On the evidence of a trend in the CO2 airborne fraction

Testing the hypothesis of the presence of a trend in the airborne fraction (AF), defined as the fraction of CO2 emissions remaining in the atmosphere, has attracted much attention, with the overall consensus that no statistical evidence is found for a trend in the data (Knorr, 2009; Gloor et al., 2010; Raupach et al., 2014; Bennedsen et al., 2019). In a paper recently published in this journal, van Marle et al. (van Marle et al., 2022) introduce a new data set for land use and land cover change CO2 emissions (LULCC), and from a Monte Carlo simulation analysis of these data, the authors conclude that there is a negative trend in the AF. We argue that the Monte Carlo design of van Marle et al. is not conducive to determine whether there is a trend in the AF and therefore presents no compelling statistical evidence. A re-examination of the data using a variety of statistical tests finds no evidence of a trend on the whole sample and some evidence of a positive trend when a break in the level of the AF is accounted for.