A recent build-up of atmospheric CO<sub>2</sub> over Europe. Part 1: observed signals and possible explanations

Abstract

We analysed interannual and decadal changes in the atmospheric CO2 concentration gradient (ΔCO2) between Europe and the Atlantic Ocean over the period 1995–2007. Fourteen measurement stations are used, with Mace-Head being used to define background conditions. The variability of ΔCO2 reflects fossil fuel emissions and natural sinks activity over Europe, as well as atmospheric transport variability. The mean ΔCO2 increased by 1–2 ppm at Eastern European stations (∼30% growth), between 1990–1995 and 2000–2005. This built up of CO2 over the continent is predominantly a winter signal. If the observed increase of ΔCO2 is explained by changes in ecosystem fluxes, a loss of about 0.46 Pg C per year would be required during 2000–2005. Even if severe droughts have impacted Western Europe in 2003 and 2005, a sustained CO2 loss of that magnitude is unlikely to be true.We sought alternative explanations for the observed CO2 build-up into transport changes and into regional redistribution of fossil fuel CO2 emissions. Boundary layer heights becoming shallower can only explain 32% of the variance of the signal. Regional changes of emissions may explain up to 27% of the build-up. More insights are given in the Aulagnier et al. companion paper.