Herbivores eat away at climate-change predictions

By David Robson Large herbivores are likely to have a bigger influence on the fate of our planet than we thought. Current climate simulations predict an increase in shrub-like vegetation in northern regions as the world heats up, and these plants should absorb some of the carbon dioxide from the atmosphere and help buffer further temperature increase. Now it seems that grazers are set to eat this potential carbon sponge. These 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 for longer than grasses, which have leafy stems that tend to decompose more quickly. “Woody plants suck carbon out of the atmosphere as they grow,” says Eric Post from Penn State University in Pennsylvania. “But they get beaten back pretty severely by large animals.” To investigate, Post placed 25 squat, glass cones in West Greenland. These had open tops that allow caribou and muskoxen 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 muskoxen 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 he suggests previous models may have overestimated the global “carbon sponge” effect of vegetation by 10%. Peter Convey, a climatologist from the British Antarctic Survey in Cambridge, agrees that 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 says. Journal reference: Proceedings of the National Academy of Science (DOI:
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