Global warming: New research suggests climate may be more sensitive to greenhouse gases than previously thought

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Convective clouds over the Florida coast. bberwyn photo.

Correcting models with new information on cloud formation leads to higher projected temperature increases

By Summit Voice

FRISCO — Climate scientists taking a closer look at the role of water vapor in cloud formation say the climate is probably more sensitive to greenhouse gases than most existing models suggest.

Based on those observations, they concluded that global temperatures could easily climb by at least 4 degrees Celsius by the end of the century if carbon dioxide emissions continue unchecked.

The research, published in the journal Nature, helps answer one of the long-standing questions about climate sensitivity — the role of cloud formation and whether this will have a positive or negative effect on global warming.

“Our research has shown climate models indicating a low temperature response to a doubling of carbon dioxide from preindustrial times are not reproducing the correct processes that lead to cloud formation,” said lead author Professor Steven Sherwood, with the University of New South Wales’ Centre of Excellence for Climate System Science.

“When the processes are correct in the climate models the level of climate sensitivity is far higher,” Sherwood said. “Previously, estimates of the sensitivity of global temperature to a doubling of carbon dioxide ranged from 1.5 to 5 degrees Celsius. This new research takes away the lower end of climate sensitivity estimates, meaning that global average temperatures will increase by 3 to 5 degrees Celsius with a doubling of carbon dioxide,” he said.

The new projections are based on observations of the relatively simple process of how updrafts form when water vapor is taken up by the atmosphere through evaporation. Sometimes, updrafts can rise 15 kilometers to form clouds that produce heavy rains. Other times, they rise just a few kilometers before returning to the surface without forming rain clouds.

When they rise only a few kilometers, they reduce total cloud cover because they pull more vapor away from the higher cloud forming regions. But water vapor is not pulled away from cloud-forming regions when only the deep 15 kilometer updraughts are present.

The researchers found climate models that show a low global temperature response to carbon dioxide do not include enough of this lower-level water vapor process. Instead they simulate nearly all updrafts as rising to 15 km and forming clouds.

When the climate models are corrected, the models produce cycles that take water vapour to a wider range of heights in the atmosphere, causing fewer clouds to form as the climate warms.

This increases the amount of sunlight and heat entering the atmosphere and, as a result, increases the sensitivity of our climate to carbon dioxide or any other perturbation.

The result is that when water vapour processes are correctly represented, the sensitivity of the climate to a doubling of carbon dioxide — which will occur in the next 50 years — means we can expect a temperature increase of at least 4 degrees Celsius by 2100.

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

“Rises in global average temperatures of this magnitude will have profound impacts on the world and the economies of many countries if we don’t urgently start to curb our emissions,” he concluded.

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