BBC News Science & Environment

Climate sensitivity to CO2 probed

By Jennifer Carpenter Science reporter, BBC News

November 25, 2011

http://www.bbc.co.uk/news/science-environment-15858603

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 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 last 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 1.7 to 2.6 degrees C.

That is a much tighter range than suggested by the Intergovernmental Panel on Climate Change (IPCC)'s 2007 report, which suggested a rise of between 2 to 4.5 degrees C.

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

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.