Conifer forests — really big greens — encroaching on Arctic tundra threaten to further accelerate warming in the far North.

Temperatures at these high latitudes already are climbing "at about twice the global average," notes F. Stuart Chapin of the University of Alaska in Fairbanks.

The newest data on the advance of northern, or boreal, forests come from the eastern slopes of Siberia's Ural Mountains. Here, north of the Arctic Circle, relatively flat mats of compressed, frozen plant matter — tundra — are the norm. This ecosystem hosts a cover of reflective snow most of the year, a feature that helps maintain the region's chilly temperatures. Throughout the past century, however, the leading edges of conifer forests have creeped some 20 to 60 meters up the mountains and begun overrunning tundra, scientists report in an upcoming Global Change Biology, now available online.