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A new study suggests that the rate of global warming from the doubling of atmospheric carbon dioxide may be less than the most extreme  estimates, and also less severe than that projected by the IPCC in 2007.

The basic greenhouse effect will raise the earth’s temperature by about 1.3 deg C. Anything larger requires putative feedback mechanisms – the chief one involving water vapour – to amplify the basic greenhouse response. The amount of warming caused by doubling the amount of carbon dioxide in the atmosphere (over pre-industrial levels) is called ‘climate sensitivity.’ In 2007, the IPCC said the climate sensitivity could be between 2 and 4.5 deg C. The mean increase in the IPCC estimates was 3.0 degrees, which was used in most climate model studies.

In a paper that has been talked about for weeks before its publication, Andreas Schmittner of Oregon State University, Corvallis, and colleagues try to obtain a better estimate for climate sensitivity by considering the Last Glacial Maximum when the last ice age was at its height.

“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,” said Schmittner. “When you reconstruct sea and land surface temperatures from the peak of the last Ice Age 21,000 years ago and compare it with climate model simulations of that period, you get a much different picture.”

Some previous studies have claimed that climate sensitivity would be much more severe – 10 deg C or higher – although these projections come with an acknowledged low probability (such extreme possibilities were seized upon by those who look at the future in terms of ‘risk.’) However studies based on data going back only to 1850 are affected by large uncertainties.

Using previously published data they compiled a global map of surface temperatures showing that the earth was, on average, 2.2 deg C cooler than today. Ice core data from the time tell us that greenhouse gas levels in the atmosphere at that time were much lower than they are today.

The researchers used the greenhouse gas concentrations during the last Glacial Maximum in a climate model with climate sensitivity as a free parameter, and adjusted it until they got the best fit to global temperature patterns. The best fit yielded a climate sensitivity of 2.4 deg C.

The researchers found that high sensitivity climate models – more than 6 deg C – suggested that the low levels of atmospheric carbon dioxide during the Last Glacial Maximum would result in a runaway effect that would have left the Earth completely ice-covered.

“Clearly, that didn’t happen,” Schmittner said. “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.”

On the other hand, models with low climate sensitivity – less than 1.3 deg C – underestimated the cooling almost everywhere at the Last Glacial Maximum, the researchers say. The closest match, with a much lower degree of uncertainty than most other studies, suggests climate sensitivity is about 2.4 degrees.

This means that projected warming this century will be at the lower end of the IPCC’s estimates, about 2.4 deg C.

This is not however a rise that can be treated as negligible. Uncertainty levels may have been underestimated because the model simulations did not take into account uncertainties arising from how cloud changes reflect sunlight, Schmittner said.

When we first looked at the paleoclimatic data, I was struck by the small cooling of the ocean,” Schmittner said. “On average, the ocean was only about 2 deg C cooler than it is today, yet the planet was completely different – huge ice sheets over North America and northern Europe, more sea ice and snow, different vegetation, lower sea levels and more dust in the air. “It shows that even very small changes in the ocean’s surface temperature can have an enormous impact elsewhere, particularly over land areas at mid- to high-latitudes,” he added.

In some ways this is a surprise result and clearly more work needs to be done. Only one climate model was considered in the study. The preliminary nature of the finding has been emphasizes by some, such as Gavin Schmidt of the NASA Goddard Institute of Space Studies who is reported to have said, “I don’t expect this to impact consensus estimates.”

But it cannot be dismissed as inconvenient. The researchers intend to repeat the calculations with several climate models.

“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 concludes.

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