Intergovernmental Panel on Climate Change: 30 years to climate calamity if we carry on blowing the carbon budget

The world's leading climate scientists have set out in detail for the first time how much more carbon dioxide humans can pour into the atmosphere without triggering dangerous levels of climate change – and concluded that more than half of that global allowance has been used up.

If people continue to emit greenhouse gases at current rates, the accumulation of carbon in the atmosphere could mean that within as little as two to three decades the world will face nearly inevitable warming of more than 2C, resulting in rising sea levels, heatwaves, droughts and more extreme weather.

This calculation of the world's "carbon budget" was one of the most striking findings of the Intergovernmental Panel on Climate Change (IPCC) , the expert panel of global scientists who on Friday produced the most comprehensive assessment yet of our knowledge of climate change at the end of their four-day meeting in Stockholm.

The 2,000-plus page report, written by 209 lead authors, also found it was "unequivocal" that global warming was happening as a result of human actions, and that without "substantial and sustained" reductions in greenhouse gas emissions we will breach the symbolic threshold of 2C of warming, which governments around the world have pledged not to do.

Ban Ki-moon, the UN secretary-general, urged world leaders to pay heed to the "world's authority on climate change" and forge a new global deal on cutting emissions. "The heat is on. Now we must act," he said.

John Kerry, the US secretary of state, said in a statement: "This is yet another wakeup call: those who deny the science or choose excuses over action are playing with fire."