HOW much will Earth warm this century? The best answer to this killer question remains broad, but a study has narrowed the range of likely temperatures – and comes down on the warmer side.
As we pump more greenhouse gases into the atmosphere, temperatures rise. The consensus among climatologists is that if the amount of carbon dioxide doubles, temperatures will rise by about 3 °C. But this figure, known as climate sensitivity, is an estimate and falls within a range of 2 to 4.5 °C, as calculated by the Intergovernmental Panel on Climate Change in 2007 (see “Climate change: It’s even worse than we thought“).
Much of the uncertainty stems from clouds, whose effects climate models struggle to simulate. Fasullo and Trenberth bypassed this problem by using satellite records of relative humidity, which influences cloud formation, instead. Humidity is unevenly distributed, and changes over the year. The pair looked at climate models with various sensitivities to see how well they reproduced these real-life changes.
Much of the uncertainty stems from clouds, whose effects climate models struggle to simulate
Models with climate sensitivities at the lower end of the scale did badly, often reporting high humidity in regions where it was low. Those with sensitivities around 4 °C did best (Science, doi.org/jqj). “We have found a new metric for evaluating climate models,” says Trenberth. “The models with a higher sensitivity are doing a better job.”
“It’s a pretty impressive step,” says Reto Knutti of the Swiss Federal Institute of Technology in Zurich, though he cautions that just one study should not be taken as gospel. Trenberth suggests the sensitivity is about 4 °C, but he says 3 °C is still “very viable”.
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