Rising levels of carbon dioxide in the atmosphere have reached a new milestone, prompting renewed warnings of the "huge risks" of climate change.
Atmospheric carbon dioxide is now at 400 parts per million (ppm), higher than at any time in human history, according to preliminary figures from the US National Oceanic and Atmospheric Association monitoring station in Hawaii.
Levels of the key greenhouse gas have risen from 270ppm before the Industrial Revolution as a result of the burning of fossil fuels and deforestation.
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Carbon dioxide was last at current levels three million years ago, when average temperatures were 2˚C to 3˚C higher and sea levels were 20 metres higher than at present, according to Bob Ward, a climate change researcher at the London School of Economics.
"We are creating a prehistoric climate in which human societies will face huge and potentially catastrophic risks," he said.
"Only by urgently reducing global emissions will we be able to bring carbon dioxide levels down and avoid the full consequences of turning back the climate clock by three million years."
Greenpeace UK executive director John Sauven said: "The implications are grave – we're facing major adverse planetary changes.
"Our seasons are shifting faster than we thought possible."
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