The probability of Earth’s climate passing a “tipping point” that could result in large impacts within the next two centuries is greater than 50 percent, according to research published in Proceedings of the National Academy of Sciences
Elmar Kriegler of the Potsdam Institute for Climate Impact Research and colleagues asked 43 climate experts to estimate the likelihood of “major changes” to five components of the climate system — the North Atlantic Ocean circulation, the Greenland and Western Antarctic ice sheets, the Amazon rainforest, and El Niño — under different warming scenarios. Based on this poll, the authors calculate a 16 percent chance that climate change will trigger at least one of the events for a 2-4°C rise in temperature, and a 56 percent change for a 4°C or higher rise.
Of the events, the experts pegged melting of the Greenland ice sheet as the most likely to occur (60 percent for a 4-8°C rise in temperature). An ice-free Greenland would raise sea levels by more than 23 feet (7 meters).
The likelihood of a permanent el Niño — which would trigger changes in rainfall and temperature around the global — or a catastrophic die-off (more than 50 percent) of the Amazon rainforest was estimated at about 50 percent under a high warming scenario.
Less likely, according to the experts, is a shutdown of the North Atlantic Ocean circulation which could trigger a deep freeze in Europe and Eastern North America or melting of the Western Antarctic ice sheet.
Kriegler et al. Imprecise probability assessment of tipping points in the climate system. PNAS Early Edition Mar 16, 2009