Governments can keep climate change in check at manageable costs but will have to cut greenhouse gas emissions to zero by 2100 to limit risks of irreversible damage, a U.N. report said on Sunday.
The 40-page synthesis, summing up 5,000 pages of work by 800 scientists already published since September 2013, said global warming was now causing more heat extremes, downpours, acidifying the oceans and pushing up sea levels.
"Science has spoken. There is no ambiguity in the message. Leaders must act, time is not on our side," U.N. Secretary-General Ban Ki-moon said in presenting the report in Copenhagen that is meant to guide global climate policy-making.
With fast action, climate change could be kept in check at manageable costs, he said, referring to a U.N. goal of limiting average temperature rises to 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial times. Temperatures are already up 0.85 C (1.4F).
The study by the Intergovernmenal Panel on Climate Change (IPCC), approved by more than 120 governments, will be the main handbook for negotiators of a U.N. deal to combat global warming due at a summit in Paris in December 2015.
To get a good chance of staying below 2C, the report's scenarios show that world emissions would have to fall by between 40 and 70 percent by 2050 from current levels and to "near zero or below in 2100".
Below zero would require extracting carbon dioxide from the atmosphere - for instance by planting forests that soak up carbon as they grow or by burying emissions from power plants that burn wood or other biomass.