HARVARD EPS 5 - Contributions to accelerating atmospheric CO2 growth

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Contributions to accelerating atmospheric CO2growthfrom economic activity, carbon intensity, andefficiency of natural sinksJosep G. Canadella,b, Corinne Le Que´re´c,d, Michael R. Raupacha, Christopher B. Fielde, Erik T. Buitenhuisc, Philippe Ciaisf,Thomas J. Conwayg, Nathan P. Gillettc, R. A. Houghtonh, and Gregg Marlandi,jaGlobal Carbon Project, Commonwealth Scientific and Industrial Research Organisation Marine and Atmospheric Research, GPO Box 3023, Canberra ACT2601, Australia;cSchool of Environment Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom;dBritish Antarctic Survey, Madingley Road,Cambridge CB3 0ET, United Kingdom;eDepartment of Global Ecology, Carnegie Institution of Washington, Stanford, CA 94305;fLaboratorie desSciences du Climat et de l’Environnement, Commissariat a L’Energie Atomique, 91191 Gif sur Yvette, France;gNational Oceanic and AtmosphericAdministration Earth System Research Laboratory, Boulder, CO 80305;hWoods Hole Research Center, Falmouth, MA 02540;iCarbon DioxideInformation Analysis Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831; andjInternational Institute for Applied SystemsAnalysis, A-2361 Laxenburg, AustriaEdited by William C. Clark, Harvard University, Cambridge, MA, and approved September 17, 2007 (received for review March 27, 2007)The growth rate of atmospheric carbon dioxide (CO2), the largesthuman contributor to human-induced climate change, is increasingrapidly. Three processes contribute to this rapid increase. Two ofthese processes concern emissions. Recent growth of the worldeconomy combined with an increase in its carbon intensity have ledto rapid growth in fossil fuel CO2emissions since 2000: comparingthe 1990s with 2000–2006, the emissions growth rate increasedfrom 1.3% to 3.3% yⴚ1. The third process is indicated by increasingevidence (P ⴝ 0.89) for a long-term (50-year) increase in theairborne fraction (AF) of CO2emissions, implying a decline in theefficiency of CO2sinks on land and oceans in absorbing anthro-pogenic emissions. Since 2000, the contributions of these threefactors to the increase in the atmospheric CO2growth rate havebeen ⬇65 ⴞ 16% from increasing global economic activity, 17 ⴞ 6%from the increasing carbon intensity of the global economy, and18 ⴞ 15% from the increase in AF. An increasing AF is consistentwith results of climate– carbon cycle models, but the magnitude ofthe observed signal appears larger than that estimated by models.All of these changes characterize a carbon cycle that is generatingstronger-than-expected and sooner-than-expected climate forcing.airborne fraction 兩 anthropogenic carbon emissions 兩 carbon– climatefeedback 兩 terrestrial and ocean carbon emissions 兩 vulnerabilities of thecarbon cycleThe rate of change of atmospheric CO2reflects the balancebet ween anthropogenic carbon emissions and the dynamicsof a number of terrestrial and ocean processes that remove oremit CO2(1, 2). The long-term evolution of this balance willdeter mine to a large extent the speed and magnitude of human-induced climate change and the mitigation requirements tost abilize atmospheric CO2c oncentrations at any given level.In recent years, components of the global carbon balance havechanged substantially with major increases in anthropogenicemissions (3) and changes in land and ocean sink fluxes due toclimate variability and change (4).In this article, we report a number of changes in the globalcarbon cycle, particularly since 2000, with major implications forcurrent and future grow th of atmospheric CO2. To quantify theimport ance of these changes, we update and analyze datasets onCO2emissions from fossil fuel combustion and cement produc-tion (FFoss), CO2emissions from land use change (FLUC), thecarbon intensity of global economic activity, and estimatedtrends in the CO2balance of the oceans and of ecosystems onland.We also quantif y the relative importance of key processesresponsible for the observed acceleration in atmospheric CO2c oncentrations. This attribution provides insights into key lever-age points for management of the carbon c ycle and also indicatesthe present significance of carbon–climate feedbacks associatedwith the long-term dynamics of natural CO2sinks and sources.Results and DiscussionGrowth in Atmospheric CO2. Global average atmospheric CO2rosef rom 280 ppm at the start of the industrial revolution (⬇1,750)to 381 ppm in 2006. The present concentration is the highestduring the last 650,000 years (5, 6) and probably during the last20 million years (7). The growth rate of global average atmo-spheric CO2for 2000–2006 was 1.93 ppm y⫺1[or 4.1 pet agramsof carbon (PgC) y⫺1, Table 1]. This rate is the highest since thebeginn ing of continuous monitoring in 1959 and is a significantincrease over growth rates in earlier decades: the average growthrates for the 1980s and the 1990s were 1.58 and 1.49 ppm y⫺1,respectively (Fig. 1).CO2Emissions. From 1850 to 2006, fossil fuel and cement emis-sions released a cumulative total of ⬇330 PgC to the atmosphere(1 PgC ⫽ 1 petagram or 109metric tons of carbon). A n additional158 PgC came from land-use-change emissions, largely defores-t ation and wood harvest (see Methods for dat a sources anduncert ainties).Fossil fuel and cement emissions (FFoss) increased f rom 7.0PgC y⫺1in 2000 to 8.4 PgC y⫺1in 2006, 35% above emissions in1990. The average FFossfor 2000–2006 was 7.6 ⫾ 0.4 PgC y⫺1(Table 1). The average proportional growth rate of FFosskincreased from 1.3% y⫺1for 1990–1999 to 3.3% y⫺1for 2000–2006 (Fig. 1B).Model-based estimates of emissions from land-use changeAuthor contributions: J.G.C., C.L.Q., M.R.R., C.B.F., and P.C. designed research; J.G.C., C.L.Q.,M.R.R., C.B.F., E.T.B., P.C., T.J.C., R.A.H., and G.M. performed research; J.G.C., C.L.Q., M.R.R.,E.T.B., P.C., T.J.C., N.P.G., R.A.H., and G.M. analyzed data; and J.G.C., C.L.Q., M.R.R., C.B.F.,P.C., and R.A.H. wrote the paper.The authors declare no conflict of interest.This article is a PNAS Direct Submission.Freely available online through the PNAS open access option.Abbreviations: AF, airborne fraction; GWP, gross world product; kgC, kilograms of carbon;PgC, petagrams of carbon.See Commentary on page 18353.bTo whom correspondence should be addressed. E-mail: [email protected] growth rates are slightly different from those in ref. 3 because the CDIAC datasetused in ref. 3 was for 2000 –2005, and the one used


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