Explaining variations in carbon uptake by European ecosystems

Atmospheric circulation determines wind patterns across the continent and thereby the amount of rainfall and temperature in different regions. In a paper published in Nature Communications, an international team of researchers involving Christian Rödenbeck from MPI for Biogeochemistry, explains the exceptionally low CO₂uptake by European ecosystems in the early 2000’s by the persistence of a particular combination of atmospheric circulation patterns. A persistent, but rather variable carbon sink in Europe.

In the global carbon cycle, land ecosystems are very important because they absorb 30% of the human CO₂ emissions and thereby strongly slow down Global Warming. Europe is a particularly relevant region because it has the highest rates of CO₂ uptake in the Northern Hemisphere. Previous studies, however, reported a decrease of CO₂ uptake in Europe in the early 2000’s, suggesting a possible saturation of the land carbon sink. Using long-term atmospheric and ecosystem data sets, the team led by Ana Bastos at the University of Lisbon, Portugal and now at LSCE, France found that the European CO₂ sink strengthened again in the late 2000s.

"We found that the strong inter-annual variability of the European carbon sink is explained by the coupled impacts of two atmospheric pressure gradients: the North-Atlantic Oscillation (NAO) and the East-Atlantic Pattern (EA). These north/south pressure gradients control wind patterns and storm-tracks in the North Atlantic and thereby the transport of warm and moist air towards Europe.” says Ana Bastos.

“The European carbon sink is enhanced when the two pressure gradients are both in negative phase (weaker pressure gradients). Under these conditions, cold winters affect most of Europe, leading to a higher snowpack in central and eastern Europe that serves as a moisture source in spring and summer.”, says Ivan Janssens one of the authors of the study, working at the University of Antwerp.

In contrast, when the NAO and the EA are in opposing phases (one strong, another weak), their effects cancel each other out and the European sink tends to be weaker.

The predominance of seven consecutive years with NAO and EA in opposing phases during the early 2000s explains the lower CO₂ uptake during that period. Luckily, European ecosystems appear to have started absorbing more CO₂ again.