A new historical record of carbon dioxide levels suggests current political targets on climate may be "playing with fire", scientists say.
Data came from samples brought up by the drilling ship Joides Resolution
Levels similar to those now commonly regarded as adequate to tackle climate change were associated with sea levels 25-40m (80-130 ft) higher than today.
Scientists write in the journal Science that this extends knowledge of the link between CO2 and climate back in time.
The last 800,000 years have been mapped relatively2 well from ice cores drilled in Antarctica, where historical temperatures and atmospheric3 content have left a series of chemical clues in the layers of ice.
But looking back further has been more problematic; and the new record contains much more precise estimates of historical records than have been available before for the 20 million year timeframe.
The new research was able to look back to the Miocene period, which began a little over 20 million years ago.
At the start of the period, carbon dioxide concentrations in the atmosphere stood at about 400 parts per million (ppm) before beginning to decline about 14 million years ago - a trend that eventually led to formation of the Antarctic icecap and perennial4（四季不断的，连续多年的） sea ice cover in the Arctic.
The high concentrations were probably sustained by（由于……蒙受损失） prolonged（持续很久的） volcanic5 activity in what is now the Columbia River basin of North America, where rock formations called flood basalts（玄武岩） relate a history of molten rock flowing routinely onto the planet's surface.
In the intervening millennia6（一千年）, CO2 concentrations have been much lower; in the last few million years they cycled between 180ppm and 280ppm in rhythm with the sequence of ice ages and warmer interglacial periods（间冰期）.
"What we have shown is that in the last period when CO2 levels were sustained at levels close to where they are today, there was no icecap on Antarctica and sea levels were 25-40m higher," said research leader Aradhna Tripati from the University of California at Los Angeles (UCLA).
"At CO2 levels that are sustained at or near modern day values, you don't need to have a major change in CO2 levels to get major changes in ice sheets," she told BBC News.
The elevated CO2 and sea levels were associated with temperatures about 3-6C (5-11F) higher than today.
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