In 1981 researchers removed a long tube of ice from the center of a glacier in southern Greenland at a site known as Dye 3. More than a mile (two kilometers) long, the deep end of the core sample had been crushed by the pressure of the ice above it and sullied by contact with rock and soil. By destroying the pattern of annual layers, this contamination seemingly made it impossible to assess the region's ancient climate. But DNA extracted from the previously ignored dirty bottom has revealed that Greenland was not only green, it boasted boreal forests like those found in Canada and Scandinavia today.

Biologist Eske Willerslev of the University of Copenhagen and an international team of colleagues discovered DNA from alder, spruce, pine and yew trees at the glacier's base as well as insects ranging from butterflies to spiders. This is the "first evidence for a forested southern Greenland," Willerslev says. And based on the tree species found, Greenland must have been warmer than 50 degrees Fahrenheit (10 degrees Celsius) in summer and never colder than one degree F (–17 degrees C) in winter, much warmer than present conditions.

Willerslev used four measures (traces of last exposure to sunlight in minerals, how long ago the amino acids were part of a living creature, the relative levels of beryllium and chlorine isotopes, and the "clocks" contained in the DNA) to date the forest to at least 400,000—and possibly as much as 800,000—years ago, the team reports in Science. That means this area of southern Greenland has been continuously coated in ice for at least that long.

The ice sheet on Greenland, therefore, is more stable than some scientists previously believed and "has not contributed to global sea level rise during the last interglacial," Willerslev says. "Importantly, it does not mean that we should not be worried about future global warming as the sea level rise of five to six meters during the last interglacial must have come from somewhere."

Glaciologist Richard Alley of Pennsylvania State University, who was not involved in the study, agrees: "Something else," possibly Antarctica, must have provided the water for global sea level rise "because this observation does not at all affect [that] estimate … only the estimate of where the water came from."

Adds team member and glaciologist Martin Sharp of the University of Alberta in Edmonton: "One could argue that this shows that natural forcing could account for the current warm conditions, but the current orbital configuration does not support this, even when other natural forcings are taken into account. One could also argue that if natural warming can deglaciate much of southern Greenland, then natural warming plus anthropogenic warming could cause even more extensive deglaciation."

Looking at an ice core from a site on the glaciated continent of Antarctica known as Dome C, scientists have stretched the climate record back 800,000 years, tracking eight successive glacial periods, according to another online report in Science. The data shows that natural variations in Earth's orbit—obliquity, or how tilted the planet is in relation to the sun—have determined global temperatures in the past, they report. "This helps us put current warming into context," Sharp notes, "but it really has nothing to say about the mechanisms driving the current warming."

Applying the DNA techniques developed by Willerslev and his colleagues to the silty bottoms of such an Antarctic core may reveal the ice-free history of that continent as well. "In Antarctica, where the ice cover is thin and thus the bedrock is very cold [about –58 degrees F (–50 degrees C)] I think you could go very far back in time," Willerslev says. "What to find? I have no idea." The past, it appears, is on ice and waiting.