Bubbles of air in the 800,000-year-old ice, drilled in the Antarctic, show levels of CO2 changing with the climate. But the present levels are out of the previous range.
"It is from air bubbles that we know for sure that carbon dioxide has increased by about 35% in the last 200 years," said Dr Eric Wolff of the British Antarctic Survey.
"We really are in a situation where something is happening that we don’t have any analogue for in our records. It is an experiment that we don’t know the result of," he added.