Missing carbon mystery: Case solved?
Nature.com, Aug. 3, 2007
Scientists claim to have located the 'missing carbon sink' in tropical forests that are absorbing around one billion tonnes more carbon than previously thought. Jane Burgermeister investigates.
They looked for it here and they looked for it there, but the carbon had vanished into thin air. So it seemed in the case of the 'missing carbon sink', a billion tonnes of human-generated carbon assumed to be absorbed by northern forests, but unaccounted for in field studies. Scientists now say they have located the missing carbon in tropical forests that are removing much higher quantities of carbon dioxide from the atmosphere than realized.
Tropical forests could be absorbing as much as one billion tonnes more carbon than previously realized.
Of the 8 billion tonnes of carbon that human activity produces each year 6.4 from fossil-fuel emissions, and 1.6 from deforestation, mainly in the tropics1 on average, 3.2 billion tonnes remain in the atmosphere, 2.2 billion tonnes are stored in the oceans and 2.6 billion tonnes are sucked up by land-based carbon sinks, mainly forests. Carbon-uptake models predict that as much as 2.4 billion tonnes of this carbon ends up in northern mid- to high-latitude forests. But scientists searching for it on the ground measuring trees and carbon exchanges between the vegetation and the atmosphere have only been able to account for about 0.7 billion tonnes there2.
Researchers led by Britton Stephens from the
Despite rapid deforestation, Stephen's team also showed that tropical forests are the net source of a mere 100 million tonnes of carbon annually, contrary to previous estimates of 1.8 billion tonnes. This suggests that carbon sequestration in the tropics is substantial enough to almost counterbalance the effects of deforestation. Stephens says "tropical forests are essentially in balance, absorbing as much carbon dioxide as they give off". One reason tropical forests could be absorbing substantially more carbon dioxide than accounted for in the models is the phenomenon known as CO2 fertilization.
Trees take in carbon dioxide to grow and when there is more of it they tend to grow faster, being fertilized by carbon dioxide. Also, as temperatures increase with climate change, soil organic matter decomposes more quickly, freeing up nutrients in the ground for forest growth.
Temperate and boreal forests are also being fertilized indirectly by nitrogen, largely from farming and fuel use, according to a study recently published a study in Nature4, led by Federico Magnani from the
Locating the 'missing carbon sink' in the tropics has implications for the entire global carbon cycle.
Other scientists have also recently come to the conclusion that northern forests, although critically important in maintaining biodiversity, might be less important in slowing climate change than tropical forests. Govindasamy Bala and Ken Caldeira found that tropical forests help cool the Earth in two ways: by storing carbon and also by reflecting the suns warming rays back to space5. "Unlike tropical forests, high latitude forests darken the Earth's surface, causing the earth to absorb more sunlight, an effect that is most pronounced in snowy regions. This darkening of the surface has a warming influence that can be stronger than the cooling influence of carbon storage in these forests," says Caldeira. This suggests that removing high-latitude forests would have a net cooling effect on the planet, whereas removal of tropical forests would result in warming.
Tropical forests are, however, rapidly disappearing. Forests in South America, Central Africa and
Whether tropical or northern forests store more carbon might ultimately be academic, though, when it comes to mitigating climate change. Stephens believes that "relying on trees to mitigate climate change is not a good long-term strategy, because the carbon they store gets returned to the atmosphere on a timescale of around 30 years when they die and decompose. Afforestation and reforestation can provide short-term sinks to slow warming and possibly give us more time to find solutions, but ultimately we need to get the carbon into the ocean or geologic reservoirs, or not emit it in the first place".
Jane Burgermeister is a freelance science writer.
5. Bala, K. et al. Proc. Natl Acad. Sci. USA 104, 65506555 (2007). | Article |
6. Working Group III Report: Mitigation of Climate Change (Intergovernmental Panel on Climate Change, 2007); http://arch.rivm.nl/env/int/ipcc/pages_media/AR4-chapters.html