Two kilometres under the ice sheet in Greenland, scientists have found the oldest samples of DNA ever recovered, providing evidence that the island was much warmer hundreds of thousands of years ago than previously thought.

The samples from trees, plants and insects of a boreal forest were taken from "silty ice" at the bottom few metres of an ice core drilled through a glacier in southern Greenland, said University of Alberta glaciologist Martin Sharp.

The evidence of forest life shows that "this part of the world was significantly warmer than most people thought," he said.

The samples have been dated to between 450,000 and 900,000 years ago; previously, there was no evidence of a warm period in Greenland any later than 2.4 million years ago.

The forest life suggests temperature in southern Greenland ranged between 10 C in summer and -17 C in winter.

The warm period was the result of natural forces, so if the current human global warming were reinforced by natural warming, "we may be heading for even bigger temperature increases than we previously thought," Sharp said.

The Earth has been cooling since the the warmest peak of the most recent ice age, about 9,000 years ago. "But on a multi-decadal timescale, at present I think the natural forcing is near neutral," he said.

Sharp is co-author of a study published Thursday in the journal Science. The researchers found the DNA in silty ice that was created when the glacier picked up organic material from the top of the underlying bedrock. The silty ice created a natural freezer that preserved the DNA.

Scientists have found older organic matter, but never found uncontaminated DNA as old as the Greenland samples.

Before the new study, DNA from animals and plants from Siberia and Alaska had been dated to 395,000 years ago.

Sharp and PhD student Joel Barker provided DNA samples from silty ice in Canadian glaciers that were just 3,000 years old. Those samples provided a control for researchers working on the Greenland samples, and supported the idea that the Greenland samples came from local forests and not from material carried by wind or water.