China's carbon emissions will start falling by 2050, its top climate change policymaker said, the first time the world's largest emitter has given such a time-frame.

Whether China will agree to some kind of cap on its emissions is a critical question ahead of global climate change talks in December in Copenhagen. Beijing argues, as do most developing countries, that developed nations should take responsibility for cutting emissions first, since global warming originated with their industrialisation.

The comments by Su Wei, director-general of the climate change department at China's planning body – the National Development and Reform Commission – signal not only increasing flexibility in Beijing's approach but also continued unreadiness to accept an emissions ceiling in the short term.