Is the airborne fraction of anthropogenic CO 2 emissions increasing?

Wolfgang Knorr

Received 18 August 2009; revised 21 September 2009; accepted 23 September 2009; published 7 November 2009.

[ 1 ] Several recent studies have highlighted the possibility that the oceans and terrestrial ecosystems have started loosing part of their ability to sequester a large proportion of the anthropogenic CO2 emissions. This is an important claim, because so far only about 40% of those emissions have stayed in the atmosphere, which has prevented additional climate change. This study re-examines the available atmospheric CO2 and emissions data including their uncertainties. It is shown that with those uncertainties, the trend in the airborne fraction since 1850 has been 0.7 ± 1.4% per decade, i.e. close to and not significantly different from zero. The analysis further shows that the statistical model of a constant airborne fraction agrees best with the available data if emissions from land use change are scaled down to 82% or less of their original estimates. Despite the predictions of coupled climate-carbon cycle models, no trend in the airborne fraction can be found. Citation: Knorr, W. one of them, the estimated trend reduces to zero. The analysis, however, does not propagate the error in the land use flux to the uncertainty in the trend estimate, nor does it include data from before the late 1950s, when direct CO2 measurements began. According to Etheridge et al. [1996], between 1850 and 1960 atmospheric CO 2 increased by 66 GtC (from 285 to 316 ppm, with 2.13 GtC per ppm [Trenberth, 1981]), which is 41% of the total emissions of 162 GtC during the same period [Boden et al., 2009; Houghton, 2008].

[ 4 ] The purpose of the present study is to extend the trend analysis of the AF back to 1850, to re-do the analysis with data uncertainties and to re-examine the use of predictors of interannual variability. The results of several approaches are compared to re-consider the significance of any observed trend in the AF.

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