The results have potential implications for the Kyoto Protocol negotiations where recent difficulties have included a lack of scientific knowledge about the strength and distribution of carbon sinks and how they vary from year to year.
Produced by a team of 30 leading carbon scientists from around the world, the paper draws on a large body of research to build up a new and comprehensive picture of carbon sinks on land.
"It is a major step forward in understanding where terrestrial carbon sinks actually are, why they are there, and how long they will operate into the future," said Dr Will Steffen, one of the authors and Executive Director of the International Geosphere-Biosphere Programme (IGBP), which coordinated the production of the paper.
The report states that carbon sinks of today’s magnitude cannot be relied on to operate steadily into the future because the key processes that give rise to the sinks in the first place are temporary and likely to diminish with time.
"Although carbon sinks have a role to play in absorbing excess carbon dioxide, it is possible that the net global terrestrial carbon sink may disappear altogether in the future," said lead author Professor David Schimel from the Max Planck Institute for Biogeochemistry in Jena, Germany.
One of the key points of the report is that a large part of the current Northern Hemispheric sink is likely to have been caused by relatively recent changes in land use and land management. For example, the North American carbon sink is thought to be primarily the result of forest regrowth after the abandonment of agricultural land during the 1980s and 1990s together with a reduction in the frequency of wild fires. In China and Europe, carbon sinks are also thought to be due to a recent increase in tree growth through reforestation programmes and improvements in forest management.
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 0.5°C 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 Niño 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 Dr Steffen.
Co-author Professor Chris Field from the Carnegie Institution of Washington sums up the situation. "Land and ocean processes have, in essence, provided a major, but far from permanent, subsidy to humans, protecting the atmosphere from many of the consequences of deforestation and burning fossil fuels."
Other key findings of the report:
· Contrary to earlier reports, the authors found no significant difference in the strength of the North American and Eurasian sinks;
· There is no evidence for a significant terrestrial carbon sink in non-tropical Southern Hemisphere regions;
· The tropics appear to be roughly in balance with respect to carbon sources and sinks, suggesting strong sink mechanisms to offset sources due to deforestation.
Journal
Nature