Global response of terrestrial ecosystem structure and function to CO₂ and climate change: results from six dynamic global vegetation models

Abstract

Summary The possible responses of ecosystem processes to rising atmospheric CO2 concentration and climate change are illustrated using six dynamic global vegetation models that explicitly represent the interactions of ecosystem carbon and water exchanges with vegetation dynamics. The models are driven by the IPCC IS92a scenario of rising CO2 (Wigley et al. 1991), and by climate changes resulting from effective CO2 concentrations corresponding to IS92a, simulated by the coupled ocean atmosphere model HadCM2‐SUL. Simulations with changing CO2 alone show a widely distributed terrestrial carbon sink of 1.4–3.8 Pg C y−1 during the 1990s, rising to 3.7–8.6 Pg C y−1 a century later. Simulations including climate change show a reduced sink both today (0.6–3.0 Pg C y−1) and a century later (0.3–6.6 Pg C y−1) as a result of the impacts of climate change on NEP of tropical and southern hemisphere ecosystems. In all models, the rate of increase of NEP begins to level off around 2030 as a consequence of the ‘diminishing return’ of physiological CO2 effects at high CO2 concentrations. Four out of the six models show a further, climate‐induced decline in NEP resulting from increased heterotrophic respiration and declining tropical NPP after 2050. Changes in vegetation structure influence the magnitude and spatial pattern of the carbon sink and, in combination with changing climate, also freshwater availability (runoff). It is shown that these changes, once set in motion, would continue to evolve for at least a century even if atmospheric CO2 concentration and climate could be instantaneously stabilized. The results should be considered illustrative in the sense that the choice of CO2 concentration scenario was arbitrary and only one climate model scenario was used. However, the results serve to indicate a range of possible biospheric responses to CO2 and climate change. They reveal major uncertainties about the response of NEP to climate change resulting, primarily, from differences in the way that modelled global NPP responds to a changing climate. The simulations illustrate, however, that the magnitude of possible biospheric influences on the carbon balance requires that this factor is taken into account for future scenarios of atmospheric CO2 and climate change.

Tropical forests and the global carbon cycle: impacts of atmospheric carbon dioxide, climate change and rate of deforestation

“…Productivity and respiration effects of increased drought are moderated by the direct effect of CO 2 increase (not shown; cf. Cramer et al 2001;Sitch et al 2003). The remaining drought stress reduces NPP slightly after about 2030 and yields increased mortality rates.…”

“…DGVMs combine representations of biogeochemical processes with representations of processes contributing to the dynamics of vegetation structure and composition. Existing DGVMs (reviewed by Cramer et al 2001) differ in their degree of complexity and suitability for different tasks. One common feature is their 'generic' formulation, which makes them suitable for broad-scale assessments in any biome.…”

“…table 1; Houghton 1999;Fearnside 2000;Malhi & Grace 2000;Achard et al 2002;DeFries et al 2002). For some future climate-change scenarios, it has been shown that tropical forests could generate an unprecedented source of carbon, even in the absence of additional anthropogenic deforestation (Cox et al 2000;Cramer et al 2001). Assessing the future conditions of the Earth system requires better quantification of the significance of both direct deforestation and climate-driven changes in biospheric carbon stocks, compared with background 'reference' conditions.…”

An Urban Parameterization for a Global Climate Model. Part I: Formulation and Evaluation for Two Cities

“…Land use/land cover change mechanisms include both transformation of natural land surfaces to those serving human needs (i.e., direct anthropogenic change; e.g., conversion of tropical forest to agriculture) as well as changes in land cover on longer time scales that are due to biogeophysical feedbacks between atmosphere and land (i.e., indirect change; Cramer et al 2001;Foley et al 2005). Global and regional models have been used extensively to investigate effects of direct and indirect land use/land cover change mechanisms on climate (Copeland et al 1996;Betts 2001;Eastman et al 2001;Pielke et al 2002;Feddema et al 2005).…”

Systematic assessment of terrestrial biogeochemistry in coupled climate–carbon models

“…These tests of coupled models build upon an extensive intercomparison and evaluation history within the terrestrial biogeochemistry and land modeling communities (Schimel et al, 1997;Cramer et al, 2001;McGuire et al, 2001;Dargaville et al, 2002;Morales et al, 2005). However, a systematic framework evaluating the coupled behavior of the land carbon system as well as the interaction between climate and land biogeochemistry has been lacking, and is needed to reduce and assess uncertainties associated with future climate change projections.…”