What has changed? Unlike other models, MIT’s Integrated Global Systems Model makes detailed assessments not just of climate science but also of the likely changes in human activity. The difference between the two outcomes arises from several factors, such as new economic data showing that our greenhouse gas emissions are unlikely – if there are no constraints – to be as low as previously thought, and new oceanic temperature data, showing that the deep oceans are not removing heat from the atmosphere as quickly as scientists expected.
Even the new figure could be an under-estimate, the team suggests, because it doesn’t account for the full range of positive feedbacks, such as melting permafrost releasing methane and carbon dioxide.
Climate change deniers hate these models. Why, they say, should we base current policy on scenarios and computer programmes rather than observable facts? But that’s the trouble with the future: you can’t observe it. If you reject the world’s most sophisticated models as a means of forecasting likely climate trends, you must suggest an alternative. What do they propose? Gut feelings? Seaweed? Chicken entrails?
Computer models are only as good as the assumptions they contain, which is why those assumptions are constantly tested and updated. No one claims to have a definitive answer; instead the models test hundreds of different likely scenarios, then find the median result. There is no attempt to make the future look either rosier or grimmer than it is.
What they give us is the best available estimate of the consequences of doing as Mr Klaus and others suggest, and letting events take their course. The MIT model suggests that even the most profligate climate change programme the world’s governments could devise would do nowhere near as much economic and humanitarian damage as our failure to act. Nothing is certain: it’s all a matter of probability. But which risk do you want to take?