A United Nations report says that governments must act faster to keep global warming in check and delays until 2030 could force them to use little-tested technologies to extract greenhouse gases from the air.
The study, drawing on work by more than 1,000 experts, said a radical shift from fossil fuels to low-carbon energy such as wind, solar or nuclear power would shave only about 0.06 of a percentage point a year off world economic growth.
"It does not cost the world to save the planet," Ottmar Edenhofer, a German scientist who is co-chair of a meeting of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), told a news conference in Berlin.
The report, endorsed by governments, is meant as the main scientific guide for nations working on a U.N. deal to be agreed in late 2015 to rein in greenhouse gas emissions that have hit repeated highs this century, led by China's industrial growth.
"We don't have the luxury of time," Rajendra Pachauri, chair of the IPCC, told Reuters, saying costs would rise sharply if strong action was delayed to 2030. "We will have to move quickly and with an unprecedented level of international cooperation."
Governments have promised to limit temperature rises to a maximum 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times to avert ever more heatwaves, floods, droughts and rising sea levels that the IPCC says are linked to man-made warming.
Such levels were still attainable, it said, but policies in place so far put the world on target for a temperature rise of up to 4.8C (8.6F) by 2100. Temperatures have already risen by about 0.8 C (1.4F) since the Industrial Revolution of the 18th and 19th centuries.
IPCC scenarios showed world emissions of greenhouse gases, mainly from burning fossil fuels, would need to peak soon and tumble by between 40 and 70 percent from 2010 levels by 2050, and then to almost zero by 2100, to keep rises below 2C.
The IPCC said that natural gas, which emits fewer greenhouse gases than coal, could get a boost until about 2050.
"Ambitious mitigation may even require removing carbon dioxide from the atmosphere," the IPCC said. Delay in acting to cut emissions until 2030 would force far greater use of such technologies, a 33-page summary for policymakers said.