Delegates for the United Nations COP21 climate change summit will soon be arriving in Paris, hoping to broker an agreement that will shape the future of our climate.

Nature special: 2015 Paris climate talks

Central to their negotiations is a temperature target: stop the world warming by more than 2 °C above preindustrial levels. Politicians established that threshold five years ago at the COP16 in Cancun, Mexico, and hope it will prevent the worst impacts of climate change.

Scientific support for this target, however, is mixed. Many consider it too high, and favour a 1.5 °C limit. Others believe it is too optimistic, or too simplistic. In this Nature Video, reporter Adam Levy investigates the basis of the 2 °C limit — and how carbon we have left to burn before we breach it.

Next video: Paris climate talks: the consequences of climate change