London, August 19 : Though current climate models predict an increase in shrub-like vegetation in northern regions, which should absorb some of the carbon dioxide (CO2) from the atmosphere, grazing animals are likely to eat them in large quantities.
According to a report in New Scientist, climate models assume that in a warmer climate, shrubs, which capture and store carbon in their woody stems, will replace grasses as the most common type of vegetation.
Wood stores carbon far longer than grasses, which have leafy stems that tend to decompose more quickly.
"Woody plants suck carbon out of the atmosphere as they grow," said Eric Post from Penn State University in Pennsylvania. "But they get beaten back pretty severely by large animals," he added.
To investigate, Post placed 25 squat, glass cones in West Greenland.
These had open tops that allow caribou and musk oxen to graze over the sides and glass to prevent a breeze from blowing across the ground, trapping warm air to mimic the effects of global warming.
Of the 25 cones, 12 were fenced off from grazing animals.
Post measured the amount of shrub-like vegetation, such as dwarf birch and willow trees, in the area over 5 years.
As expected, shrub biomass increased by 85% in the warmed areas compared with control areas. But the caribou and musk oxen reduced this increase by 19% in unfenced areas.
Post hasn't yet plugged his data into a climate simulation, and he suggests the true impact may be hard to predict, since he has only studied one area with just two species of grazers.
In addition, it's currently not clear how the populations of grazing animals will change with global warming.
But as a conservative guess, Post suggests previous models may have overestimated the global "carbon sponge" effect of vegetation by 10%.
According to Peter Convey, a climatologist from the British Antarctic Survey in Cambridge, it's difficult to estimate exactly what effect this could have on future climate simulations.
"As soon as you kick an ecosystem, it's difficult to know exactly how it will respond. But biological systems are poorly represented in climate models - anything that improves the accuracy is positive," he said.