The rapid rise in greenhouse gases over the past century is unprecedented in at least 800,000 years, according to a study of the oldest Antarctic ice core.
Air bubbles trapped in ice for hundreds of thousands of years have revealed that the way humans are changing the composition of the
atmosphere has no known natural parallel.
Scientists at the British Antarctic Survey in Cambridge found there had been eight cycles of atmospheric change in the past 800,000 years when carbon dioxide and methane rose. Each time, the world experienced relatively high temperatures.
However, existing levels of carbon dioxide and methane were far higher than anything seen before, said Dr Eric Wolff.
"Over the past 200 years, human activity has increased carbon dioxide to well outside the natural range and we have no analogue for what will happen next," Wolff said.
The ice core was drilled from a thick area of ice known as Dome C. It is nearly 3.2km long and reaches to a depth where air became trapped in ice that formed 800,000 years ago.
"Carbon dioxide has increased by about 35 per cent in the past 200 years. Before that 200 years, which is when man's been influencing the atmosphere, it was pretty steady to within 5 per cent," Wolff said.
The core shows that carbon dioxide was always between 180 parts per million (ppm) and 300ppm during the 800,000 years. However, now it is 380 ppm. Methane was previously never higher than 750 parts per billion (ppb) but now it stands at 1780ppb.
The rate of change is even more dramatic, with increases in carbon dioxide never exceeding 30ppm in 1000 years - and yet now carbon dioxide has risen by 30ppm in 17 years.
"The rate of change is probably the most scary thing because it means that the earth systems can't cope with it," Wolff told a British Association meeting.
"We have little capacity to adapt to changes that are much faster than anything in human experience."
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