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In the often-controversial field of global climate change, it is well-established that the amount of carbon dioxide (CO₂) in the atmosphere is rising. Since the industrial revolution, the CO₂ concentration in the atmosphere has increased from approximately 290 parts per million by volume (ppmv) to greater than 360 ppmv today. This has been largely the result of fossil fuel burning, with an additional contribution from land-use change. It is expected that atmospheric CO₂ levels will continue to rise, and during the century could well exceed 500 ppmv (see the latest reports of the Intergovernmental Panel on Climate Change for projections of future concentrations). It is worth noting that the 2.9 ppmv increase in CO₂ atmospheric concentration from 1997 to1998 observed at the Mauna Loa Observatory in Hawaii is the largest annual increase since measurements began in 1958.

In addition to whatever climate changes (global warming, regional climate changes, accelerated sea-level rise) may be caused by increasing atmospheric CO₂, it is known that atmospheric CO₂ directly affects plant photosynthesis and water use, thereby potentially affecting vegetation and ecosystems, both managed and unmanaged (e.g., crops and forests). Analyzing the responses of vegetation and ecosystems to such increasing concentrations of CO₂ in the atmosphere is a key component of global change research. Numerous CO₂ -enrichment studies in greenhouses, growth chambers, and open-top chambers have suggested that growth of many plants could increase about 30% on average with a doubling of the atmospheric CO₂ concentration, in the short-term (up to a few years, to most). However, the applicability of such work to the growth of plants outdoors, under natural conditions and for longer periods, has been seriously questioned. The free-air CO₂ enrichment (FACE) approach represents the state of the science in exposing plants and ecosystems to levels of atmospheric CO₂ that may be reached during the coming 50 years. The approach eliminates many of the more significant limitations inherent in greenhouse and chamber systems, especially for large/tall vegetation such as trees. Information from FACE research on how crop, forest, and other ecosystems will react can help anticipate the impacts (both positive and negative) of future global change. Will there be changes in the rate at which crops and trees grow over the next hundred years? Will the storage of carbon aboveground and belowground (including soils) change? Could feedbacks from carbon storage in terrestrial ecosystems alter global atmospheric CO₂ concentrations?

FACE research facilities provide a platform from which to seek answers to these and many more questions concerning the response of ecosystems to increasing atmospheric CO₂ concentrations.

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