Climate change could be worse than anyone thought

Expressed in the language of science, the paper does not generate a feeling of great alarm: “Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing.

“Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to five degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate.

“The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models...

“The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than three degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.”

Yet these comments, in a paper published in Nature early last month, suggest that climate change could be far worse than even the Intergovernmental Panel on Climate Change – IPCC – predicted, with global temperatures set to rise by at least four degrees by 2100.

The study by Australian and French climate scientists looked at the effect of clouds on Earth and found that as the planet warmed, clouds formed and this caused temperatures to rise further in an ever-upward spiral.

Lead researcher Professor Steven Sherwood, of the climate change research centre at the University of New South Wales, said a four degree temperature rise “would likely be catastrophic rather than simply dangerous.

“For example, it would make life difficult if not impossible in much of the tropics, and would guarantee the eventual melting of the Greenland ice sheet and some of the Antarctic ice sheet,” Sherwood told the Guardian newspaper.

He said that unless emissions of greenhouse gases were cut, the planet would heat up by a minimum of four degrees by 2100, twice the level the world's governments consider to be dangerous. Sherwood and his two French colleagues note that the way clouds affect global warming has been the biggest mystery surrounding future climate change.

“This study breaks new ground twice: first by identifying what is controlling the cloud changes and second by strongly discounting the lowest estimates of future global warming in favour of the higher and more damaging estimates.”

For two decades, estimates of global warming have run from 1.5 degrees to five degrees. The new research has narrowed that range down to between three and five degrees by closely examining the biggest cause of uncertainty: clouds.

"Climate sceptics like to criticise climate models for getting things wrong, and we are the first to admit they are not perfect," Sherwood said. "But what we are finding is that the mistakes are being made by the models which predict less warming, not those that predict more."

He said his team's work on the role of clouds could not definitively rule out that future temperature rises would be at the lower end of projections.

"But for that to be the case, one would need to invoke some new dimension to the problem involving a major missing ingredient for which we currently have no evidence. Such a thing is not out of the question but requires a lot of faith."