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ABSTRACT

Averaged annual accumulation of CO2 in the atmosphere, dCa/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), dCa/dt increases are well represented by linear regression, using direct monitoring records from 1990 or 1965, also from pre-industrial times using archived air. In contrast, modelled atmospheric trends due to reported emissions dCE/dt (assuming historically consistent oceanic and terrestrial uptake mechanisms), agree with dCa/dt or dCa/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.