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HARVARD EPS 5 - Forests and Climate Change

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DOI: 10.1126/science.1155121 , 1444 (2008); 320Science et al.Gordon B. Bonan,and the Climate Benefits of ForestsForests and Climate Change: Forcings, Feedbacks,This copy is for your personal, non-commercial use only.. clicking herecolleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others. herefollowing the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles (this information is current as of November 6, 2010 ):The following resources related to this article are available online at of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, can be found at: Supporting Online Materialfound at: can berelated to this articleA list of selected additional articles on the Science Web sites, 14 of which can be accessed for free: cites 45 articlesThis article 103 article(s) on the ISI Web of Science. cited byThis article has been 8 articles hosted by HighWire Press; see: cited byThis article has been Science : subject collectionsThis article appears in the following registered trademark of AAAS. is aScience2008 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on November 6, 2010 www.sciencemag.orgDownloaded fromREVIEWForests and Climate Change:Forcings, Feedbacks, and theClimate Benefits of ForestsGordon B. BonanThe world’s forests influence climate through physical, chemical, and biological processes thataffect planetary energetics, the hydrologic cycle, and atmospheric composition. These complex andnonlinear forest-atmosphere interactions can dampen or amplify anthropogenic climate change.Tropical, temperate, and boreal reforestation and afforestation attenuate global warming throughcarbon sequestration. Biogeophysical feedbacks can enhance or diminish this negative climateforcing. Tropical forests mitigate warming through evaporative cooling, but the low albedo ofboreal forests is a positive climate forcing. The evaporative effect of temperate forests isunclear. The net climate forcing from these and other processes is not known. Forests are undertremendous pressure from global change. Interdisciplinary science that integrates knowledge of themany interacting climate services of forests with the impacts of global change is necessary toidentify and understand as yet unexplored feedbacks in the Earth system and the potential offorests to mitigate climate change.Forests cover ~42 million km2in tropical,temperate, and boreal lands, ~30% of theland surface (Fig. 1A). These forests pro-vide ecological, economic, social, and aestheticservices to natural systems and humankind (1),including refuges for biodiversity, provision offood, medicinal, and forest products, regulation ofthe hydrologic cycle, protection of soil resources,recreational uses, spiritual needs, and aesthetic val-ues. Additionally , forests influence climate throughexchanges of energy, water, carbon dioxide, andother chemical species with the atmosphere.Forests store ~ 45% of terrestrial carbon (Fig.1B), contribute ~50% of terrestrial net primaryproduction (2), and can sequester large amountsof carbon annually (Fig. 1C). Carbon uptake byforests contributed to a “residual” 2.6 Pg C year−1terrestrial carbon sink in the 1990s, ~33% of anthro-pogenic carbon emission from fossil fuel and land-use change (3). Forests have low surface albedo andcan mask the high albedo of snow (Fig. 1D),which contributes to planetary warmin g t h r o ughincreased solar heating of land. Forests sustain thehydrologic cycle through evapotranspiration, whichcools climate through feedbacks with clouds andprecipitation. The ratio of evapotranspiration toavailable energy is generally low in forest com-pared with some crops and lower in conifer forestthan in deciduous broadleaf forest (Fig. 1E).That forests influence climate has long beenpostulated. From the onset of European settle-ment of North America, it was believed that clear-ing of forests for cultivation, wood products, andsettlement altered climate (4). Today, scientistshave a diverse array of methodologies, includingeddy covariance flux towers, free-air CO2enrich-ment systems, satellite sensors, and mathematicalmodels to investigate the coupling between for-ests and the atmosphere. It is now understood thatforests and human uses of forests provide im-portant climate forcings and feedbacks (3), thatcli mat e change may adversely affect ecosystemfunctions (5), and that forests can be managed tomitigate climate change (6). What is lacking, how-eve r, is science that integrates the many interact-ing climate services of forests with the impacts ofglobal change to inform climate change mitigationpolicy.Accordingly, this article reviews biosphere-atmosphere interactions in tropical, temperate, andboreal forests. Emphasis is placed on biogeo-physical processes (albedo and evapotranspiration)(7), their comparison with biogeochemical pro-cesses (carbon cycle) (8), and alteration of forest-atmosphere coupling through biogeographicalprocesses (land use and vegetation dynamics) (9).The Ecology of Climate ModelsThe influence of forests on large-scale climateis difficult to establish directly through obser-vations. Careful examination of climatic datacan sometimes reveal an ecological influence,such as the effect of leaf emergence on spring-time evapotranspiration and air temperature.Eddy covariance flux towers and field exper-iments provide local-scale insight to forest-atmosphere interactions, and advances in remotesensing science can aid extrapolation of thisknowledge to larger spatial scales. More often,however , our understanding of how forests af-fect climate comes from atmospheric models andtheir numerical parameterizations of

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