The Lateral Carbon Pump, and the European Carbon Balance

Comparing atmospheric inversion estimates of the carbon fluxes of continents with bottom-up estimates (Pacala et al. 2001; Janssens et al. 2003; Peylin et al. 2005) is no easy task because (1) inversion fluxes always contain a certain amount of a priori information from bottom-up studies, so that the two approaches are not independent, (2) the time period for which inversion models and bottom-up estimates are produced is generally not the same, in the presence of substantial interannual variability, and (3) lateral carbon displacement makes some bottom-up estimates differ from inversions. Lateral displacement processes form a “carbon pump” which moves carbon away from the area where CO2 was fixed from the atmosphere by photosynthesis with a very small additional sink from rock weathering. Lateral pumping of carbon implies that regional changes in carbon storage must differ from regional mean CO2 fluxes (Tans et al. 1995; Sarmiento and Sundquist 1992).