The Intergovernmental Panel on Climate Change (IPCC) has released its Working Group III report in Berlin.
The report warns that energy efficiency improvements have not kept up with economic growth and says delaying action until 2030 could force reliance on technologies to extract greenhouse gases from the air.
"We have a window of opportunity for the next decade, and maximum the next two decades" to act at moderate costs, Ottmar Edenhofer, co-chair of a Berlin meeting of the IPCC, said.
"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."
The report is meant as the main scientific guide for nations working on a UN deal to be agreed in late 2015 to rein in world greenhouse gas emissions that have hit repeated highs.
Governments have promised to limit temperature rises to a maximum 2 degrees Celsius 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.
IPCC scenarios showed that world emissions of greenhouse gases would need to peak soon and tumble by between 40 and 70 per cent 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.
'Little-tested' options may be needed if action delayed
If countries delay, the world will have to deploy little-tested options, Dr Edenhofer said.
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 UN'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.
Environment Minister Greg Hunt, who is trying to scrap carbon pricing, said in a statement that the Federal Government was committed to taking strong and effective action to reduce emissions.
He said the Government would consider further targets in 2015, depending on what the rest of the world does.
Cost of action on climate change
The IPCC says it is at least 95 per cent probable that man-made emissions, rather than natural variations, are the main cause of warming.
Professor Frank Jotzo is an economist and one of the lead authors of the social, economic and ethical concepts and methods chapter.
He says efforts to mitigate climate change may cost about 6 per cent of gross domestic product (GDP) by 2050.
"What that means is that on an annualised basis, that would be almost impossible to see on a chart because economies will keep growing even with very ambitious climate change action," he said.
More than 10 per cent of the world's increasing emissions came from transport.
Lead author of the transport chapter Professor Peter Newman said given Australia's remoteness and the distance between cities, there was still a great challenge ahead to reduce emissions in the sector.
"There's still issues with freight transport and aircraft," he said.
"They're both growing substantially and don't show any great sign of peaking and declining as they must."
Even the IPCC's chairman, Rajendra Pachauri, was keeping with the transport theme.
"The high-speed mitigation train would need to leave the station soon and all of global society have to get on board," he said.
The European Union is already on board and has set up an emissions trading scheme, the world's biggest carbon market.
But it has faced significant problems and the report says cap-and-trade systems in the short-term have had a limited environmental effect because of caps that have not proved to be constraining.
Professor Frank Jotzo says despite the issues experienced overseas, that approach is still the way to go.
"The answer quite unambiguously if you ask the economics community, as well as if you ask the IPCC, would be economic instruments: carbon pricing, emissions trading, carbon taxes are the centrepiece of cost-effective action," he said.