US fossil-CO2 emissions from 2010 - 2019 based on measurements of atmospheric radiocarbon

Despite inventory-based (“bottom-up”) estimates of fossil-fuel emissions likely being accurate to within 10% for most developed countries, estimates of emissions based on atmospheric measurements are needed to bolster confidence in bottom-up estimates as well as possibly reduce uncertainty and provide emissions estimates closer to real time.  In the case of CO2, however, measurements of CO2 concentrations alone, whether from in situ or remote sensing approaches, are not sufficient to determine fossil emissions because of the confounding influence of terrestrial ecosystem CO2 exchange which influences, and even dominate, spatio-temporal gradients of CO2.  The rarest isotope of C, 14C (radiocarbon), is completely absent from fossil CO2 because of radioactive decay allowing us to use precise measurements of atmospheric CO2 and its 14C:C ratio to isolate the fossil contribution to observed gradients.  Within the context of an atmospheric inverse model, CO2 and 14C:C measurements provide constraints that yield optimized fluxes of both fossil and ecosystem CO2.  Using a global, but US-focused, radiocarbon measurement and modeling system developed over the past 20 years, we are now able to estimate seasonal fossil CO2 emissions for different regions the U.S. between 2010 and 2019 with an estimated national, annual random error of ~ 2%.   Our top-down estimates for 2010 and 2015 show a significant decrease in US fossil CO2 emissions in line with, yet independent from, the inventory-based estimates from the US EPA.  Despite some limitations of top-down emissions related to systematic errors in atmospheric transport models, we expect that top-down year to year flux changes should be robust with respect to transport error, as has been shown previously in model intercomparison studies.  Indeed, while our 2015 – 2010 decrease matches the EPA well, mean emissions are offset, although within 95% confidence intervals. Spatially, our results show that the eastern third of the US dominates national emissions and also dominates the observed decrease, consistent with the understanding that US emissions have decreased since their peak mainly as a result of a transition from coal- to gas-fired power plants, which are dominantly in this area.  Here, we will compare our 2010-2019 emissions estimates with both the EPA’s official US inventory and other bottom-up estimates such as the Vulcan data product that were not used as a priori information in our inverse modeling system.  We will also discuss the steps required to enhance our US-focused system and the extent to which our system might be applied to regions in other parts of the world.