Observations of radiocarbon (¹⁴C) in atmospheric CO₂ confirm decline in U.S. emissions of fossil fuel CO₂ (FFCO₂) between 2010 and 2015

Abstract. All emissions pathways aimed at stabilizing global temperatures at the internationally agreed target of 1.5–2 °C above pre-industrial levels require steep cuts in global CO₂ emissions. Reliable emissions tracking is therefore essential to monitoring progress towards related mitigation goals, especially for the world’s largest emitters. Here we make use of atmospheric measurements of Δ¹⁴CO₂ and the dual-tracer Δ¹⁴CO₂:CO₂ assimilation and inversion system previously developed by our group to estimate annual and monthly CO₂ emissions from fossil fuel use and cement production (FFCO₂) for the U.S. for 2010 and 2015, the first two individual years for which large numbers of atmospheric Δ¹⁴CO₂ measurements are available. Ensemble-mean national FFCO₂ totals obtained from a 9-member suite of inverse results are larger than reported by the U.S. Environmental Protection Agency (EPA), but overlap at their respective 2σ ensemble-wide model spreads (inversions) and reported 95 % confidence intervals (EPA) in both years. In contrast, the inverse results agree with both annual totals and 16 of 24 derived monthly totals from the Vulcan 3.0 emissions data product with 1σ, ensemble wide. Central estimates of the change in U.S. FFCO₂ emission between 2010 and 2015 range from -5.1 % (EPA), -5.8 % (Vulcan), and -7.8 % (this work), providing a first confirmation of an expected national FFCO₂ decline based on atmospheric observations. The development of a reliable emissions tracking system based directly on atmospheric observations, as detailed here, may take on additional scientific and policy relevance given the recent interruption of EPA reporting.