A new study of three ice cores from Greenland documents the warming of the large ice sheet at the end of the last ice age -- resolving a long-standing paradox¹ over when that warming occurred. Large ice sheets covered North America and northern Europe some 20,000 years ago during the coldest part of the ice age, when global average temperatures were about four degrees Celsius² (or seven degrees Fahrenheit) colder than during pre-industrial times. And then changes in Earth's orbit around the sun increased the solar energy reaching Greenland. Beginning some 18,000 years ago, release of carbon from the deep ocean led to a graduate rise in atmospheric³ carbon dioxide (CO2).

 

Yet past analysis of ice cores from Greenland did not show any warming response as would be expected from an increase in CO2 and solar energy flux⁴, the researchers note.

 

In this new study, funded by the National Science Foundation and published this week in the journal Science, scientists reconstructed air temperatures by examining ratios of nitrogen isotopes⁶ in air trapped within the ice instead of isotopes in the ice itself, which had been used in past studies.

 

Not only did the new analysis detect significant warming in response to increasing atmospheric CO2, it documents a warming trend at a rate closely matching what climate change models predict should have happened as Earth shifted out of its ice age, according to lead author Christo Buizert, a postdoctoral researcher at Oregon State University and lead author on the Science article.

 

"The Greenland isotope⁵ records from the ice itself suggest that temperatures 12,000 years ago during the so-called Younger Dryas period near the end of the ice age were virtually the same in Greenland as they were 18,000 years ago when much of the northern hemisphere was still covered in ice," Buizert said. "That never made much sense because between 18,000 and 12,000 years ago atmospheric CO2 levels rose quite a bit."

 

"But when you reconstruct the temperature history using nitrogen isotope ratios as a proxy⁷ for temperature, you get a much different picture," Buizert pointed⁸ out. "The nitrogen-based temperature record shows that by 12,000 years ago, Greenland temperatures had already warmed by about five degrees (Celsius), very close to what climate models predict should have happened, given the conditions."

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