* Projected economic losses omit catastrophic risks
* Risks include runaway ice melt, coral reef collapse
* IPCC report to guide governments in combating warming
By Alister Doyle, Environment Correspondent
OSLO, March 28 (Reuters) - Many governments want sterner warnings of probable economic damage from global warming in a draft U.N. report due on Monday, saying that existing estimates of trillions of dollars in losses are only part of the picture.
A final draft before talks this week among governments and scientists in Japan projected that warming would cut economic output by between 0.2 and 2.0 percent a year by damaging human health, disrupting water supplies and raising sea levels.
But many countries reckon that is an underestimate because it excludes risks of catastrophic changes, such as a runaway melt of Greenland's ice, collapse of coral reefs or a drying of the Amazon rainforest that could cause massive economic losses.
Trying to address the objections, an updated draft text from the meeting on Friday, obtained by Reuters, adds that impact estimates "do not yet account for catastrophic changes, tipping points, and many other details."
"The quoted figures of 0.2 to 2.0 percent of GDP (gross domestic product) are at best an under-estimate, and at worst completely meaningless," a note by the British government said before the meeting, faulting the draft for omitting many risks.
And Nicaragua said the projected range of GDP losses should be raised to 6 percent. Some estimates of losses do not include projected changes in the frequency of extreme events, from heatwaves to hurricanes.
"Economists argue that it's difficult to estimate risks of catastrophic events, so they leave them out. But that's like saying the risk is zero, which isn't true," said Bob Ward, communications director of the Grantham Research Institute on Climate Change and the Environment at the London School of Economics.
The U.N. Intergovernmental Panel on Climate Change (IPCC) said that only the final summary for policymakers that will be issued on Monday is valid. IPCC meetings are closed to the media and drafts are not meant to be published.
"Comments about the unfinished work of IPCC are of little value," IPCC vice-chair Jan-Pascal van Ypersele wrote in a Tweet. "Only the final version...matters."
ECONOMICS AT HEART
Even so, the IPCC's debate about the economic risks goes to the heart of whether governments will feel they have to step up action to limit rising world emissions of greenhouse gases, mainly from burning fossil fuels.
Governments have promised to agree a U.N. pact to combat climate change at a summit in late 2015 in Paris, and IPCC reports are their main guide.
The projected 0.2 to 2.0 percent range for economic losses is based on warming of 2.5 degrees Celsius (4.5 Fahrenheit) above pre-industrial times, more than a 2.0C (3.6F) ceiling set by almost 200 governments for limiting heatwaves, floods, droughts and rising seas.
The range was drawn from a sub-chapter co-led by Richard Tol, an economist at the University of Sussex in England who is often at odds with scientific colleagues for saying that moderate global warming may have economic benefits.
He said this week he had pulled out from the 70-strong team writing the draft summary, saying he viewed parts as alarmist.
Other IPCC scientists say that bleak conclusions are fully justified and that Tol exaggerates the benefits of warming, including that crops will grow better in some regions or that fewer elderly people will die from winter cold. (Reporting By Alister Doyle)