Global warming may lead to an ice-free Arctic sooner than previously thought, a new study suggests.

The research comes from an international team of researchers who examined sediment cores from Lake El'gygytgyn in 2009, which is the oldest deep lake in the Arctic region of northeast Russia. They found that about 3.6 million years ago, when carbon dioxide levels were not much higher than they are now, the Arctic underwent a prolonged period of warm temperatures.

"There was probably no sea ice, and the whole Arctic was pretty well forested, so it was a very different world," lead author and professor in the department of geosciences at the University of Massachusetts Amherst Julie Brigham-Grette told LiveScience. "So, how did we go from that to the tundra that we have today, and what does this tell us about the future?"

In coming to their conclusion, the researchers examined fossil pollen in the sediment core, finding traces of hemlock and Douglas fir. By learning more about the time period's vegetation, they were able to gain a clearer understanding of the Arctic's climate and precipitation. The scientists knew that carbon dioxide levels during this warm period were similar to what they are now.

""We can see that the Arctic is quite sensitive to CO2 changes, and levels in the Pliocene were thought to be similar to today," Brigham-Grette said. "Some of the changes we see going on now — sea ice melting, tree lines migrating and glaciers with tremendous ablation rate — suggest that we're heading back to the Pliocene."

"The ice is melting at all elevations," geological sciences professor at the University of Colorado Boulder Gifford Miller added. "Even if there is no additional warming, it's only a matter of time before the ice is all gone."

The study was published on Thursday May 9 in the journal Science.