Potential future changes in water limitations of the terrestrial biosphere

This study explores the effects of atmospheric CO2 enrichment and climate change on soil moisture (W (r) ) and biome-level water limitation (L (TA)), using a dynamic global vegetation and water balance model forced by five different scenarios of change in temperature, precipitation, radiation, and atmospheric CO2 concentration, all based on the same IS92a emission scenario. L (TA) is defined as an index that quantifies the degree to which transpiration and photosynthesis are co-limited by soil water shortage (high values indicate low water limitation). Soil moisture decreases in many regions by 2071-2100 compared to 1961-1990, though the regional pattern of change differs substantially among the scenarios due primarily to differences in GCM-specific precipitation changes. In terms of L (TA), ecosystems in northern temperate latitudes are at greatest risk of increasing water limitation, while in most other latitudes L (TA) tends to increase (but again varies the regional pattern of change among the scenarios). The frequently opposite direction of change in W (r) and L (TA) suggests that decreases in W (r) are not necessarily felt by actual vegetation, which is attributable mainly to the physiological vegetation response to elevated CO2. Without this beneficial effect, the sign of change in L (TA) would be reversed from predominantly positive to predominantly negative.