Global temperatures to rise 4 degrees by 2100

Global average temperatures will rise at least 4 degrees Celsius by 2100 and potentially more than 8 degrees by 2200 if carbon dioxide emissions are not reduced, a new research has warned.
Scientists found global climate is more sensitive to carbon dioxide than most previous estimates.
The research also appears to 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 Professor Steven Sherwood, lead author from the University of New South Wales.

"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," said Sherwood.
The key to this narrower but much higher estimate can be found in the real world observations around the role of water vapour in cloud formation, researchers said.

Observations show when water vapour is taken up by the atmosphere through evaporation, the updraughts can either rise to 15 km to form clouds that produce heavy rains or rise just a few kilometres before returning to the surface without forming rain clouds.

When updraughts rise only a few kilometres they reduce total cloud cover because they pull more vapour away from the higher cloud forming regions.
However, water vapour is not pulled away from cloud forming regions when only deep 15km 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 vapour process.
When the processes in climate models are corrected to match the observations in the real world, 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.
The study was published in the journal Nature.