Important Components of the Earth System Might Be Reaching Tipping Points
07.02.2008 - Other
Where the Intergovernmental Panel on Climate Change (IPCC) could lead non specialists to think of climate change as a smooth transition, scientists now warn that many components of the Earth system could display non-linear behaviour and transitions under human (anthropogenic) climate forcing.
In a comprehensive study published in the Proceedings of the National Academy of Sciences, scientists identify nine areas that risk passing critical thresholds or "tipping points", beyond which they will not recover. Amongst these areas, Arctic sea ice and the Greenland ice sheet are regarded as being most immediately in peril.
Arctic sea ice
Scientists calculate Arctic sea ice will go into irreversible decline once temperatures rise between 0.5C and 2C above those of the beginning of the century, a threshold that may already have been crossed. The area coverage of both summer and winter Arctic sea ice are declining, summer sea ice more markedly. Additionally, the ice has thinned significantly over a large area. Analysis have also showed that positive ice-albedo feedback dominates over external forcing in causing the thinning and shrinkage since around 1988. When ice melts, it is replaced by water, the dark colour of which absorbs solar rays instead of reflecting them back, which leads to further ice melting. This type of positive feedback loop has got a greater influence on ice cover than the mere fact that global temperatures are increasing.
Greenland ice sheet
There is already a 50% chance that the Greenland ice sheet will soon begin melting unstoppably, although it could take hundreds of years to melt completely. The timescale for the ice sheet to melt is at least 300 years and is often given as roughly 1000 years. However, the Greenland ice sheet contains what would be equivalent to 7m of global sea-level rise, an amount which dwarfs other contributors such as glaciers.
West Antarctic ice sheet (WAIS)
Additionally, the West Antarctic Ice Sheet (WAIS) is thought to be less vulnerable to warming than the Greenland Ice Sheet but a threshold could still be reached this century. Measurements of the western Antarctic ice sheet show that the balance between snowfall and melting has shifted in favour of melting.
