The God Species_ How the Planet Can Survive the Age of Humans - Mark Lynas [33]
Another candidate on the tipping-point list is the Amazonian rain forest. For years now many scientists have warned that global warming could trigger a collapse of the forest if rising temperatures lead to severe drought in western Brazil. This scenario seems even more of a danger given the recent droughts experienced in Amazonia in both 2005 and 2010, when entire river systems in this normally wet forest dried up for hundreds of kilometers. The problem here is that models don’t concur: Some show a warmer Amazon getting wetter, while the most pessimistic forecasts for Amazon dieback are based on the projections of just one model, the HadCM3 model produced by the U.K. Met Office’s Hadley Centre. However, half of the 19 different models examined by a team of scientists led by Oxford University’s Yadvinder Mahli in 2009 did show a shift toward more seasonal forest, and a quarter showed that the rain forest could dry out sufficiently to collapse into a savannah-type ecosystem instead.38 Keeping global temperatures below 3°C—very likely if our 350 ppm planetary boundary is achieved—should be enough to avoid this transition, but just as important will be respecting the other planetary boundaries on land use and biodiversity loss. The Amazon rain forest today is probably more threatened by deforestation and agriculture than it is by rising temperatures.
If the Amazon rain forest did collapse, huge quantities of carbon would be released in the process, giving a further boost to global warming. But the biggest carbon stores of all lie not in the tropics, but in the subpolar continental regions where frozen permafrost holds enormous carbon stores tens of meters thick in Siberia and other high-latitude land areas. The threat to permafrost stability is possibly global warming’s biggest tipping point, because if this frozen carbon store begins to thaw, vast quantities of both carbon dioxide and methane will be released. According to a 2008 study in the journal BioScience, the carbon locked up in the Northern permafrost zone totals more than 1.5 trillion tonnes, double the entire carbon content of the atmosphere.39 Even if only 10 percent of this permafrost thaws, another 80 ppm of CO2 will have accumulated in the atmosphere by 2100, raising the planet’s temperature by an additional 0.7 degrees40—and making the eventual attainment of the 350 ppm climate change boundary much more difficult.
Scientists have already begun watching with some alarm a recent upward trend in atmospheric methane, some of which may be coming from the Arctic.41 Not all this methane—a greenhouse gas 25 times more potent than CO2—is likely to bubble out of swamps on land; vastly more is contained in subsea sediments in the form of ice-like methane hydrates. If these hydrates melt rapidly as the oceans warm up, then all global warming bets are off—a scenario that has already sparked scary newspaper headlines. So how afraid should we be? Researchers have already reported seeps of methane leaking from the seabed offshore from eastern Siberia and the Norwegian Arctic islands of Svalbard, in both cases possibly in response to warmer ocean waters.42 But the experts are cautious. “Methane sells newspapers, but it’s not the big story,” writes David Archer on the excellent RealClimate blog.43 “CO2 is plenty to be frightened of, while methane is frosting on the cake.”
Work by Archer and colleagues modeling the Earth’s response to climate change suggests that methane hydrate release could add another half-degree or so to the total warming,