Global warming feedbacks on terrestrial carbon uptake under the Intergovernmental Panel on Climate …
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GLOBAL WARMING FEEDBACKS ON TERRESTRIAL CARBON UPTAKE Page 3.
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GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 15, NO. 4, PAGES 891 – 907, DECEMBER 2001
Global warming feedbacks on terrestrial carbon uptake under the Intergovernmental Panel on Climate Change (IPCC) emission scenarios
Fortunat Joos,1 I. Colin Prentice,2 Stephen Sitch,3 Robert Meyer,1 Georg Hooss,4 Gian-Kasper Plattner,1 Stefan Gerber,1 and Klaus Hasselmann4
Abstract. A coupled physical-biogeochemical climate model that includes a dynamic global vegetation model and a representation of a coupled atmosphere-ocean general circulation model is driven by the nonintervention emission scenarios recently developed by the Intergovernmental Panel on Climate Change (IPCC). Atmospheric CO2, carbon sinks, radiative forcing by greenhouse gases (GHGs) and aerosols, changes in the fields of surface-air temperature, precipitation, cloud cover, ocean thermal expansion, and vegetation structure are projected. Up to 2100, atmospheric CO2 increases to 540 ppm for the lowest and to 960 ppm for the highest emission scenario analyzed. Sensitivity analyses suggest an uncertainty in these projections of À10 to +30% for a given emission scenario. Radiative forcing is estimated to increase between 3 and 8 W m-2 between now and 2100. Simulated warmer conditions in North America and Eurasia affect ecosystem structure: boreal trees expand poleward in high latitudes and are partly replaced by temperate trees and grasses at lower latitudes. The consequences for terrestrial carbon storage depend on the assumed sensitivity of climate to radiative forcing, the sensitivity of soil respiration to temperature, and the rate of increase in radiative forcing by both CO2 and other GHGs. In the most extreme cases, the terrestrial biosphere becomes a source of carbon during the second half of the century. High GHG emissions and high contributions of non-CO2 agents to radiative forcing favor a transient terrestrial carbon source by enhancing warming and the associated release of soil carbon.
1. Introduction
The urgency of climate policy actions depends on how future concentrations of greenhouse gases (GHGs) and global climate change are likely to evolve in the absence of GHG emission control and how global climate change may affect the world’s ecosystems and the services they provide. Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas (GHG). Currently, only about half of the anthropogenic CO2 emission stays airborne. The rest is taken up by the terrestrial biosphere and the ocean. Global climate change due to increased GHG concentrations has the potential to reduce these natural CO2 sinks [e.g., Cao and Woodward, 1998; Sarmiento and Le Quere, 1996; Joos et al., 1999b; ´´ Cramer et al., 2000] and to affect ecosystem structure [Smith and Shugart, 1993] in ways that could enhance or reduce carbon uptake [Cramer et al., 2001; Smith and Shugart, 1993; Cox et al., 2000]. However, projections of future atmospheric CO2 and climate are rendered uncertain by our understanding of how the mechanisms driving oceanic and terrestrial carbon sequestration are influenced by a changing environment. We investigated possible feedbacks between global climate change and the terrestrial system and their impacts on projections
1 2
Climate and Environmental Physics, Bern, Switzerland. Max Planck Institute for Biogeochemistry, Jena, Germany. 3 Potsdam Institute for Climate Impact Research, Potsdam, Germany. 4 Max Planck Institute for Meteorology, Hamburg, Germany. Copyright 2001 by the American Geophysical Union. Paper number 2000GB001375. 0886-6236/01/2000GB001375$12.00
of future changes in climate and ecosystem structure for the new emission scenarios developed by the writing team of the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) [Nakicenovic et al., 2000]. The ´ ´ feedbacks involve atmospheric CO2, other radiative forcing agents, changes in surface temperature, the hydrological cycle, and the carbon cycle. Rising atmospheric concentrations of CO2 and other GHGs lead to increased radiative forcing, higher surface-air temperatures, and changes in the hydrological cycle [Houghton et al., 1996]. Such changes may cause increased respiration of the carbon stored in soil and litter owing to higher bacterial activities at higher temperatures [Lloyd and Taylor, 1994; Rustad, 2000; Cox et al., 2000], reduced net primary production because of excessively high temperatures and/or reduced water availability [Cramer et al., 2001], and dieback of extant forests in response to heat or drought stress [Cramer et al., 2001; Smith and Shugart, 1993; Cox et al., 2000], thereby offsetting the carbon uptake stimulated by the increase in atmospheric CO2 [Farquhar et al., 1980] and, in some regions, by climate change. Although there is still considerable uncertainty about the relative magnitudes of these processes, paleodata clearly indicate that terrestrial carbon storage has varied under different climatic regimes [e.g., Crowley, 1995] and that the terrestrial system can show a substantial response to climatic shifts within a few decades [e.g., MacDonald et al., 1993; Mayle and Cwyner, 1995; Birks and Ammann, 2000]. Most modeling studies to date have only partly addressed the feedback loops described above by prescribing atmospheric CO2 and climate in dynamic global vegetation models (DGVMs) [Cramer et al., 2001] or in simpler terrestrial carbon models that prescribe the ecosystem distribution as constant in time [Cao and Woodward, 1998; Meyer et al., 1999]. These studies show that the
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