* Rich nations would need to halve emissions by 2030
* Cuts to meet shrinking carbon "budget" beyond those planned
By Environment Correspondent Alister Doyle
WARSAW, Nov 21 (Reuters) - Governments are shying away from their own warnings that the world has only a fast-shrinking budget of carbon emissions left to use to avoid damaging global warming, frightened off at U.N. climate talks by the radical cuts it would require.
To keep warming to a level that most have defined as manageable, developed nations would have roughly to halve their greenhouse gas emissions by 2030 from 2010 levels, far beyond most governments' plans, said Niklas Hoehne of research group Ecofys.
The global action required, by both rich and poor nations, "is very, very ambitious" compared to current pledges, Hoehne said.
Just two months ago, 110 governments endorsed findings by the U.N.'s Intergovernmental Panel on Climate Change (IPCC) in Stockholm saying that the world had emitted about 515 billion tonnes of carbon since the Industrial Revolution.
The IPCC estimated the total cumulative budget could not exceed a trillion tonnes to allow a good chance of keeping a rise in temperatures to 2 degrees Celsius (3.6 Fahrenheit) in order to avoid ever more frequent and intense heatwaves, floods, droughts and rising sea levels.
Once in the atmosphere, many greenhouse gases remain there for decades or even centuries, and on current trends of rising emissions, the trillion tonnes will be reached in a few decades.
But now, at the first meeting since Stockholm working on the outlines of a U.N. deal to slow global warming due to be agreed in 2015, the budget has barely been mentioned.
Myles Allen, of Oxford University, said the implications were just too radical for governments already struggling to cut emissions.
MORE RADICAL MEASURES
Working towards the budget would require a shift towards radical ways to suck carbon dioxide from the air, ensuring that carbon emitted by power plants was captured and stored, and setting out to leave oil and gas in the ground, he said.
But many developed nations are more focused on spurring their sluggish economic growth than on fighting climate change.
And whatever sense of urgency they may have had has been alleviated by the side effects of financial crisis and economic slowdown in the West - which have helped to brake a rise in emissions - and by a probably short-lived brief slowdown in the pace of warming this century.
A related idea by developing nations to ask the IPCC to examine historical responsibility for causing global warming, as a guide to future action in sharing out emissions, is also a minefield at the Warsaw talks.
Developing nations see it as a way to underline the fact that the rich have burnt most fossil fuels since the Industrial Revolution. Rich nations say it would take too long to figure out, and that blame is constantly shifting.
The proposal also raises awkward political questions.
Is Britain, for instance, responsible for India's emissions before independence in 1947? Should heat-trapping methane from rice paddies in China be measured along with Europe's industrial emissions in the 19th century?
Robert Stavins, director of the Harvard Environmental Economics Program, said it would be disastrous to try to apportion historical blame. He also said the IPCC's carbon budget had not been intended as a policy guide.
But these are anyway areas where the Warsaw conference is not going to go.
"The mood here is 'don't tread on anybody's toes'," said Hans Joachim Schellnhuber, founding director of the Potsdam Institute for Climate Impact Research.
"But I am sure that on the Titanic, many people were treading on each other's toes."
(Editing by Kevin Liffey)