Wind turbines used to generate electricity are seen at a wind farm in Guazhou, 950km (590 miles) northwest of Lanzhou, Gansu Province September 15, 2013.
Faster action is needed to keep global warming to agreed limits and delays until 2030 could force reliance on technologies to extract greenhouse gases from the air, a UN report said on Sunday.
The study, drawing on the work of more than 1,000 experts, said a shift from fossil fuels to low-carbon energy such as wind, solar or nuclear power was affordable and would shave only about 0.06 percentage point a year off world economic growth.
"We have a window of opportunity for the next decade, and maximum the next two decades" to act at moderate costs, said Ottmar Edenhofer, co-chair of a Berlin meeting of the Intergovernmental Panel on Climate Change (IPCC).
"I'm not saying it's costless. I'm not saying climate policy is a free lunch. But it's a lunch worthwhile to buy," he said.
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 world greenhouse gas emissions that have hit repeated highs, led by China's industrial growth.
Governments have promised to limit temperature rises to a maximum 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times to avert ever more heat waves, floods, droughts and rising sea levels that the IPCC says are linked to man-made warming.
IPCC scenarios showed that world emissions of greenhouse gases would need to peak soon and tumble by between 40 and 70 percent from 2010 levels by 2050, and then close to zero by 2100, to keep temperatures below 2C.
Such cuts are far deeper than most governments are planning.
"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.
If countries delay, the world will have to deploy little-tested options, said Edenhofer, a German scientist from the Potsdam Institute for Climate Impact Research.
One method mentioned by the IPCC is to burn wood, crops or other biomass to generate electricity and capture the greenhouse gases from the exhaust fumes and bury them underground.
The experimental technology would reduce the amount of carbon in a natural cycle of plant growth and decay. But there are risks, for instance that vast areas of land will be needed to grow biomass, displacing crops and pushing up food prices.
Simpler methods to extract greenhouse gases from the air are to plant trees, which soak up greenhouse gases as they grow.
The IPCC report is the third and final part of a massive United Nations series, updating science for the first time since 2007. A summary of findings will be issued in October.
The U.N.'s climate chief, Christiana Figueres, said the world should step up action to cut emissions. "We cannot play a waiting game where we bet on future technological miracles to emerge and save the day," she said in a statement.
U.S. Secretary of State John Kerry said that every year the world defers action, the costs only grow.
"These technologies can cut carbon pollution while growing economic opportunity at the same time," he said in a statement.
"This report makes very clear we face an issue of global willpower, not capacity."
The IPCC says it is at least 95 percent probable that man-made emissions, rather than natural variations, are the main cause of warming. But many voters are doubtful and few governments have policies consistent with a 2C target.
Low-carbon energies, which accounted for 17 percent of world energy supplies in 2010, would have to triple or quadruple their share by 2050, displacing conventional fossil fuels as the top source of energy, IPCC scenarios showed.
Low-carbon energy can include coal-, natural gas or oil-fired power plants if they use carbon capture and storage (CCS) to bury emissions underground. That technology, however, is mostly experimental.
Saskatchewan Power Corp in Canada will start a $1.35 billion Boundary Dam coal-fired CCS project this year, capturing a million tonnes annually of carbon dioxide in what it says is the world's first post-combustion coal-fired CCS project.
Oil and gas firms say they are tackling global warming. On March 31, Exxon Mobil Corp said that all energy sources, including fossil fuels, had to be exploited to meet growing world demand.
Environmentalists said the focus should be on shifting to renewables rather than nuclear power or CCS. "We need to put our money into the future ... with a focus on renewables and energy efficiency," said Samantha Smith of the WWF conservation group.