Despite some claims that North America is a "carbon sink," the continent does not absorb as much carbon dioxide as it releases through the burning of fossil fuels, according to an article in the journal Science.
Researchers at Colorado State University and other institutions undertook a computer simulation study of carbon absorption and release over 1895-1993. They found that carbon uptake via natural processes came nowhere near matching the release of carbon dioxide produced by fossil fuel burning.
"Releasing and reabsorbing carbon is the principal force behind climate change, and for decades researchers have been concerned because a substantial part of the carbon emitted from the combustion of fossil fuels can’t be accounted for," said Dennis Ojima, senior research scientist at Colorado State’s Natural Resource Ecology Laboratory. "Scientists have been unable to provide accurate data about this." Some researchers have used atmospheric concentrations of carbon dioxide to infer where carbon sinks and sources are occurring. One such model now being questioned suggested that North America so dominates terrestrial sinks that ecosystem uptake of carbon dioxide almost balances the continent’s huge level of emissions. The new model and data indicate this is incorrect; carbon dioxide fertilization (the stimulation of plant growth by increasing atmospheric carbon dioxide), once thought the sole cause of carbon storage, accounts for only a third of the observed total, with land-use change being an important factor.
The Science article is written by lead author David Schimel, a researcher at Colorado State’s Natural Resource Ecology Laboratory and now co-director at the Max-Planck-Institute for Biogeochemistry in Germany; Ojima; William Parton, professor of rangeland ecosystem science; and Robin Kelly, a research associate with the Natural Resource Ecology Laboratory. They worked with colleagues from the National Center for Atmospheric Research, Marine Biological Lab, University of Montana, University of Lund in Sweden, U.S. Forest Service, University of Virginia and the University of Sheffield in the United Kingdom.
The Vegetation and Ecosystem Modeling and Analysis Project, using state-of-the-art data and models, shows that increasing carbon dioxide and climate trends result in a small uptake of carbon–a tenth of what the atmospheric analyses indicate. Analyses of data from forests and other sources suggest carbon uptake about three times larger than that modeled, an uptake still much smaller than the atmospheric analysis suggested.
The low estimates from the models and inventories suggest that North American ecosystems, while storing carbon, are doing so in much smaller quantities than what is being emitted by fossil fuel use. Carbon uptake is widely distributed around the continent in forests, grasslands, farmlands and other ecosystems, implying other regions such as Europe or boreal forests also must be important.
Investigators found changes were highly variable and dependent on weather, showing that carbon storage is very sensitive to the climate. Ecosystem carbon storage will respond to future climate change.
Another implication of the high, year-to-year variability relates to the Kyoto protocol, the international agreement to limit fossil fuel emissions. Under the protocol, national carbon budgets must be put together, including estimates of carbon uptake in deliberately managed forests. However, the model shows not only that ecosystem uptake varies but that it can move between uptake and release from year to year, complicating the use of reporting intervals.
Until recently, scientists and policymakers assumed that carbon dioxide fertilization was the main vehicle for carbon dioxide uptake. Recently, other mechanisms have been identified. In particular, the regrowth of forests on abandoned farmland, a major process in the United States, and in previously harvested forests reabsorbs carbon lost during earlier decades. For the United States, at least, this accounts for as much or more of an effect as carbon dioxide fertilization. The problem is that, when trees mature or are harvested, future carbon dioxide levels will become more difficult to predict.
"The results of this model are important scientifically because they offer the first comparison of the large-scale effects of carbon dioxide relative to estimates of actual forest growth," Parton said. "The results confirm a growing suspicion of many of us ecologists that carbon dioxide fertilization is merely one of a number of processes resulting in the total uptake of carbon by ecosystems, and they contradict the suggestion that North American ecosystem uptake balances its emissions from fossil fuel.
"The other finding is that policymakers must keep in mind that ecosystem carbon uptake can vary annually by large amounts and that short-term measurement could be very misleading."