The earth's land-based ecosystems absorbed all of the carbon released by deforestation plus another 1.4 billion tons emitted by fossil fuel burning during the 1990s, but we can't rely on this convenient uptake to head off global warming in the future.
This is the conclusion of a new study published this week in the journal Nature. Carbon dioxide (CO2) is the primary greenhouse gas entering the atmosphere from human activities.
"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 the paper's lead author, David Schimel, of the National Center for Atmospheric Research (NCAR).
Fossil-fuel burning, cement manufacture, and deforestation gave off about 7 billion tons of carbon per year during the 1980s and 8 billion tons 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 equaled that taken up by land ecosystems into various "sinks."
During the 1990s, the balance tipped, and 1.4 billion tons more 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 buildup.
"Forests can only replace farms for so long," explains 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 fertilization will peak as well, 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.
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 Niño/Southern Oscillation and its widespread impacts.
For the tropics, scientists expected computer models to show a large increase in atmospheric carbon dioxide released by deforestation during recent decades. In fact, 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.
Previous attempts to evaluate carbon uptake in North America compared to that occurring in Europe and Asia have been controversial. In this paper, the authors conclude that the 1990s sink was roughly split between Eurasia and North America, with Eurasia slightly leading.
Because they used only atmospheric data in their analyses, the authors caution that the resulting distribution pattern is highly uncertain. Even so, it appears consistent with independent analyses of satellite vegetation data. The uptake patterns across the continents also make sense physically: they appear to be driven by broad climate patterns interacting with historic human management of ecosystems.
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 extensive programs in foresting and reforesting.
David Schimel's portion of this research was funded by NASA and by the National Science Foundation, NCAR's primary sponsor.
[Contact: David Schimel, Anatta]