AsianScientist (Jan. 6, 2014) – Global average temperatures will rise at least 4°C by year 2100 and potentially more than 8°C by year 2200 if carbon dioxide emissions are not reduced, according to new research published in Nature.
In this study, 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.
“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,”said Professor Steven Sherwood, lead author from the University of New South Wales’ Center of Excellence for Climate System Science.
The key to this narrower but much higher estimate can be found in the real world observations around the role of water vapor in cloud formation.
Observations show when water vapor 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 kilometers before returning to the surface without forming rain clouds.
When updraughts rise only a few kilometers they reduce total cloud cover because they pull more vapor away from the higher cloud forming regions. 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 updraughts as rising to 15 km and forming clouds.
However, real world observations show this behavior is wrong. When the processes in climate models are corrected to match the observations in the real world, the models produce cycles that take water vapor 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.
“Climate skeptics 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 Sherwood.
The article can be found at: Shiogama H et al. (2014) Climate science: Clouds of uncertainty.
Source: University of New South Wales; Photo: woodleywonderworks/Flickr/CC.
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