Solution to cloud riddle reveals hotter future

The Newsroom at the University of New South Wales: Global average temperatures will rise at least 4°C by 2100 and potentially more than 8°C by 2200 if carbon dioxide emissions are not reduced, according to new research published in Nature that shows our climate is more sensitive to carbon dioxide than most previous estimates.  The research could solve one of the great unknowns of 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 from UNSW's Centre of Excellence for Climate System Science, Professor Steven Sherwood.

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

...When the processes are correct in the climate model, this produces cycles that take water vapour to a wider range of heights in the atmosphere, causing fewer clouds to form in a warmer climate. This increases the amount of sunlight and heat entering the atmosphere and increases the sensitivity of our climate to carbon dioxide or any other perturbation.

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 3°C and more likely 4°C by 2100.

“Climate sceptics like to criticise 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 Professor 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."...

A lenticular cloud above Hawaii, shot by Rootmeansquare, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported license