Climate change impacts on ecosystems and the terrestrial carbon sink: a new assessment

Climate output from the UK Hadley Centre's HadCM2 and HadCM3 experiments fo r the period 1860 to 2100, with IS92a greenhouse gas forcing, together with predicted patterns of N deposition and increasing CO2, were input (offline ) to the dynamic vegetation model, Hybrid v4.1 (Friend et al., 1997; Friend and White, 1999). This model represents biogeochemical, biophysical and bi ogeographical processes, coupling the carbon, nitrogen and water cycles on a sub-daily timestep, simulating potential vegetation and transient changes in annual growth and competition between eight generalized plant types in response to climate. Global vegetation carbon was predicted to rise from about 600 to 800 PgC (o r to 650 PgC for HadCM3) while the soil carbon pool of about 1100 PgC decre ased by about 8%. By the 2080s, climate change caused a partial loss of Ama zonian rainforest, C-4 grasslands and temperate forest in areas of southern Europe and eastern USA, but an expansion in the boreal forest area. These changes were accompanied by a decrease in net primary productivity (NPP) of vegetation in many tropical areas, southern Europe and eastern USA tin res ponse to warming and a decrease in rainfall), but an increase in NPP of bor eal forests. Global NPP increased from 45 to 50 PgC y(-1) in the 1990s to a bout 65 PgC y(-1) in the 2080s (about 58 PgC y(-1) for HadCM3). Global net ecosystem productivity (NEP) increased from about 1.3 PgC y(-1) in the 1990 s to about 3.6 PgC y(-1) in the 2030s and then declined to zero by 2100 owi ng to a loss of carbon from declining forests in the tropics and at warm te mperate latitudes - despite strengthening of the carbon sink at northern hi gh latitudes. HadCM3 gave a mon erratic temporal evolution of NEP than HadC M2, with a dramatic collapse in NEP in the 2050s. (C) 1999 Elsevier Science Ltd. All rights reserved.