Contributions to accelerating atmospheric CO 2 growth from economic activity, carbon intensity, and efficiency of natural sinks

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Abstract
The growth rate of atmospheric carbon dioxide (CO 2 ), 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 CO 2 emissions, implying a decline in the efficiency of CO 2 sinks on land and oceans in absorbing anthropogenic emissions. Since 2000, the contributions of these three factors to the increase in the atmospheric CO 2 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.