Global warming will displace millions of people, trigger falling crop yields, stoke conflict and cost trillions of dollars in lost economic output, a United Nations report will warn.

A draft of the report - to be finalised this month by the Intergovernmental Panel on Climate Change (IPCC) - obtained by The Independent in Britain, says "hundreds of millions of people" will be forced to relocate because of coastal flooding and land loss as sea levels rise.

Changing conditions will also threaten food security, with crop yields to drop as much as 2 per cent per decade for the rest of the century. Food demand was on course to rise 14 per cent per decade until 2050, the Independent said.

Poverty and economic shocks from climate change will spur migration, increasing the risks of violence from protests and from civil or international conflicts, according to the draft of the report on impacts, adaptation and vulnerability.

Macquarie University's Professor Lesley Hughes, a lead author of the report, declined to comment on its contents but said scientists had increasing confidence the climate was shifting, both from research and a rise in observations of extreme events.

"The climate system is very different from what it was three or four decades ago," Professor Hughes said, citing the incidence of heatwaves and droughts.

The summary of the report runs for 76 pages. It notes the number of papers on adaptation to climate change had doubled in the five years to 2010, adding to the material to be assessed by the report's authors.

Among the projections likely to attract scrutiny from reviewers when the final report of the IPCC's Working Group II is settled on in Japan is the draft's estimate that annual global gross domestic product will drop by 0.2 to 2 per cent if temperatures rise 2.5 degrees above pre-industrial levels.

Global mean temperatures have already risen about 0.9 degrees, with at least a 2-degree rise increasingly likely as carbon dioxide emissions from the burning of fossil fuels and land clearing continue to rise, scientists say.

Based on 2012 figures of GDP at $US71.8 trillion the annual loss would be as much as $US1.4 trillion, The Independent said. Other reports suggest the real impacts will be much greater in part because models struggle to capture the full costs of climate change.