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Do US ecosystems balance US fossil fuel use?

Max Planck Society : 10 March, 2000  (Technical Article)
Atmospheric carbon dioxide gradients suggest that uptake of carbon dioxide by US forests may balance US fossil fuel use. New model calculations and data, published in Science (17 March 2000) by David Schimel of the Max Planck Institute for Biogeochemistry in Jena/Germany and a group of international researchers suggest lower carbon uptake. Carbon dioxide fertilization, once thought to be the only cause of carbon storage, accounted for only a third of the observed total.
The carbon cycle is the main driving force of climate change, and for decades, researchers and policymakers have been concerned because a substantial part of the carbon emitted from fossil fuels remained unaccounted for. Even if only one major process remains unknown, projections of future atmospheric CO2 are highly uncertain. A number of researchers internationally have used gradients of CO2 concentrations in the atmosphere to infer where sources and sinks of carbon dioxide are occurring. One such analysis suggested that North America dominates terrestrial sinks, so that ecosystem uptake actually amost balances North America’s huge fossil fuel emissions.

Researchers led by Prof. David Schimel, Director at the Max-Planck-Institute for Biogeochemistry in Jena, with team members from the National Center for Atmospheric Research (USA), Marine Biological Lab (USA), Colorado State University (USA), University of Montana (USA), University of Lund (Sweden), US Forest Service, University of Virginia (USA) and the University of Sheffield (UK) have modeled the ecosystem carbon budget of the US using state-of-the-art data and models. The results from this project called VEMAP, the Vegetation and Ecosystem Modeling and Analysis Project, show that increasing carbon dioxide and climate trends result in small uptake of carbon, only a tenth of what the atmospheric analyses suggest. Analyses of data from forest inventory measurements and other direct data suggest an uptake about three times larger than the modeled flux, an uptake still much smaller than the atmospheric analysis implied.

The low estimates from the models and inventories suggest that ecosystems in the US, and North America, are storing carbon, but much less than is being emitted by fossil fuel use. The uptake is widely distributed around the US, in forests, grasslands, farmlands and other ecosystems. This result implies that other regions of the world - Europe, Siberia or the tropical forests, must also be important.

The uptake of carbon was modeled from 1895 to 1993. Over this period, the fluxes were highly variable as the weather varied from one year to the next. This shows that carbon storage is very sensitive to the climate and if there is climate change in the future, ecosystem carbon storage will respond. In general, warmer and drier conditions caused carbon release to the atmosphere. The high variability of carbon storage from one year to the next has another implication. Under the Kyoto protocol, which is the international agreement to limit fossil fuel emissions, “commitment periods” are identified during which national carbon budgets must be compiled, including estimates of carbon uptake in deliberately managed forests. However, the models show that the amount of ecosystem uptake is greatly variable and can even go from uptake to release from year to year! Therefore, depending on the weather, the ecosystem balance in a given country could be positive or negative during the commitment period, and this will complicate the use of reporting intervals.

Until recently, scientists and policymakers have assumed that CO2 fertilization was the main mechanism for carbon uptake by ecosystems. Carbon fertilization is the stimulation of plant growth by increasing atmospheric CO2, a well-known phenomenon. Recently, other mechanisms have also been identified as having global importance. In particular, the regrowth of forests on abandoned farmland (a major process in the the US) and in previously harvested forests replaces carbon lost during previous decades. The VEMAP result suggests that, for the US at least, this process is between 100 and 200 percent of the CO2-effect. This is important for the prediction of future CO2 because while CO2 fertilization will continue for the foreseeable future, regrowth of trees will slow or cease when forests reach maturity or are harvested.

The results of the VEMAP activity are important scientifically because they are the first results that quantify the large-scale effects of the CO2 effect relative to quantitative estimates of actual forest growth. They confirm the growing suspicion of ecologists that CO2 fertilization is merely one of a number of processes resulting in the total uptake of carbon by ecosystems. They contradict the suggestion that North American ecosystem uptake balances its emissions of fossil fuel. The results suggest that when policies are made that requiring knowing ecosystem carbon uptake, policymakers must be aware that ecosystem carbon uptake can vary from year to year by large amounts. Therefore, short-term measurement periods could be very misleading. The high variability of ecosystem processes demonstrate why long term ecological measurements and research is so critical and urgently needed.
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