Abstract. Since the first estimate of global CO2 emissions was
published in 1894, important progress has been made in the development of
estimation methods while the number of available datasets has grown. The
existence of parallel efforts should lead to improved accuracy and
understanding of emissions estimates, but there remains significant
deviation between estimates and relatively poor understanding of the reasons
for this. Here I describe the most important global emissions datasets
available today and – by way of global, large-emitter, and case examples – quantitatively compare their estimates, exploring the reasons for
differences. In many cases differences in emissions come down to differences
in system boundaries: which emissions sources are included and which are
omitted. With minimal work in harmonising these system boundaries across
datasets, the range of estimates of global emissions drops to 5 %, and
further work on harmonisation would likely result in an even lower range,
without changing the data. Some potential errors were found, and some
discrepancies remain unexplained, but it is shown to be inappropriate to
conclude that uncertainty in emissions is high simply because estimates
exhibit a wide range. While “true” emissions cannot be known, by comparing
different datasets methodically, differences that result from system
boundaries and allocation approaches can be highlighted and set aside to
enable identification of true differences, and potential errors. This must
be an important way forward in improving global datasets of CO2
emissions. Data used to generate Figs. 3–18 are available at
https://doi.org/10.5281/zenodo.3687042 (Andrew, 2020).