China has unveiled its first firm target for limiting greenhouse gas emissions, two weeks before a global summit on climate change in Copenhagen.
Beijing said it would aim to reduce its "carbon intensity" by 40-45% by the year 2020, compared with 2005 levels.
Carbon intensity, China's preferred measurement, is the amount of carbon dioxide emitted for each unit of GDP.
But our correspondent says it does not mean China's overall levels of carbon dioxide will start falling.
Its economy is still growing and is mostly fuelled by polluting coal, says the BBC's Quentin Sommerville in Beijing.
It will be at least a couple of decades before China's emissions peak, so it is likely to remain the largest polluter for some time to come, he adds.
But greenhouse gas emissions in China have not been rising as fast as its economy has been growing.