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.
With fast action, climate change could be kept in check at manageable costs, said U.N. Secretary-General Ban Ki-moon, 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 Intergovernmental 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.
To cut emissions, the report points to options including energy efficiency, renewable energies from wind to solar power, nuclear energy or coal-fired power plants where carbon dioxide is stripped from the exhaust fumes and buried underground.
Without extra efforts to cut emissions, "warming by the end of the 21st century will bring high risks of severe, widespread, and irreversible impacts globally," the IPCC said.
"Irreversible" could mean, for instance, a runaway melt of Greenland's vast ice sheets that could swamp coastal regions and cities or disruptions to monsoons vital for growing food.
Deep cuts in emissions would reduce global growth in consumption of goods and services, the economic yardstick used by the IPCC, by just 0.06 percentage point a year below annual projected growth of 1.6 to 3.0 percent, it said.
So far, major emitters are far from curbs on emissions on a scale outlined by the IPCC. China, the United States and the European Union are top emitters.
The report also says that it is at least 95 percent sure that manmade emissions of greenhouse gases, rather than natural variations in the climate, are the main cause of warming since 1950, up from 90 percent in a previous assessment in 2007.
PHOTO: U.N. Secretary-General Ban Ki-moon (L) and Intergovernmental Panel on Climate Change (IPCC) Chairman Rajendra Pachauri present the AR5 Synthesis Report during a news conference in Copenhagen November 2, 2014.
CREDIT: REUTERS/NIELS AHLMANN OLESEN/SCANPIX DENMARK.