Seven years of recent European net terrestrial carbon dioxide exchange constrained by atmospheric observations

Abstract

We present an estimate of net ecosystem exchange (NEE) of CO₂ in Europe for the years 2001 through 2007. It is derived with a data assimilation that uses a large set of atmospheric CO₂ mole fraction observations (<70 000) to guide relatively simple descriptions of terrestrial and oceanic net exchange, while fossil fuel and fire emissions are prescribed. Weekly terrestrial sources and sinks are optimized (i.e., a flux inversion) for a set of 18 large ecosystems across Europe in which prescribed climate, weather, and surface characteristics introduce finer scale gradients. We find that the terrestrial biosphere in Europe absorbed a net average of 2212165 TgC yr22121 over the period considered. This uptake is predominantly in non-EU countries, and is found in the northern coniferous (221294 TgC/yr) and mixed forests (221230 TgC yr22121) as well as the forest/field complexes of eastern Europe (221285 TgC yr22121). An optimistic uncertainty estimate derived using three biosphere models suggests the uptake to be in a range of 2212122 to 2212258 TgC yr22121, while a more conservative estimate derived from the a-posteriori covariance estimates is 2212165±437 TgC yr22121. Note however that uncertainties are hard to estimate given the nature of the system and are likely to be significantly larger than this. Interannual variability in NEE includes a reduction in uptake due to the 2003 drought followed by three years of more than average uptake. The largest anomaly of NEE occurred in 2005 concurrent with increased seasonal cycles of observed CO₂. We speculate these changes to result from the strong negative phase of the North Atlantic Oscillation in 2005 that lead to favorable summer growth conditions, and altered horizontal and vertical mixing in the atmosphere. All our results are available through http://www.carbontracker.e

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