Uncertainty in the prediction of the climate response to rising levels of GHG

According to a study published in Nature on January 27, 2005, the climate response to rising levels of greenhouse gases could be much worse than predicted. The study, carried out by Pr Stainforth's team, shows the uncertainty in the predictions. The range of possibilities for future climate evolution needs to be taken into account when planning climate change mitigation and adaptation strategies. According to the study, statistical estimates of model response uncertainty, based on observations of recent climate change, admit climate sensitivities (i.e. the equilibrium response of global mean temperature to doubling levels of atmospheric carbon dioxide) substantially greater than 5°C. The timescale would depend on how quickly the doubling of CO2 was reached, but large rises would be on a scale of a century at least from now. However, such strong responses are not used in the ranges for future climate change predictions because they have not been seen in general circulation models. In this study, the authors present results from the "climateprediction.net" experiment, where over 90 000 participants from more than 140 countries ran a simulation on their personal computer. Each participant was allocated a particular set of parameter perturbations and initial conditions enabling them to run one simulation, i.e. one member of the grand ensemble, examining what happens to the global climate if levels of carbon dioxide in the atmosphere double from pre-industrial levels - such levels are expected to be reached around the middle of this century unless deep cuts are made in greenhouse gas emissions. This experiment was the first multi-thousand-member grand ensemble of simulations using a general circulation model. The climate sensitivities for doubled CO2 concentrations, found using the initial 2 000 results received, range from less than 2°C to more than 11°C and most sensitivities cluster around 3.4°C. The range of sensitivities is more than twice that found in the general circulation model used in the IPCC Third Assessment Report (1.4-5.8°C range for 1990-2100). According to the authors, 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. The project is still ongoing and simulations are still being carried out. Future experiments will include a grand ensemble of transient simulations of the years 1950-2100 using a model with a fully dynamic ocean.