Climate scientists funded by the US government have announced new research in which they have established that the various doomsday global warming scenarios are in fact extremely unlikely to occur, and that the scenarios considered likeliest - and used for planning by the world's governments - are overly pessimistic.

The new study improves upon previous results by including data from the remote past, rather than only examining records from recent times.

"Many previous climate sensitivity studies have looked at the past only from 1850 through today, and not fully integrated paleoclimate date, especially on a global scale," says Andreas Schmittner, professor at the College of Earth, Ocean, and Atmospheric Sciences at Oregon State uni.

"When you reconstruct sea and land surface temperatures from the peak of the last Ice Age 21,000 years ago – which is referred to as the Last Glacial Maximum – and compare it with climate model simulations of that period, you get a much different picture.

"If these paleoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought," Schmittner adds.

The baseline assumption of climate science at the moment is that given a doubling of atmospheric CO 2 compared to pre-industrial levels the most probable result is that the Earth would see a surface temperature rise average of 3°C - and that there would be a significant chance of much bigger, perhaps fatal rises.

Schmittner and his colleagues' analysis says that the planet's climate simply can't be this sensitive to CO 2 changes, however, or much more extreme events should have occurred at certain points in the past - and they did not. For instance, if the climate were sensitive enough that doubled CO 2 could mean catastrophic warming, the low carbon levels seen 21,000 years ago should have resulted in an equally lifeless iceball planet.

"Clearly, that didn't happen," Schmittner says. "Though the Earth then was covered by much more ice and snow than it is today, the ice sheets didn't extend beyond latitudes of about 40 degrees, and the tropics and subtropics were largely ice-free – except at high altitudes. These high-sensitivity models overestimate cooling."

According to the new improved analysis, the most probable result as and when double CO 2 occurs is actually a rise of just 2.3°C - only just above the 2°C limit which international climate efforts are seeking to stay within. Plainly there's no great need to fear a rise above 450 parts per million (ppm) CO 2 , as people currently do - in fact there's no likely prospect of getting near a 2°C temperature rise for a century or more at present rates of CO 2 increase (rising about about 2 ppm/year at the moment from a level of 390-odd). And Schmittner and his colleagues' results show a much tighter grouping of possible futures, too, so the scope for way-out doomsday scenarios is hugely reduced.

The Australian quotes Schmittner as saying: "Now these very large changes (predicted for the coming decades) can be ruled out, and we have some room to breathe and time to figure out solutions to the problem."

The new study is published in Science. The research was funded by the US National Science Foundation. ®