Global temperatures could increase by 4C as early as the 2060s, raising the risk of major impacts such as sea level rises, loss of coral reefs and the collapse of farming systems in some parts of the world, it has been warned.
As the latest round of UN climate talks gets under way, scientists said that even with strong political will, the chances of changing energy systems fast enough to limit global warming to less than 2C is “slim” and eventual temperature rises of 3C or 4C are much more likely.
A recent study by the UN Environment Programme showed the voluntary pledges by countries to curb emissions under the Copenhagen Accord, drawn up at the climate talks last year, will not be sufficient to keep global temperatures from rising by more than 2C – widely recognised as a “guard rail” below which dangerous climate change could mostly be avoided.
Delivering the pledges – many of which, including the EU’s, cover a range of ambition on cutting emissions – would put the world on track for warming of anywhere between 2.5C and 5C, while doing nothing could see global temperatures rise by 7C by 2100.
The series of papers published in a special edition of the Philosophical Transactions of the Royal Society A, looked at the impacts of a 4C world – which would be far greater than those incurred if temperatures rose only 2C above pre-industrial levels.
Scientists from the University of Exeter and the Met Office warned that if high levels of emissions continued, temperature rises of 4C would be reached in the 2070s, but with strong “feedbacks” – for example reductions in the ability of warmer oceans to absorb carbon dioxide from the atmosphere, amplifying the warming – temperatures could rise by 4C by the 2060s.
Authors of other papers in the journal said that land areas warmed more than the oceans, so an overall increase of 4C could see much higher temperature rises in certain parts of the world, including the northern forests and the Arctic.
A series of “thresholds” could be crossed, including the permanent absence of summer ice in the Arctic, loss of most of the tropical coral reefs, melting of the permafrost which would in turn give out yet more greenhouse gases and dieback of the Amazon forest.
Sea levels could rise by between half a metre and two metres by 2100, although rises of more than one metre were much less likely.
And the costs of protecting low-lying and coastal areas from the sea could reach 270 billion dollars (£173 billion) a year by 2100, with countries such as Bangladesh and Vietnam having to depend on money from richer countries to defend their land.
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