Over the past decade scientists thought they had figured out how to protect humanity from the worst dangers of climate change. Keeping planetary warming below two degrees Celsius (3.6 degrees Fahrenheit) would, it was thought, avoid such perils as catastrophic sea-level rise and searing droughts. Staying below two degrees C would require limiting the level of heat-trapping carbon dioxide in the atmosphere to 450 parts per million (ppm), up from today’s 395 ppm and the preindustrial era’s 280 ppm.
Now it appears that the assessment was too optimistic. The latest data from across the globe show that the planet is changing faster than expected. More sea ice around the Arctic Ocean is disappearing than had been forecast. Regions of permafrost across Alaska and Siberia are spewing out more methane, the potent greenhouse gas, than models had predicted. Ice shelves in West Antarctica are breaking up more quickly than once thought possible, and the glaciers they held back on adjacent land are sliding faster into the sea. Extreme weather events, such as floods and the heat wave that gripped much of the U.S. in the summer of 2012 are on the rise, too. The conclusion? “As scientists, we cannot say that if we stay below two degrees of warming everything will be fine,” says Stefan Rahmstorf, a professor of physics of the oceans at the University of Potsdam in Germany.
The X factors that may be pushing the earth into an era of rapid climate change are long-hypothesized feedback loops that may be starting to kick in. Less sea ice, for example, allows the sun to warm the ocean water more, which melts even more sea ice. Greater permafrost melting puts more CO2and methane into the atmosphere, which in turn causes further permafrost melting, and so on.
The potential for faster feedbacks has turned some scientists into vocal Cassandras. Those experts are saying that even if nations do suddenly get serious about reducing greenhouse gas emissions enough to stay under the 450-ppm limit, which seems increasingly unlikely, that could be too little, too late. Unless the world slashes CO2 levels back to 350 ppm, “we will have started a process that is out of humanity’s control,” warns James E. Hansen, director of the nasa Goddard Institute for Space Studies. Sea levels might climb as much as five meters this century, he says. That would submerge coastal cities from Miami to Bangkok. Meanwhile increased heat and drought could bring massive famines. “The consequences are almost unthinkable,” Hansen continues. We could be on the verge of a rapid, irreversible leap to a much warmer world.
Read on at Scientific American.