On Thursday, China, the planet's number one carbon polluter and most populous nation, put its first-ever emissions targets on the table.

It vowed to cut carbon intensity, measured per unit of GDP, by 40-to-45 percent from 2005 levels within a decade.

The objective -- which is voluntary -- came less than 24 hours after the US, second in the emitters' ranks, spelt out its first specific emissions proposals in the international arena.

Washington announced that, relative to a 2005 benchmark, it would reduce carbon emissions by 17 percent by 2020, 30 percent by 2025, 42 percent by 2030 and ultimately 83 percent by 2050.

With the exception of India, all major emitters have now declared their hands going into the December 7-18 Copenhagen climate talks, tasked with forging a durable solution to global warming.

UN climate chief Yvo de Boer said the freshly-unveiled US and Chinese positions "can unlock two of the last doors to a comprehensive agreement," although more work was needed.

"These are ambitious numbers," Laurence Tubiana, a French climate negotiator and policy analyst, said of the Chinese proposal.

"When one looks at historical coupling of economic growth and energy use in industrialised countries, we have never seen this kind of delinking," she said by phone.

Taken together, she said the two announcements were "very positive" and bring the long-stalled climate talks closer to a widely-held goal of preventing warming by more than 2.0 degrees Celsius (3.7 Fahrenheit) compared to preindustrial levels.

But not close enough, said Jim Watson, who co-chairs the energy programme at Britain's Tyndall Centre for Climate Change Research.

The Chinese plan "is a significant effort, but you have to bear in mind that it's in line with a historic trajectory as well," he said.

China has already made big gains in energy efficiency over the last 15 years for reasons that had little to do with global warming, he said.

"If you look at what is driving this energy intensity target progress, it is very much about energy security," notably scaling back oil imports, domestic coal demand and inefficiency in heavy industry, he said.

The US numbers, Watson added, are well below the contribution needed, according to the UN Intergovernmental Panel on Climate Change (IPCC), to reach the 2 C (3.6 F) target: a 25-to-40 percent cut by industrialised countries by 2020, compared to 1990.

The US target for 2020 would be the equivalent of only a four percent cut against this benchmark.

"We are still quite a long way from giving ourselves a good chance of remaining within two degrees," Watson said.

French climate expert Jean Jouzel agreed. "From the perspective of a climate scientist, we are still far off the mark," he told AFP.

For Niklas Hoehne, director of Energy and Climate Policy at Ecofys in Germany, rich nation commitments as a whole are lagging in the run-up to Copenhagen.

He put their 2020 pledges at a cut of eight to 12 percent over 1990, even if expected use of forests, as claimed carbon "sinks," are included.

According to Hoehne's calculation, emerging economies need to brake their emissions trends by between 15 and 30 percent by 2020 over a "business-as-usual" scenario to help meet the 2 C (3.6 F) goal.

Developing countries "are quite close to the range, it's very encouraging to see so many actions," he said. "Developed countries' pledges are far behind what is necessary for stabilising the climate."

De Boer said that "continued strong ambition and leadership" were needed to close the gap at Copenhagen.

"In particular, we still await clarity from industrialised nations on the provision of large-scale finance to developing countries for immediate and long-term climate action."

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