* Keeping warming to U.N. limit means rich halving emissions
* Asia should stabilise emissions at current levels by 2030
* Alternative is to overshoot U.N. temperature goals, then cut
By Alister Doyle, Environment Correspondent
OSLO, April 1 (Reuters) - The world will need far tougher curbs on greenhouse gases, by both developed nations and emerging economies, to keep global warming from exceeding a promised ceiling, a draft U.N. report shows.
Rich nations led by the United States would have to halve their emissions by 2030 from 2010 levels to keep warming below an agreed 2 degree Celsius (3.6 Fahrenheit) ceiling above pre-industrial times, according to the draft obtained by Reuters.
Asia, including China and India, would have to limit emissions to around 2010 levels by 2030 as part of a global shareout, a tough goal for countries that say they need to burn more fossil fuels to help end poverty.
"Stabilizing greenhouse gas concentrations will require large-scale transformations in human societies," according to chapter 6 of a report by the Intergovernmental Panel on Climate Change (IPCC) due for release in Berlin in mid-April.
Most governments are not planning such tough curbs, fearing they would be economically crippling. Temperatures are on track to exceed the ceiling, set by almost 200 nations in 2010, of 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times.
Even so, the curbs mark a shift in debate about climate change, which has focused most on action by rich emitters.
"The implications for all the big emitters are pretty stark," said Alden Meyer, of the Union of Concerned Scientists. "All of them now have something to worry about." Like others interviewed, he had not seen the draft.
Developing nations have often quoted the previous IPCC report, in 2007, which said industrialised nations should cut emissions by between 25 and 40 percent below 1990 levels by 2020. It did not outline such clear goals for emerging nations.
Cuts by the rich are well short of 25-40 percent. The European Union, the most ambitious of big rich nations, is considering cuts of 40 percent below 1990 levels by 2030.
The Berlin report about solving climate change follows an IPCC report about impacts of warming issued in Japan on Monday that said the world was in many cases ill-prepared for severe and perhaps irreversible change.
Monday's IPCC report said that climate change was affecting all parts of the globe and could damage food production, brake economic growth and even aggravate armed conflicts.
The reports will guide work on a deal to combat climate change due to be agreed at a summit in Paris in late 2015.
Under the IPCC scenario, former Soviet bloc nations would have to cut emissions by a third by 2030 from 2010, Latin America would have to cut overall while the Middle East and Africa could raise emissions slightly.
It does not set goals for individual nations, only groups.
An alternative is to let temperatures overshoot the 2 degrees Celsius target rise while developing technology to cool the planet, such as extracting greenhouse gases from the air, the draft says.
But overshoot is controversial, especially among poor nations most at risk from heatwaves, floods, droughts and rising seas. Also, new technologies may not work.
Marlene Moses, chair of the Alliance of Small Island States that groups some of the most vulnerable nations, said technologies exist for rapid cuts in emissions with a shift to renewable energies from fossil fuels.
Cuts in emissions "also help reduce poverty, improve public health, and build energy, food, and water security for vulnerable communities," she told Reuters.
The IPCC says that it is at least 95 percent probable that human activities, rather than natural variations in the climate, are the dominant cause of recent climate change. Opinion polls show that voters in many nations are far less certain. (Reporting By Alister Doyle; Editing by Tom Heneghan)