Two scenarios highlighted in the report, the third in a UN series in 2007 that will guide policymakers, say the costs of limiting emissions of greenhouse gases could mean a loss of 0.2 or 0.6 percent of global gross domestic product (GDP) in 2030.

Some models show that measures such as greater efficiency in burning fossil fuels such as oil and coal could even mean a small net boost to the world economy, it said.

The most stringent scenario assessed, demanding that governments ensure that global greenhouse gas emissions start falling within 15 years, would cost 3 percent of GDP by 2030.

The conclusions broadly support those in a report last year by former World Bank chief economist Nicholas Stern, who estimated that costs of acting now to slow global warming were about one percent of global output, against a far larger 5 to 20 percent if the world delayed action.

The UN draft says there is "significant economic potential" for cuts in greenhouse gas emissions, mainly from burning fossil fuels, "sufficient to offset growth of global emissions or to reduce emissions below current levels".

WIND, NUCLEAR

The Intergovernmental Panel on Climate Change (IPCC) draft, seen by Reuters, says easily achieved curbs include better use of fossil fuels, shifts to energies such as wind, solar or nuclear power and better management of forestry and farming.

Economic benefits in addition to energy savings include better health from less pollution, less damage to agriculture from acid rain and greater energy security by cutting imports.

The report, "Mitigation of Climate Change" and drawing on the work of 2,500 scientists, says time is running out.

"Mitigation efforts over the next two to three decades will determine to a large extent the long-term global mean temperature increase and the corresponding climate change impacts that can be avoided", it said.

US President George W. Bush pulled out in 2001 of the Kyoto Protocol, the main UN plan for fighting global warming until 2012, arguing that its caps on emissions would be too costly and wrongly excluded developing nations until 2012.

One official US study forecast that Kyoto could, in the worst case, cost up to 4.2 percent of US GDP by 2010. The IPCC report will be considered by the Group of Eight industrial powers at a summit in June.

The IPCC scenario of a 0.2 percent loss in GDP in 2030 is based on stabilising greenhouse gases at 650 parts per million (ppm) in the atmosphere by 2030, up from about 430 ppm now. The gases trap the sun's heat in the atmosphere.

UN "best estimates" show that might bring a temperature rise of 3.2-4.0 C (5.8-3.2 F) over pre-industrial times. Tighter curbs would cost ever more.

The most stringent scenario, costing 3 percent of GDP, would limit greenhouse gases to 445-535 ppm by 2030, inside a range likely to bring a 2-2.4 Celsius (3.6-4.3F) rise.

Greenhouse gas emissions have risen by 70 percent between 1970 and 2004 and are expected to rise by a further 25 to 90 percent by 2030 from 2000 without new restraints, driven mainly by growth in developing nations such as China and India.

That would mark a switch: in 2000, rich nations accounted for 20 percent of world population and 46 percent of emissions.