As a forest re-grows after disturbance or through active planting programmes, significant amounts of CO2 are absorbed. However, once a forest matures, the amount of CO2 absorbed is roughly equal to the amount released, thus the sink will diminish with time as forests mature.

Further causes for the current global sink are the fertilising effect of increased levels of CO2 and nitrogen, but these effects are expected to saturate and thus will also not continue to enhance plant growth indefinitely.

The paper represents a major advance in terms of reconciling two different approaches to measuring the strength of carbon sinks and sources. Historically, researchers have utilised two types of measurements: 1) direct measurement of CO2 in the atmosphere, and 2) on-the-ground measurements on the basis of forest growth and soil uptake etc. Until now, these two techniques have provided inconsistent results. In this study the authors show that, on the broad scale, they are consistent.

The authors point out that there are many regional differences in the strength of terrestrial carbon sinks. Much of Siberia, for example, has been warming at a rate of approximately 0.5C per decade since the 1960s and an increase in wild fires and insect damage appears to have converted this region from a sink into a temporary carbon source with considerable year-to-year variability.

There is also considerable annual variability in sink strength associated with climatic variations such as the El Nio Southern Oscillation in tropical and extra-tropical regions. Globally, there appears to be a net release of carbon to the atmosphere during warm, dry years and a net uptake during cooler years. This observation gives a hint of how terrestrial sinks may respond to longer term climate changes such as increased temperatures," said D
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Contact: Susannah Eliott
Susannah@igbp.kva.se
46-8-6739-556
International Geosphere-Biosphere Programme
7-Nov-2001