Spreading greenery

White spruce Dwarf birch Willow Green alder

At most of the sites, there were definite and sometimes dramatic increases in the size of individual shrubs - mostly alder, birch and willow - and also in the area they covered.

In some cases, shrub cover of 10% had doubled since the first surveys.

"Our study area is in a location where human and natural disturbances are minimal," the researchers write in the scientific journal Nature, "so we attribute much of the increase in the abundance of shrubs to the recent change in climate".

The scientists say the extra biomass could absorb a significant amount of carbon dioxide - the most common greenhouse gas blamed for global warming.

'Unpredictable effects'

Studies of ancient peat deposits suggest that the same thing happened 8,000 years ago, at the end of the last ice age.

Even with the extra plants - which are too small to be valuable as timber - the region beyond Alaska's northernmost mountains could support few people.

If vegetation at lower latitudes, such as agricultural crops, also moves north in response to global warming, more farmland might be developed in areas that are now wilderness.

But the researchers stress that the Arctic regions cannot be expected to compensate for all the unpredictable effects of climate change.