Carbon dioxide

　　Carbon dioxide (CO2) is emitted in a number of ways. It is emitted naturally through the carbon cycle and through human activities like the burning of fossil fuels.
Natural sources of CO2 occur within the carbon cycle where billions of tons of atmospheric CO2 are removed from the atmosphere by oceans and growing plants, also known as ‘sinks,’ and are emitted back into the atmosphere annually through natural processes also known as ‘sources.’ When in balance, the total carbon dioxide emissions and removals from the entire carbon cycle are roughly equal.
Since the Industrial Revolution in the 1700’s, human activities, such as the burning of oil, coal and gas, and deforestation, have increased CO2 concentrations in the atmosphere. In 2005, global atmospheric concentrations of CO2 were 35% higher than they were before the Industrial Revolution. For more information on CO2 trends in the atmosphere, visit the page on Atmosphere Changes.
Greenhouse gases occur naturally in the Earth’s atmosphere in addition to being emitted through human activities. This natural “carbon cycle” includes carbon dioxide used in plants during photosynthesis and the exchange of carbon dioxide between the atmosphere and the oceans.
The primary natural processes that release CO2 into the atmosphere (sources) and that remove CO2 from the atmosphere (sinks) are:
Animal and plant respiration, by which oxygen and nutrients are converted into CO2 and energy, and plant photosynthesis by which CO2 is removed from the atmosphere and stored as carbon in plant biomass;
Ocean-atmosphere exchange, in which the oceans absorb and release CO2 at the sea surface; and
Volcanic eruptions, which release carbon from rocks deep in the Earth’s crust (this source is very small).

ARS plant report the global warming



ARS plant physiologist Jack Morgan

 ARS plant physiologist Jack Morgan leads the study, which uses both CO2 pipelines and thermal infrared heaters to simulate global warming conditions predicted for the end of the century: 600 parts per million (ppm) of CO2-in comparison to today's average 390 ppm-and day/night temperatures raised by 3 and 5 degrees Fahrenheit, respectively. Warmer temperatures increase water loss to the atmosphere, leading to drier soils. In contrast, higher CO2 levels cause leaf stomatal pores to partly close, lessening the amount of water vapor that escapes and the amount of water plants draw from soil. This newly released study finds that CO2 does more to counterbalance warming-induced water loss than previously expected. In fact, simulations of levels of warming and CO2 predicted for later this century demonstrated no net change in soil water, and actually increased levels of plant growth for warm-season grasses.

"By combining higher temperatures with elevated CO2 levels in an experiment on actual rangeland, these scientists are in the process of developing the scientific knowledge base to help prepare managers of the world's rangelands for what is likely to happen as climate changes in the future," said Edward B. Knipling, administrator of the Agricultural Research Service (ARS), USDA's principal intramural scientific research agency. The results cover the first four years of the eight-year Prairie Heating and CO2 Enrichment (PHACE) experiment on native northern mixed grass rangeland. The study is being conducted by the ARS Rangeland Resources Research Unit (RRRU) at the High Plains Grasslands Research Station near Cheyenne, Wyo.

Based on these findings, warmer temperatures would likely play a role in changing the relative success of various grass types. "Only the warm-season grasses had their growth boosted higher by CO2 and warmer temperatures," Morgan said. "If this leads to a competitive advantage for warm-season grasses, it may increase the challenges faced by ranchers who desire cool-season grasses for early-season forage".
Elise Pendall and David Williams at the University of Wyoming at Laramie and Matthew Wallenstein at Colorado State University at Fort Collins also are participating in the study, which will be completed in 2013. Retired ARS soil scientist Bruce Kimball, designer of the infrared heater system, is helping conduct the study. Kimball serves as a research collaborator at the ARS U.S.

Arid-Land Agricultural Research Center in Maricopa, Ariz.Grass-dominated, dry rangelands account for approximately a third of the Earth's land surface, providing most of the forage eaten by livestock. This research, the first of its kind on this scale for rangelands, supports the USDA priority of helping farmers and ranchers throughout the United States and the rest of the world best adapt production practices to variable climate patterns.