Uncertainty in predictions of the climate response to rising levels of greenhouse gases

Climate models: net gains In the climate prediction .net project, thousands of individuals have volunteered spare computing capacity on their PCs to help quantify uncertainty in the way our climate responds to increasing levels of greenhouse gases. By running a state-of-the-art climate model thousands of times, it is possible to find out how the model responds to slight changes in the approximations of physical processes that cannot be calculated explicitly. The first batch of results has now been analysed, and surface temperature changes in simulations that capture the climate realistically are ranging from below 2 °C to more than 11 °C. These represent the possible long-term change, averaged over the whole planet, as a result of doubling the levels of atmospheric carbon dioxide in the model. This is the first time that complex models have been found with such a wide range of responses. Their existence will help in quantifying the risks associated with climate change on a regional level. The range of possibilities for future climate evolution 1 , 2 , 3 needs to be taken into account when planning climate change mitigation and adaptation strategies. This requires ensembles of multi-decadal simulations to assess both chaotic climate variability and model response uncertainty 4 , 5 , 6 , 7 , 8 , 9 . Statistical estimates of model response uncertainty, based on observations of recent climate change 10 , 11 , 12 , 13 , admit climate sensitivities—defined as the equilibrium response of global mean temperature to doubling levels of atmospheric carbon dioxide—substantially greater than 5 K. But such strong responses are not used in ranges for future climate change 14 because they have not been seen in general circulation models. Here we present results from the ‘climate prediction .net’ experiment, the first multi-thousand-member grand ensemble of simulations using a general circulation model and thereby explicitly resolving regional details 15 , 16 , 17 , 18 , 19 , 20 , 21 . We find model versions as realistic as other state-of-the-art climate models but with climate sensitivities ranging from less than 2 K to more than 11 K. Models with such extreme sensitivities are critical for the study of the full range of possible responses of the climate system to rising greenhouse gas levels, and for assessing the risks associated with specific targets for stabilizing these levels.