Scientists call for action to tackle CO2 levels

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".

Carbon dioxide is regarded as the most important of the manmade greenhouse gases blamed for raising the temperature on the planet over recent decades.

Human sources come principally from the burning of fossil fuels such as coal, oil and gas.