* Top emitters signal new intent to tackle warming
* Huge uncertainties about when, at what level, China may cap
By Alister Doyle and Ben Garside
OSLO/BONN, June 11 (Reuters) - China's hints that it will cap its soaring greenhouse gas emissions and a U.S. plan to cut emissions in the power sector, while representing a shift, do not add up to a strong cure for global warming by the world's top two emitters.
Other nations have hailed Washington and Beijing for a newfound commitment to tackle climate change. Governments are working on a deal, due in Paris in late 2015, to slow rising temperatures to avert more heatwaves, floods and rising seas.
Yet after a rush of enthusiasm, there are uncertainties - especially about China.
On June 2, Washington said it planned to cut emissions from existing power plants by 30 percent below 2005 levels by 2030 as the centrepiece of climate action by President Barack Obama. Chinese officials followed up by saying Beijing is considering a cap, for the first time, on the nation's emissions.
European Climate Commissioner Connie Hedegaard cautioned against celebrating about China, whose carbon dioxide emissions have almost quadrupled since 1990 amid stellar economic growth.
"I am sure that China is sincere in wanting to deliver cuts and cap but the key thing is, when is that going to happen?," she said on the sidelines of June 5-14 talks in Bonn among climate negotiators from about 170 nations.
"We have a saying in Danish that 'you have to see the man before you take off your hat'," she added.
So far, absolute ceilings on emissions are reserved for industrialised nations under U.N. climate plans until 2020. Emerging nations led by China and India have softer goals, to slow the rise in emissions relative to economic growth.
CHANGE OF TONE
Many experts say a change of tone is important, even though the U.S. and Chinese plans fall far short of cuts needed to reach a universal goal to limit temperature rises to 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times.
A U.N. panel of scientists says that would require cutting global emissions by 40 to 70 percent under 2010 levels by 2050, deeper than almost any nations are planning.
"There are very clear signals from the two largest emitters that they are shifting to lower carbon ... there is a whole different level of intent," said Jennifer Morgan of the U.S.-based World Resources Institute think tank.
Christiana Figueres, head of the U.N. Climate Change Secretariat, expressed confidence that China will set a cap.
"They understand that their economy is not competitive if they continue the rise in emissions and they understand that their population needs much better-quality air," she said.
But the signs are not yet conclusive.
In Beijing on Monday, a senior government official said any near-term regulation in China would allow future emissions growth. Last week, another adviser had suggested a cap could be set in the next five-year plan from 2016.
China's top climate negotiator Xie Zhenhua said in Bonn last week that Beijing was working to cap emissions as early as possible but that Chinese experts were divided about how.
42 PERCENT CUT
Hedegaard said the U.S. plan of a 30 percent cut in the power sector was "not enough" for a U.N. deal in Paris, which is due to enter into effect from 2020. The EU is considering a deeper cut by 2030, of 40 percent below 1990 levels.
"If I compare to Europe ... the sector that has to deliver most is the power sector," she said.
Obama's original pledge in 2009, of a cut in U.S. emissions of 17 percent below 2005 levels by 2020, noted that pending U.S. legislation implied a cut of 42 percent below 2005 levels by 2030.
That legislation failed in the Senate and its level of ambition for 2030 now looks unlikely. Some scientists even doubt Washington's assurances that the power sector proposal puts it on target for the 2020 goal.
"We've checked and re-checked. The U.S. plan is not enough to achieve the 2020 target," said Bill Hare of Climate Analytics, who compiled a Climate Action Tracker last week with other researchers, based mainly on regulations rather than government intentions.
The White House contested CAT's findings, pointing to standards for new power plants, vehicles, appliances and actions to reduce other gases. "We are on track," a spokesman said.
The European Union says its greenhouse gas emissions fell 19.2 percent below 1990 levels by 2012. The United States, which has had stronger economic growth and a rising population, says its emissions are 4.3 percent above 1990 levels. (Editing by Dale Hudson)