New paper looks to the past to estimate the future

  • 25 Nov 2011, 10:20
  • Verity Payne

A new scientific paper released yesterday in the journal Science offers a revised estimate of climate sensitivity - the amount of increase in the global mean temperature expected from a doubling of atmospheric carbon dioxide - suggesting that it may be smaller than some previous estimates.

The paper hit the blogosphere earlier this month, with climate skeptic bloggers making much of it at the time. Now it is gaining a fair bit of media coverage, with some provocative headlines including " Climate change fears 'have been exaggerated' say scientists who claim apocalyptic predictions are unlikely" (the Mail), and the Australian's " Climate forecasts 'exaggerated': Science journal". The Economist and the BBC have more sober headlines, with "Good news at last?" and "Climate sensitivity to CO2 probed" respectively.

So, what does the paper show? Scientists used a climate model to reconstruct land and sea surface temperatures from the Last Glacial Maximum (LGM), around twenty thousand years ago, when certain aspects of Earth's climate were quite different. There was about a third less atmospheric carbon dioxide than before the Industrial Revolution, sea levels were lower because northern latitudes were covered in ice and snow, there was less rainfall, and there was more dust in the air.

The team used a model to examine this environment and work out what implications it had on how the climate changes as atmospheric CO2 levels change.

Based on the modelling, the group came up with a new best guess for climate sensitivity of 2.3 °C (with a two in three chance that it lies between 1.7 and 2.6 °C). This is a little lower than some previous estimates. Notably, the Intergovernmental Panel on Climate Change (IPCC)'s 2007 report (AR4) assessed all of the previous estimates and suggested that climate sensitivity is likely (two in three chance) to lie between 2 and 4.5 °C. They concluded that it was unlikely to be below 1.5 °C, but that values above 4.5 °C could not be ruled out.

So why is this range of estimates slightly lower than previous estimated ranges? Andreas Schmittner, an Oregon State University researcher and lead author on the Science article explains:

"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... 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."

Other estimates of climate sensitivity are shown in the image below:

Climate _Sensitivity _Summary

The new results are not actually so very different from what previous studies have found as you can see from the image, but perhaps the most interesting finding of the new study is that the higher end estimates of previous studies (over 4.5 °C) are effectively ruled out. This is not in agreement with previous studies, which couldn't rule out such higher climate sensitivities - as the image shows.

This should be great news, given the rate at which greenhouse gases are being released into the atmosphere by human activity. Schmittner explains:

"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"

There are the usual scientific uncertainties in the paper, for example, in this case the group's climate model simulations did not take into account uncertainties arising from how cloud changes reflect sunlight. And this study is one of many investigating climate sensitivity, so although it is an important result, it is important not to overemphasise this finding at the expense of all the previous work in this area.

This is precisely what some areas of the blogosphere did in their early online discussions of the article. And, as the Economist puts it, overhyping the study in this way is pretty hypocritical:

"...Some sceptics complain about the way ancient data of this type were used to construct a different but related piece of climate science: the so-called hockey-stick model, which suggests that temperatures have risen suddenly since the beginning of the industrial revolution. It will be interesting to see if such sceptics are willing to be equally sceptical about ancient data when they support their point of view."

Usefully, the team involved have taken time to explain the work to bloggers at Planet 3, where they also address some of the ways that their work has already been misrepresented or overhyped online, in a pretty fair and even handed manner.

The scientists involved in the study take pains to point out that our continued increasing emissions of atmospheric carbon dioxide will have serious impacts. But, if the study's findings are correct, Schmittner says, we may get a small bit of breathing space:

"our study implies that we still have time to prevent that from happening, if we make a concerted effort to change course soon."


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