By David Fogarty and Alister Doyle

(Reuters) - As the nations of the world struggle in Doha to agree even modest targets to tackle global warming, the cuts needed in rising greenhouse gas emissions grow ever deeper, more costly and less likely to be achieved.

U.N. talks have delivered only small emissions curbs in 20 years, even as power stations, cars and factories pump out more and more heat-trapping gases.

An overriding long-term goal set by all nations two years ago to keep temperature rises to less than 2 degrees Celsius (3.6 F) above levels prior to the Industrial Revolution is fast slipping away.

"The possibility of keeping warming to below 2 degrees has almost vanished," Pep Canadell, head of the Global Carbon Project at Australia's Commonwealth Scientific and Research Organization, told Reuters.

Disagreements mean the U.N. climate talks in Doha, Qatar, that run until December 7 have scant chance of making meaningful progress. The talks are aimed at reaching a new deal to start by 2020 to slow climate change in the form of more floods, droughts, rising sea levels and severe storms like Hurricane Sandy that lashed the U.S. Northeast last month.

Global emissions of carbon dioxide (CO2), the main greenhouse gas, have risen 50 percent since 1990 and the pace of growth has picked up since 2000, Canadell said. In the past decade, emissions have grown about 3 percent a year despite an economic slowdown, up from 1 percent during the 1990s.

Based on current emissions growth and rapid industrial expansion in developing nations, emissions are expected to keep growing by about 3 percent a year over the next decade.

For the talks to have any chance of success in the long run, emissions must quickly stop rising and then begin to fall. Temperatures have already risen by 0.8 C (1.4 F) since pre-industrial times.

"The alarm bells are going off all over the place. There's a disconnect between the outside world and the lack of urgency in these halls," Alden Meyer of the Union of Concerned Scientists said at the Doha talks.

Nearly 1,200 coal-fired power plants, among the biggest emitters, are proposed around the globe, with three-quarters of them planned for China and India, a study by the Washington-based World Resources Institute think-tank said last week.

Emissions from China, the world's top carbon polluter, are growing 8 to 9 percent a year and are now about 50 percent higher than those of the United States. And China's carbon emissions are not expected to peak until 2030.

POLLUTION

In some projections, global emissions will need to go into reverse by mid-century, with the world sucking more carbon out of the air than it puts in, if warming is to be kept to below 2 C.

And air pollution, mostly particles from fossil fuel use, may be masking the warming by dimming sunshine.

"Those aerosols today hide about one-third of the effect of greenhouse gases," Jean-Pascal van Ypersele, vice-chairman of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), told Reuters.

Without that pollution, a breach of the 2 degree threshold might already be inevitable, he said.

The latest IPCC report, in 2007, said keeping greenhouse gas concentrations low would cost less than 3 percent of world gross domestic product by 2030. So far, the panel has not assessed the costs of delays, said Rajendra Pachauri, chairman of the panel.

The report also said that world emissions of greenhouse gases would need to peak by 2015 to give a good chance of keeping the average temperature rise to below 2 C.

But deep disagreement on future emissions cuts between rich and poor nations has delayed the start of a new global pact until 2020, undermining the chances of a robust extension in Doha of the existing plan, the Kyoto Protocol, which obliges almost 40 rich nations to cut emissions until the end of 2012. Continued...