Modelling changes in VOC emission in response to climate change in the continental United States

Global Change Biology (1999) 5, 791±806 Modelling changes in VOC emission in response to climate change in the continental United States J O H N V . H . C O N S T A B L E , * A L E X B . G U E N T H E R , ² D A V I D S . S C H I M E L ³ and RUSSELL K. MONSON* *Department of EPO Biology, University of Colorado, Boulder, CO 80309±0334, ²Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307, ³Climate System Modelling Program, University Corporation for Atmospheric Research, Box 3000, Boulder, CO 80307, USA Abstract The alteration of climate is driven not only by anthropogenic activities, but also by biosphere processes that change in conjunction with climate. Emission of volatile organic compounds (VOCs) from vegetation may be particularly sensitive to changes in climate and may play an important role in climate forcing through their in¯uence on the atmospheric oxidative balance, greenhouse gas concentration, and the formation of aerosols. Using the VEMAP vegetation database and associated vegetation responses to climate change, this study examined the independent and combined effects of simulated changes in temperature, CO 2 concentration, and vegetation distribution on annual emissions of isoprene, monoterpenes, and other reactive VOCs (ORVOCs) from potential vegetation of the continental United States. Temperature effects were modelled according to the direct in¯uence of temperature on enzymatic isoprene production and the vapour pressure of monoterpenes and ORVOCs. The effect of elevated CO 2 concentration was modelled according to increases in foliar biomass per unit of emitting surface area. The effects of vegetation distribution re¯ects simulated changes in species spatial distribution and areal coverage by 21 different vegetation classes. Simulated climate warming associated with a doubled atmospheric CO 2 concentration enhanced total modelled VOC emission by 81.8% (iso- prene + 82.1%, monoterpenes + 81.6%, ORVOC + 81.1%), whereas a simulated doubled CO 2 alone enhanced total modelled VOC emission by only + 11.8% (isoprene + 13.7%, monoterpenes + 4.1%, ORVOC + 11.7%). A simulated redistribution of vegetation in response to altered temperatures and precipitation patterns caused total modelled VOC emission to decline by 10.4% (isoprene ± 11.7%, monoterpenes ± 18.6%, ORVOC 0.0%) driven by a decline in area covered by vegetation classes emitting VOCs at high rates. Thus, the positive effect of leaf-level adjustments to elevated CO 2 (i.e. increases in foliar biomass) is balanced by the negative effect of ecosystem-level adjustments to climate (i.e. decreases in areal coverage of species emitting VOC at high rates). Keywords: climate change, isoprene, modelling, monoterpenes, VOC emission. Received 4 June 1998; resubmitted 3 November 1998 and accepted 16 January 1999 Introduction The emission of volatile organic compounds (VOCs, including isoprene, monoterpenes, and others) from terrestrial vegetation plays an important role in the concentrations and lifetimes of atmospheric trace gases Correspondence and present address: Department of Bio- logical Sciences, University of Illinois at Chicago, 845 West Taylor St., Chicago, IL 60607, USA, fax +1/312 413 2435, e-mail constabj@tigger.cc.uic.edu. Blackwell Science Ltd. including CO (Hatakeyama et al. 1991), ozone (Liu et al. 1987), aerosols (Andreae & Crutzen 1997), and OH radical (Jacob & Wofsy 1988), affecting the global carbon cycle (Fehsenfeld et al. 1992). Changes in the concentra- tions of these gases are important for issues relating to local and regional ozone pollution and potential in¯u- ence on climate change. These concerns have prompted the development of national and global inventories of the emission of VOCs into the atmosphere from natural sources which contribute to more than two thirds of the