BBC: Scientists are calling on world leaders to take action on climate change after carbon dioxide levels in the atmosphere broke through a symbolic threshold.
Daily CO2 readings at a US government agency lab on Hawaii have topped 400 parts per million for the first time.
Sir Brian Hoskins, the head of climate change at the UK-based Royal Society, said the figure should "jolt governments into action".
China and the US have made a commitment to co-operate on clean technology.
But BBC environment...