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 CO 2 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 Ϸ65 ؎ 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 T he rate of change of atmospheric CO 2 reflects the balance between anthropogenic carbon emissions and the dynamics of a number of terrestrial and ocean processes that remove or emit CO 2 (1, 2). The long-term evolution of this balance will determine to a large extent the speed and magnitude of humaninduced climate change and the mitigation requirements to stabilize atmospheric CO 2 concentrations at any given level.In recent years, components of the global carbon balance have changed substantially with major increases in anthropogenic emissions (3) and changes in land and ocean sink fluxes due to climate variability and change (4).In this article, we report a number of changes in the global carbon cycle, particularly since 2000, with major implications for current and future growth of atmospheric CO 2 . To quantify the importance of these changes, we update and analyze datasets on CO 2 emissions from fossil fuel combustion and cement production (F Foss ), CO 2 emissions from land use change (F LUC ), the carbon intensity of global economic activity, and estimated trends in the CO 2 balance of the oceans and of ecosystems on land.We also quantify the relative importance of key processes responsible for the observed acceleration in atmospheric CO 2 concentrations. This attribution provides insights into key leverage points for management of the carbon cycle and also indicates the present significance of carbon-climate feedbacks associated with the long-term dynamics of natural CO 2 sinks and sources.

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carbon intensity of economy ͉ carbon intensity of energy ͉ emissions scenarios ͉ fossil fuels ͉ Kaya identity