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BIOLOGICAL SCIENCES / SUSTAINABILITY SCIENCE
Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks
aGlobal Carbon Project, Commonwealth Scientific and Industrial Research Organisation Marine and Atmospheric Research, GPO Box 3023, Canberra ACT 2601, 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 des Sciences du Climat et de l'Environnement, Commissariat a L'Energie Atomique, 91191 Gif sur Yvette, France; gNational Oceanic and Atmospheric Administration Earth System Research Laboratory, Boulder, CO 80305; hWoods Hole Research Center, Falmouth, MA 02540; iCarbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831; and jInternational Institute for Applied Systems Analysis, A-2361 Laxenburg, Austria
Edited 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 largest human contributor to human-induced climate change, is increasing rapidly. Three processes contribute to this rapid increase. Two of these processes concern emissions. Recent growth of the world economy combined with an increase in its carbon intensity have led to rapid growth in fossil fuel CO2 emissions since 2000: comparing the 1990s with 2000–2006, the emissions growth rate increased from 1.3% to 3.3% y–1. The third process is indicated by increasing evidence (P = 0.89) for a long-term (50-year) increase in the airborne fraction (AF) of CO2 emissions, implying a decline in the efficiency of CO2 sinks on land and oceans in absorbing anthropogenic emissions. Since 2000, the contributions of these three factors to the increase in the atmospheric CO2 growth rate have been 
65 ± 16% from increasing global economic activity, 17 ± 6% from the increasing carbon intensity of the global economy, and 18 ± 15% from the increase in AF. An increasing AF is consistent with results of climate–carbon cycle models, but the magnitude of the observed signal appears larger than that estimated by models. All of these changes characterize a carbon cycle that is generating stronger-than-expected and sooner-than-expected climate forcing.
airborne fraction | anthropogenic carbon emissions | carbon–climate feedback | terrestrial and ocean carbon emissions | vulnerabilities of the carbon cycle
Author 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.
See Commentary on page 18353.
k These growth rates are slightly different from those in ref. 3 because the CDIAC dataset used in ref. 3 was for 2000–2005, and the one used here is for 2000–2006. This update involved revisions to global emissions data for the 1990s, as well as the addition of 2006 data, mainly to resolve earlier discrepancies in emissions data from China (supporting information figure 10 in ref. 3).
This article contains supporting information online at www.pnas.org/cgi/content/full/0702737104/DC1.
l The GWP data used throughout this paper are based on market exchange rates (MER). In ref. 3, we show that our main conclusions, particularly the reversal of the trend in Fig. 1A, are evident using either the MER or purchasing power parity definition for GWP.
bTo whom correspondence should be addressed. E-mail: pep.canadell{at}csiro.au
© 2007 by The National Academy of Sciences of the USA
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