Climate scientists are surer than ever that human activity is causing global warming, according to leaked drafts of a major UN report, but they are finding it harder than expected to predict the impact in specific regions in coming decades.
The uncertainty is frustrating for government planners: the report by the Intergovernmental Panel on Climate Change (IPCC) is the main guide for states weighing multi-billion-dollar shifts to renewable energy from fossil fuels, for coastal regions considering extra sea defences or crop breeders developing heat-resistant strains.
Drafts seen by Reuters of the study by the UN panel of experts, due to be published next month, say it is at least 95 per cent likely that human activities – chiefly the burning of fossil fuels – are the main cause of global warming since the 1950s.
That is up from at least 90 per cent in the last report in 2007, 66 per cent in 2001, and just over 50 per cent in 1995, steadily squeezing out the arguments by a small minority of scientists that natural variations in the climate might be to blame.
That shifts the debate on to the extent of temperature rises and the likely impacts, from manageable to catastrophic. Governments have agreed to work out an international deal by the end of 2015 to rein in rising emissions.
“We have got quite a bit more certain that climate change...is largely man-made,” said Reto Knutti, a professor at the Swiss Federal Institute of Technology in Zurich.
“We’re less certain than many would hope about the local impacts.”
And gauging how warming would affect nature, from crops to fish stocks, was also proving hard since it goes far beyond physics.
“You can’t write an equation for a tree,” he said.
The IPCC report, the first of three to be released in 2013 and 2014, will face intense scrutiny, particularly after the panel admitted a mistake in the 2007 study which wrongly predicted that all Himalayan glaciers could melt by 2035. Experts say the error far overestimated the melt and might have been based on a misreading of 2350.