UA ECOL 596L - The role of ecosystem-atmosphere interactions

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Theor. Appl. Climatol. 78, 157–175 (2004)DOI 10.1007/s00704-004-0050-y1Hadley Centre for Climate Prediction & Research, Met Office, Exeter, UK2Department of Meteorology, Reading University, UK, now at Met Office Hadley Centre, UK3Centre for Ecology and Hydrology, OX, UKThe role of ecosystem-atmosphere interactionsin simulated Amazonian precipitation decreaseand forest dieback under global climate warmingR. A. Betts1,P.M.Cox1, M. Collins2, P. P. Harris3, C. Huntingford3, and C. D. Jones1With 10 FiguresReceived March 13, 2003; revised September 1, 2003; accepted October 10, 2003Published online May 6, 2004 # Springer-Verlag 2004SummaryA suite of simulations with the HadCM3LC coupledclimate-carbon cycle model is used to examine the variousforcings and feedbacks involved in the simulated precipita-tion decrease and forest dieback. Rising atmospheric CO2is found to contribute 20% to the precipitation reductionthrough the physiological forcing of stomatal closure, with80% of the reduction being seen when stomatal closure wasexcluded and only radiative forcing by CO2was included.The forest dieback exerts two positive feedbacks on theprecipitation reduction; a biogeophysical feedback throughreduced forest cover suppressing local evaporative waterrecycling, and a biogeochemical feedback through therelease of CO2contributing to an accelerated global warm-ing. The precipitation reduction is enhanced by 20% by thebiogeophysical feedback, and 5% by the carbon cycle feed-back from the forest dieback. This analysis helps to explainwhy the Amazonian precipitation reduction simulated byHadCM3LC is more extreme than that simulated in otherGCMs; in the fully-coupled, climate-carbon cycle simula-tion, approximately half of the precipitation reduction inAmazonia is attributable to a combination of physiologicalforcing and biogeophysical and global carbon cycle feed-backs, which are generally not included in other GCMsimulations of future climate change. The analysis alsodemonstrates the potential contribution of regional-scaleclimate and ecosystem change to uncertainties in globalCO2and climate change projections. Moreover, the impor-tance of feedbacks suggests that a human-induced increasein forest vulnerability to climate change may have implica-tions for regional and global scale climate sensitivity.1. IntroductionThe Hadley Centre coupled climate-carbon cyclemodel HadCM3LC (Cox et al., 2000) includes asubmodel of vegetation dynamics and ecosystem-atmosphere carbon exchange. When driven bythe IPCC IS92a emissions scenario, HadCM3LCsimulates major losses of forest cover in theAmazon Basin over the next 50–100 years. Themodel simulates a large decrease in precipitationover much of Amazonia, initiated by an El Ni~nno-like pattern of sea-surface warming in the PacificOcean (Cox et al., this issue). Associated with thisdrying is a large reduction in the coverage ofbroadleaf tree. Across large areas of Amazonia,tree cover is either reduced to savanna proportionsor replaced entirely with shrubs and grasses. Inpart of the basin, even grasses can no longer besupported and the simulated land cover becomessemi-desert.Clearly, this forest ‘‘dieback’’ has catastrophicimplications for the ecology and socio-economicsof Amazonia. It also potentially contributes to amajor positive feedback on atmospheric CO2riseand global climate change (Cox et al., 2000). It istherefore vital to assess the reliability of thesesimulations as indicators of actual future change.Such an assessment requires an understanding andquantification of the processes involved in the cli-mate and vegetation change, so that the impor-tance of uncertainties in representing differentcomponents of the system can be established.This paper examines a number of mechanismsthrough which the forest itself contributes to thesimulated precipitation changes. Both feedbackand forcing mechanisms are examined. While afeedback modifies the changes initiated by anexternal perturbation, a forcing is the direct per-turbation itself. Vegetation can be involved inboth kinds of contribution to climate change.As well as exerting a radiative forcing on theclimate system, increasing concentration of atmo-spheric CO2may also exert a forcing throughdirect effects on plant physiology. A number ofstudies have shown that plant stomata may openless under higher CO2concentrations (Field et al.,1995), which directly reduces the flux of mois-ture from the surface to the atmosphere (Sellerset al., 1996). This can warm the air near the sur-face by increasing the ratio of sensible heat fluxto latent heat flux. In a region such as Amazoniawhere the much of the moisture for precipitationis supplied by evaporation from the land surface,reduced stomatal opening may also contributeto decreased precipitation. Although this can bepartly offset by increases in leaf area (Bettset al., 1997), this offset may not be total. Whileit is yet to be established whether such responsesare universal, it is possible that rising CO2con-centrations could therefore exert two forcings onAmazonian precipitation; (i) radiative forcing ofglobal climate modifying atmospheric circulationpatterns, and (ii) physiological forcing modifyingnear-surface temperature and also reducing thesupply of moisture for precipitation. The possiblerelative importance of these two potential forc-ings on Amazonian precipitation is comparedhere.However, the two forcings exerted by CO2may result in different impacts on the forestitself. While the radiative forcing will affect eco-systems only indirectly via climate, the physio-logical forcing will also affect plant functioningdirectly through increased water use efficiencyand fertilization of photosynthesis. Therefore,the two CO2forcings are also compared in termsof their final effects on forest dieback.The climate of Amazonia can also be affectedby changes in the extent of forest cover. Obser-vational and modelling studies in Amazoniasuggest that the nature of the land cover can sig-nificantly influence the surface energy and mois-ture budgets and the atmospheric circulation.Therefore, reduced forest cover resulting fromdecreasing precipitation could exert feedbackson the precipitation reduction through changesin the physical properties of the land surface.Furthermore, changes in forest cover mayaffect the rate of CO2rise and hence provide afeedback on global climate change. If thestrength of the precipitation reduction relates tothe magnitude of the global


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