ice coverage is in decline: In January 2011 the NSIDC announced that the sea ice extent for that month was the lowest in the satellite record, with the Labrador Sea and much of western Greenland’s coast remaining completely unfrozen.12 The year of what I call A-Day, the late summer day at some time in the future when not a fleck of the North Polar floating ice remains, has been suggested by one modeling study as likely to arrive in 2037, but if recent years are anything to go by this could shift closer by as much as a decade.13

A-Day will be a momentous date for the Earth, for it will be the first time in at least five thousand years that the Arctic Ocean has been without any summertime sea ice.14 This will in turn alter the heat balance of the planet and the circulation of the atmosphere: Without its shiny cap of frigid ice, the Arctic Ocean can absorb a lot more solar heat in summer and release much more in winter, changing storm tracks and weather patterns. The resulting prognosis is not for straightforward warming everywhere: One model projection by scientists working in Germany, published in November 2010, suggested that disappearing sea ice in the Arctic Ocean north of Scandinavia and Siberia could in fact drive colder winters in Europe. The researchers proposed that warmer unfrozen waters in the north could drive a change in wind patterns that allows cold easterly winds to sweep down into Europe and Russia, and that this may have helped cause the colder winters of 2005–6, 2009–10, and 2010–11 in both Europe and eastern North America, which have seen snowstorms and frosts even as the Arctic basked in unprecedented winter warmth. “Our results imply that several recent severe winters do not conflict [with] the global warming picture but rather supplement it,” they concluded in the Journal of Geophysical Research.15

The disappearance of the Arctic ice will eliminate an entire marine ecosystem. Currently algae growing