China is likely to agree to a quantified target to limit its greenhouse gas emissions after 2020, said a senior expert with a government think tank.

But this depends on the outcome of climate change negotiations and China's level of development by that time, said Xu Huaqing, a researcher from the Energy Research Institute affiliated with the National Development and Reform Commission, the government body that oversees climate change issues in China.

This is the first time that China has mentioned a possible timetable regarding its greenhouse gas emissions.

China had previously pledged to cut carbon emissions per unit of economic growth by 40 to 45 percent by 2020 from 2005 levels.

Xu said China would need to spend over 1 billion yuan ($157 million) on energy conservation and developing renewable energy by 2020 to honor its commitment to reduce carbon intensity.

China's carbon emissions are still set to grow at a rapid pace in the coming years, as the country's coal-based energy structure is unlikely to change in the short term, and the country's industrialization and urbanization continue, Xu said.

"The most optimistic studies I've seen say that China's carbon emissions will peak around 2030," he said, "while the majority estimate the peak will come between 2035 and 2045".

"So it is more reasonable for China to set a post-2020 target to restrict its carbon emissions, rather than a reduction goal," he said.

But the Chinese government is already considering controlling total energy consumption in the 12th Five-Year Plan (2011-2015), according to Xu.

"The goal is to restrict total energy consumption at around 4 to 4.2 billion tons of coal equivalent by 2015," said Xu.

The country consumed 3.2 billion tons of standard coal in 2010, about 46 percent of the global total.

The National Energy Bureau is calculating detailed targets for different regions based on their own development levels, according to Xu.

Non-fossil fuels will be exempted from the cap as a policy incentive to encourage the use of renewable energy.

And the bureau is also mulling a plan to set regional electricity consumption caps, according to Xu.

"This is because electricity consumption is relatively easy to monitor, report and verify, and it is also very closely linked with coal consumption," he explained, ruling out special caps on coal consumption.

Meanwhile, leaders from major environmental groups have sent a letter to US Secretary of State Hillary Clinton, expressing concern about the negotiating positions held by the United States and urging the country to take a leadership role in the two-week long climate talks in Durban.

The letter, signed by 16 executives from major non-governmental organizations such as Greenpeace and Oxfam, criticized the US President Barack Obama for failing to commit to his promise.

"Once I take office, you can be sure that the United States will once again engage vigorously in these negotiations and help lead the world toward a new era on global cooperation on climate change," Obama said in a 2008 speech.

However, three years later, the US risks being viewed not as a global leader on climate change, but as "a major obstacle to progress", the letter said.