2003, a black year

But for how long? How will the ecosystems – notably forestry ones – behave over the course of the 21st century, when the Intergovernmental Panel on Climate Change (IPCC) projections are expecting a temperature rise of two to six degrees from now until the year 2100? At this stage nobody can provide an answer with any certainty. Nevertheless, various measurements carried out on the occasion of extreme events, such as the drought which struck Europe in 2003, provide some elements of a response. And they are not necessarily reassuring. ‘That particular Summer the behaviour of the ecosystems in a way went into a reverse process,’ observes Marc Aubinet. ‘Instead of sequestering carbon, the terrestrial ecosystems released 500 Tgs of carbon into the atmosphere, in other terms a quantity which is as good as the equivalent of what is sequestered over two and a half years!’ The increase in the frequency of such extreme events thus risks significantly diminishing the sequestration potential of terrestrial ecosystems.’

What exactly happened in 2003? It appears that two phenomena played in opposite directions. On the one hand, the drought slowed down micro-organism activity, leading to a reduction of respiration on the part of forest ecosystems and, through that, a reduction of CO₂ emissions. But on the other hand it also slowed down carbon assimilation by the plants. The latter phenomenon is thought to have had greater weight than the former, leading finally to a net carbon emission. This phenomenon was not experienced everywhere: very paradoxically 2003 resulted in one of the largest sequestrations of carbon out of the thirteen years studied at Vielsalm. Why? The exact causes of this unusual behaviour remain to be discovered.

This grey area is far from being the only one. In effect, even putting aside extreme events such as that of 2003, the observation of carbon fluxes reveals significant annual fluctuations. Thus, at Vielsalm, over the same time lapse (thirteen years), the sequestration by a hectare of forest varied over a range moving from 3.8 to 6.4 tons of carbon per year. To understand what it is we are talking about in concrete terms, it is like the carbon discharge of a small cylinder car being driven, according to the year, from between 131,000 to 221,000 kilometres. ‘At the present time the causes of these variations remain largely unknown,’ acknowledges Marc Aubinet. ‘They are not in any case directly connected to average climate conditions.’