Greenland began heating up around 19,000 years ago at the end of the last ice age, just like the rest of the northern hemisphere, researchers said Thursday in a report that resolves a paradox over when that warming happened.

Previous studies had suggested this warming went back only 12,000 years, according to the study published in the US journal Science.

Huge sheets of ice covered North America and northern Europe some 20,000 years ago during the coldest part of the ice age. At the time, global average temperatures were about four degrees Celsius (seven degrees Fahrenheit) colder than during pre-industrial times.

Then, changes in the Earth's orbit around the sun increased solar energy reaching Greenland beginning some 19,000 years ago, causing the release of carbon from the deep ocean. This led to a gradual rise in atmospheric carbon dioxide (CO2).

In the past, studies of ice cores from Greenland did not show any warming response as would be expected from an increase in CO2 and solar energy flux, said lead author Christo Buizert of Oregon State University.

In the new study scientists reconstructed air temperatures by examining ratios of nitrogen isotopes in air trapped within the ice, rather than in the ice itself, which had been used in past studies.

The new method did in fact detect significant warming in response to increasing atmospheric CO2.

According to this analysis of the period going from 19,000 years ago to 12,000 years ago, Greenland heated up by about five degrees. This is very close to what climate models predict, the researchers said.

This rise heralded the start of the so-called Holocene period, which was warm and stable and allowed human civilization to develop.

"The last deglaciation is a natural example of global warming and climate change," Buizert said. "It is very important to study this period because it can help us better understand the climate system and how sensitive the surface temperature is to atmospheric CO2."