Almost half the carbon dioxide (CO2) emissions related to human activity builds up in the atmosphere and contributes to global warming. The remainder is stored in the oceans and terrestrial ecosystems, such as forests or grasslands. "Terrestrial and aquatic ecosystems halve the effects of global warming and making sure they are properly managed is one of the best ways of attenuating climate change," explains Philippe Ciais, Deputy Director of LSCE. "A closer look, however, at the detailed inventory of sinks and sources of the three main greenhouse gases (CO2, methane, nitrous oxide) shows that the reduction obtained is not really that significant in Europe."

An international study, in which several LSCE researchers took part, has produced the first balance of greenhouse gases exchanged by the atmosphere and the European continent. The team succeeded in improving on previous balances based on two separate estimations. The first was obtained from direct measurements of greenhouse gas fluxes on the scale of plots of land, which were then combined with models of how the biosphere functions. In the second method, referred to as a reverse approach, the locations of carbon sinks and sources were deduced from differences in concentrations measured at various spots around the world. The new inventory confirms the existence of a significant carbon sink in Europe's forests and grasslands, which offset about 15% of emissions related to fossil energy combustion.

Philippe Ciais goes on to explain: "As all European ecosystems are "managed", however, they emit nitrous oxide due to the fertilisation of grasslands and crops, as well as methane from peatlands or enteric fermentation in ruminants."

The inventory reveals that methane and nitrous oxide emissions almost totally cancel out the action of the biospheric CO2 sink, with the result that Europe's terrestrial ecosystems can only absorb the equivalent of 2% of household, industrial and transport-related emissions.

"Our findings suggest that cutting down on methane and nitrous emissions induced by soil management should be our priority if we want Europe's terrestrial ecosystems to play a more effective role in curbing global warming," concludes Philippe Ciais.

Reference: Schulze et al 2009. Importance of methane and nitrous oxide for Europe’s terrestrial greenhouse-gas balance. Nature Geoscience, 2 December 2009.