25 November 2011 Last updated at 10:48 ET

CO2 climate sensitivity 'overestimated'

By Jennifer Carpenter Science reporter, BBC News

Global temperatures could be less sensitive to changing atmospheric carbon dioxide (CO2) levels than previously thought, a study suggests.

The researchers said people should still expect to see "drastic changes" in climate worldwide, but that the risk was a little less imminent.

The results are published in Science.

Previous climate models have tended to used meteorological measurements from the past 150 years to estimate the climate's sensitivity to rising CO2.

From these models, scientists find it difficult to narrow their projections down to a single figure with any certainty, and instead project a range of temperatures that they expect, given a doubling of atmospheric CO2 from pre-industrial levels.

The new analysis, which incorporates palaeoclimate data into existing models, attempts to project future temperatures with a little more certainty.

Lead author Andreas Schmittner from Oregon State University, US, explained that by looking at surface temperatures during the most recent ice age - 21,000 years ago - when humans were having no impact on global temperatures, he, and his colleagues show that this period was not as cold as previous estimates suggest.

"This implies that the effect of CO2 on climate is less than previously thought," he explained.

By incorporating this newly discovered "climate insensitivity" into their models, the international team was able to reduce uncertainty in its future climate projections.

The new models predict that given a doubling in CO2 levels from pre-industrial levels, the Earth's surface temperatures will rise by 1.7C to 2.6C (3.1F to 4.7F).

That is a much tighter range than the one produced by the Intergovernmental Panel on Climate Change's (IPCC) 2007 report, which suggested a rise of between 2.0C to 4.5C.

The new analysis also reduces the expected rise in average surface temperatures to just over 2C, from 3C.

The authors stress the results do not mean threat from human-induced climate change should be treated any less seriously, explained palaeoclimatologist Antoni Rosell-Mele from the Autonomous University of Barcelona, who is a member of the team that came up with the new estimates.

But it does mean that to induce large-scale warming of the planet, leading to widespread catastrophic consequences, we would have to increase CO2 more than we are going to do in the near future, he said.

"But we don't want that to happen at any time, right?"

"At least, given that no one is doing very much around the planet [about] mitigating CO2 emissions, we have a bit more time," he remarked.

Whether these results mean that the global temperatures will be less responsive to falling CO2 is unclear. "I don't think we know that, to be honest," remarked Dr Rosell-Mele.

Gabriele Hegerl, from the University of Edinburgh, is cautious about the result in her perspective piece published in the same issue of Science.

She says that this is just one particular climate model, and "future work with a range of models would serve to strengthen the result".

Climatologist Andrey Ganopolski, from Potsdam Institute for Climate Impact Research, Germany, went further and said that he would not make such a strong conclusion based on this data.

"The results of this paper are the result of the analysis of [a] cold climate during the glacial maximum (the most recent ice age)," he told BBC News.

"There is evidence the relationship between CO2 and surface temperatures is likely to be different [during] very cold periods than warmer."

Scientists, he said, would therefore prefer to analyse periods of the Earth's history that are much warmer than now when making their projections about future temperatures.

However, although good data exists for the last million years, temperatures during this time have been either similar to present, or colder.

"One should be very careful about using cold climates to [construct] the future," he added.