On the evidence of a trend in the CO2 airborne fraction

In their recent publication, van Marle et al.5 introduce a new dataset 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. To carry out hypothesis testing using a Monte Carlo approach, the data-generating process is simulated under the null hypothesis (no trend), and the test statistic is calculated for a large number of simulated trajectories. [...]the frequency of detected trends in the design of van Marle et al. cannot be interpreted as a probability under either hypothesis. Online content Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41586-023-05871-6.