Faster than expected

Sea levels and related surface temperatures are rising faster than predicted. In 2001, the IPCC predicted an annual rise of less than 2mm in the next decade. In 2007, the IPCC reported that between 1993 and 2006, sea levels actually rose by 3.3mm a year.

 

An annual rise of 3.3mm is right at the upper limit of the IPCC's predictions and highlights how difficult it is to predict the extent of human impact. If sea levels continue to rise at the upper end of the predicted range we will experience an 88cm rise in sea levels by the end of the century.

 

Tipping point

 

Some scientists and insurance companies acknowledge the possibility of reaching a 'tipping point'. This term describes a situation where surface temperature and sea-level dramatically rise over a very short period of time.

 

A tipping point would occur if the Earth's surface temperature reached a point where it catalysed many major events simultaneously. These events would then drastically exacerbate global warming in a very short amount of time.

Examples include:

 

• The releasing of large quantities of stored greenhouse gases from thawing permafrost.
• The disintegration of the Greenland and West Antarctica ice sheets raising sea levels and decreasing the amount of solar energy being reflected back into the atmosphere.

Runaway collapse

 

This last point is of greatest concern to some eminent scientists. As James Hansen, head of NASA's Goddard Institute for Space Studies in New York, commented in New Scientist magazine (25 July 2007):  "So why do I think a sea level rise of metres would be a near certainty if greenhouse gas emissions keep increasing? Because while the growth of great ice sheets takes millennia, the disintegration of ice sheets is a wet process that can proceed rapidly"

 

He concludes that if ice sheet melting were to add 1 centimetre to sea level for the decade 2005 - 2015, and if this rate of melting doubled each decade until the West Antarctic ice sheet were depleted, there would be a sea level rise of over 5 metres by 2095.