Image courtesy of the U.S. Global Change Research Program

An international group of climate experts have identified nine areas that are in the greatest danger of passing critical threshholds or "tipping points" beyond which they will not recover due to climate change. Three areas that are at the top of the list are the Arctic sea ice, the Greenland ice sheet and the dieback of the Amazon rainforests. Also mentioned were the boreal forests, El Nino, the western Antarctica Ice Sheet and the African and Indian monsoons. The study appears in the Proceedings of the National Academy of Sciences.

Although the scientists cannot be sure precisely when each region will reach the point of no return, their assessment warns it may already be too late to save Arctic sea ice and the Greenland ice sheet, which they regard as the most immediately in peril. By some estimates, there will not be any sea ice in the summer months within 25 years, according to the Guardian Unlimited article.

"There's a perception that global warming is something that will happen smoothly into the future, but some of these ecosystems go into an abrupt decline when warming reaches a certain threshold," said Tim Lenton, an environmental scientist at the University of East Anglia and lead author of the study.

"If we know when the different tipping points are, we can use them to inform targets to limit global warming. It gives us something to aim for," he added.

Dr. Klaus Keller, an assistant professor of Geosciences at Penn State spoke about climate thresholds at the "Focus the Nation" teach-in that I attended at Penn State last Thursday. Dr. Keller is also a contributing author to the fourth assessment of the IPCC report. Dr. Keller co-authored the chapter titled "Assessing Key Vulnerabilities and The Risk from Climate Change".

In Dr. Keller's short presentation he talked about examples of potent climate threshold such as...

--The Greenland Ice sheet melting (as mentioned in the article)
--Coral bleaching
--El Nino
--The possible shutdown of the North Atlantic Meridional Overturning Circulation (MOC) which is a critical ocean circulation that transfers heat from the lower latitudes to the higher latitudes and has a large impact on the climate of the earth. But, according to Dr. Keller, the IPCC says that it is unlikely (1-10% chance) that the MOC will undergo a large abrubt transition in the 21st century.