A project running almost 10,000 climate simulations on volunteers' home computers has found that a global warming of 3 degrees Celsius by 2050 is 'equally plausible' as a rise of 1.4 degrees.
The study, the first to run so many simulations using a complex atmosphere-ocean climate model, addresses some of the uncertainties that previous forecasts, using simpler models or only a few dozen simulations, may have over-looked.
Importantly, the forecast range is derived from models that accurately reproduce observed temperature changes over the last 50 years.
The results suggest that the world is very likely to cross the '2 degrees barrier' at some point this century if emissions continue unabated, and that those planning for the impacts of climate change need to consider the possibility of warming of up to 3 degrees (above the 1961-1990 average) by 2050 even on a mid-range emission scenario. This is a faster rate of warming than most other models predict.
A report of the research is published in Nature Geoscience.
'It's only by running such a large number of simulations -- with model versions deliberately chosen to display a range of behaviour -- that you can get a handle on the uncertainty present in a complex system such as our climate,' said Dr Dan Rowlands of Oxford University's Department of Physics, lead author of the paper. 'Our work was only possible because thousands of people donated their home computer time to run these simulations.'
'Most forecasts of global warming are based on the range of results that different groups around the world happen to contribute to a model comparison. These groups don't set out to explore the full range of uncertainty, which is why studies like ours are needed,' said Professor Myles Allen of the School of Geography and Environment and Department of Physics, Oxford University, an author of the paper.
Dr Ben Booth, Senior Climate Scientist at the Met Office Hadley Centre, an author of the paper, said: 'There have been substantial efforts within the international community to quantify and understand the consequence of climate uncertainties for future projections. Perhaps the most ambitious effort to date, this work illustrates how the citizen science movement is making an important contribution to this field.
'Co-author Professor Dave Frame of Victoria University of Wellington, Visiting Fellow of Oxford University's Smith School of Enterprise and the Environment, said: 'Ensembles like this are an innovative way of exploring a range of possible futures, and provide an exciting new resource for the climate adaptation and impact communities.'
The model used in the project was supplied by the UK Met Office and the work was supported by the Natural Environment Research Council (NERC), the European Union FP6 WATCH and ENSEMBLES projects, the Oxford Martin School, the Smith School of Enterprise and the Environment, and Microsoft Research.
The research was made possible because volunteers donated time to run the simulations on their home computers through climateprediction.net as part of the BBC Climate Change Experiment.
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