Global temperatures could rise substantially more because of increases in carbon dioxide in the atmosphere than previously thought, according to a new study by US and Chinese scientists.
The researchers used a long-term model for assessing climate change, confirming a similar British study released this month that said calculations for man-made global warming may be underestimated by between 30 and 50 percent.
The new study published online by Nature Geoscience focused on a period three to five million years ago - the most recent episode of sustained global warming with geography similar to today's, a Yale University statement said.
This was in order to look at the Earth's long-term sensitivity to climate fluctuation, including in changes to continental icesheets and vegetation cover on land.
More common estimates for climate change are based on relatively rapid feedback to increases in carbon dioxide, such as changes to sea ice and atmospheric water vapour.
Using sediment drilled from the ocean floor, the scientists' reconstruction of carbon dioxide concentrations found that "a relatively small rise in CO2 levels was associated with substantial global warming 4.5 million years ago."
They also found that the global temperature was between two and three degrees Celsius (3.6 and 5.4 degrees Fahrenheit) higher than today even though carbon dioxide levels were similar to the current ones, the statement said.
"This work and other ancient climate reconstructions reveal that Earth's climate is more sensitive to atmospheric carbon dioxide than is discussed in political circles," said the paper's lead author, Yale's Mark Pagani.
"Since there is no indication that the future will behave differently than the past, we should expect a couple of degrees of continued warming even if we held CO2 concentrations at the current level," he said in the statement.
The study was published on the heels of a 12-day UN conference in Copenhagen that was aimed at providing a durable solution to the greenhouse-gas problem and its disastrous consequences but was labelled a failure by critics.
The meeting set a commitment to limit global warming to two degrees Celsius (3.6 Fahrenheit), but did not spell out the important stepping stones - global emissions targets for 2020 or 2050 - for getting there.
The British study released on December 6 had also researched the Pliocene era, between three to five million years ago.