UK researchers predict 4 °C rise within decades.
The planet could warm by 4 °C as early as 2060 if greenhouse-gas emissions are not curbed quickly and dramatically, according to a UK government-backed study.
The research, commissioned by the UK Department of Energy and Climate Change in London from the Met Office Hadley Centre in Exeter, looks in detail at the future high-emissions scenario that was considered the worst case in the 2007 report from the Intergovernmental Panel on Climate Change (IPCC).
Much analysis and policy debate has focused on avoiding global warming of more than 2 °C above pre-industrial temperatures — widely considered to be the threshold for dangerous climate change. But emissions have continued to rise rapidly and the latest findings on the carbon cycle have strengthened the view that in future, less of our CO2 emissions will be absorbed by natural sinks. So a much greater temperature rise is increasingly plausible, says Richard Betts, who presented the study at the conference 4 Degrees and Beyond in Oxford, UK, on 28 September.
"Now we know that emissions are at the upper end of what the IPCC projected a decade ago, it justifies taking the higher-emissions scenario more seriously," says Betts. Moreover, he says, evidence is accumulating that warming will weaken natural carbon sinks that so far have been taking up 50% of the greenhouse gases produced from the burning of fossil fuels, speeding up warming even further.
Betts's group applied a complex climate model to the IPCC's highest-emissions scenario that incorporated the effects of weakening sinks. Depending on how much the sinks weaken — a key source of uncertainty — they found that temperatures of 4 °C higher than pre-industrial levels would probably be reached in the 2070s, and perhaps by 2060.
In climate simulations where average global temperature rises 4 °C or more, oceans warm less than that average and land areas more — by 7 °C in many areas, Betts reports. Temperatures could shoot up by up to 10 °C in western and southern Africa and by that much or more in the Arctic, and decreases in rainfall of 20% or more would be widespread in parts of Africa, Australia, the Mediterranean and Central America.
"It's an extreme scenario, but one that is plausible," says Betts. "When millions of people's lives are at stake, it's worthwhile thinking about extreme, plausible scenarios."
To avoid a rise of 4 °C, emissions must peak and then steeply decline within the next 30 years, says Betts. To stay under 2 °C, that needs to happen in a decade. But even if ambitious action is agreed at this year's United Nations climate conference in Copenhagen, he says, it will take years to implement. This puts policy-makers between a rock and a hard place, says Kevin Anderson of the Tyndall Centre for Climate Change Research in Norwich, UK. "Mitigating for 2 °C is much more challenging than was previously thought, but adapting to 4 °C is also extremely challenging," Anderson says. "There is no easy way out."
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Barnett, A. Climate sizzle could come soon. Nature (2009). https://doi.org/10.1038/news.2009.959