Has global warming slowed right down?

Posted by Kate Taylor

Global warming could be a lot less extreme than feared, according to a new study which finds that worldwide temperatures have levelled off.

"These results are truly sensational," says Dr Caroline Leck of Stockholm University, who wasn't involved in the research. "If confirmed by other studies, this could have far-reaching impacts on efforts to achieve the political targets for climate."

According to the Norwegian team, while Earth's mean surface temperature climbed sharply through the 1990s, this increase has levelled off nearly completely at its 2000 level. Ocean warming also appears to have stabilised somewhat, despite the fact that CO2 and other emissions are still on the rise.

The Intergovernmental Panel on Climate Change (IPCC) predicts that the result of doubled atmospheric CO2 levels would probably be between 2°C and 4.5°C. The new study, however, gives a figure of 1.9°C as the most likely level of warming.

"In our project we have worked on finding out the overall effect of all known feedback mechanisms," says project manager Terje Berntsen of the University of Oslo.

"We used a method that enables us to view the entire earth as one giant 'laboratory' where humankind has been conducting a collective experiment through our emissions of greenhouse gases and particulates, deforestation, and other activities that affect climate."

The team entered all the factors contributing to human-induced climate forcings since 1750 into their model, along with fluctuations in climate caused by natural factors such as volcanic eruptions and solar activity. They also entered measurements of temperatures taken in the air, on ground, and in the oceans.

When they applied their model and statistics to analyse temperature readings from the air and ocean for the period ending in 2000, they found that climate sensitivity to a doubling of atmospheric CO2 concentration would most likely be 3.7°C - somewhat higher than the IPCC prognosis.

But the researchers were surprised when they entered temperatures and other data from 2000-2010, finding that climate sensitivity was greatly reduced to just 1.9°C.

"The Earth's mean temperature rose sharply during the 1990s. This may have caused us to overestimate climate sensitivity," says Berntsen.

"We are most likely witnessing natural fluctuations in the climate system - changes that can occur over several decades - and which are coming on top of a long-term warming. The natural changes resulted in a rapid global temperature rise in the 1990s, whereas the natural variations between 2000 and 2010 may have resulted in the levelling off we are observing now."

Berntsen emphasises that the findings mustn't lead to complacency. But, he says, they indicate that it may be more feasible to achieve global climate targets than previously thought.