Differences between trends in atmospheric CO₂ and the reported trends in anthropogenic CO₂ emissions

Abstract

Averaged annual accumulation of CO₂ in the atmosphere, dC_a/dt, has been slowing from peak growth in 2002/2003 associated with anomalous climate-induced emissions at high northern latitudes. This slowing is widespread but determined with greatest certainty in the largest well-mixed portion of the global troposphere (30◦S–90◦S). We rely on atmospheric mixing for global integration and selection of atmospheric data for spatial representativeness. Prior to 2002/2003, after empirical adjustment for perturbations associated with ENSO and volcanic activity (EV), dC_a/dt increases are well represented by linear regression, using direct monitoring records from 1990 or 1965, also from preindustrial times using archived air. In contrast,modelled atmospheric trends due to reported emissions dC_E/dt (assuming historically consistent oceanic and terrestrial uptake mechanisms), agree with dC_a/dt or dC_a/dt-EV up until 1990, are near-stable through the 1990s and increase by 29% between 2000 and 2008. Using atmospheric constraints based on trends in both dCa/dt-EV and interhemispheric gradient, the differences between trends in dCE/dt and atmospheric CO₂ growth are most simply explained as an artefact of underestimating 1994–2003 emissions by around 6%. This is achieved with a near constant post-1965 airborne fraction; otherwise unusually complicated sink changes are required for the period.