Forest ‘sinks’ absorbed more carbon in 1990s than in 1980s despite deforestation

Land-based ecosystems absorbed all of the carbon released by deforestation plus another 1.4 billion tonnes emitted by fossil fuel burning during the 1990s, but this “convenient uptake” cannot be relied on to head off global warming in the future, researchers have said.

“We could easily see this robust transfer of carbon out of the atmosphere and into land-based ecosystems that occurred in the 1990s slow-down in the future,” says David Schimel of the US’ National Center for Atmospheric Research (NCAR), who is the lead author of new research on carbon ‘sinks’, the importance of which is imperative in the light of the finalisation of talks on implementing the Kyoto Protocol (see this week’s ‘Europe’ section).

Fossil-fuel burning, cement manufacture, and deforestation gave off about 7 billion tonnes of carbon per year during the 1980s and 8 billion tonnes annually during the 1990s, about half of it ending up in the earth’s atmosphere, according to the study. In the 1980s the amount of carbon released to the atmosphere from deforestation about equalled that taken up by land ecosystems into various ‘sinks’. However, during the 1990s the balance tipped, and an additional 1.4 billion tonnes of carbon ended up in the land-based biosphere than in the atmosphere, despite continuing deforestation.

Land-use changes in the Northern Hemisphere have been partly responsible for carbon uptake during the 1990s. In the United States, trees and other growth expanded on abandoned agricultural land, while a reduction in fires allowed forests to spread. Enhanced plant growth spurred by increasing carbon dioxide and nitrogen deposits – a process more noticeable in Europe and Asia – also helped clear the air of CO2 build-up.

“Forests can only replace farms for so long,” says Schimel. “Eventually new trees and grasses reach maturity and soak up less carbon dioxide. Similarly, there’s a limit to how much forests can fill in and spread, even with successful fire suppression.” The boost in CO2 and nitrogen fertilisation will also peak, though at high levels. Over time the effects of climate change on ecosystems will probably reduce sinks globally, write the authors. Meanwhile, carbon dioxide emissions are expected to continue to rise because of human activities. Several other recent studies (see related story and related story) have warned about the effectiveness of carbon ‘sinks’, especially favoured by the current US administration in its climate change policy (see related story).

An unusually large uptake of atmospheric carbon in the early 1990s was due to the climate’s natural variability, researchers suspect. Globally there appears to be a net release of carbon into the atmosphere during warm, dry years and a net uptake during cooler years. Recently, evidence has grown linking changing levels of atmospheric CO2 to the El Nino/Southern Oscillation and its widespread impacts.

For the tropics, the researchers expected computer models to show a large increase in atmospheric carbon dioxide released by deforestation during recent decades, but no such increase emerged, indicating a potentially large sink in the low latitudes. Local-scale studies show carbon absorption by a range of mature tropical forest types, but the authors warn that such processes may not be true of the entire region, since tropical ecosystems vary widely. The lack of data, both atmospheric and ecological, combined with a complex meteorology, make estimates of tropical fluxes highly uncertain, they caution.

Carbon accumulates at higher rates in intensively managed ecosystems and those recovering from disturbance, the researchers note. For example, Chinese inventory studies of continental plant growth show a major carbon sink resulting from its extensive reforestation and afforestation programmes, making it the world’s biggest planter of forests (see related story).

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