The dominant role of semi-arid ecosystems in the trend and variability of the land CO 2 sink

The difference is found at the margins The terrestrial biosphere absorbs about a quarter of all anthropogenic carbon dioxide emissions, but the amount that they take up varies from year to year. Why? Combining models and observations, Ahlström et al. found that marginal ecosystems—semiarid savannas and low-latitude shrublands—are responsible for most of the variability. Biological productivity in these semiarid regions is water-limited and strongly associated with variations in precipitation, unlike wetter tropical areas. Understanding carbon uptake by these marginal lands may help to improve predictions of variations in the global carbon cycle. Science , this issue p. 895