Arlington, Va. -- As spring turns into summer, scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and other institutions will fly a C-130 research aircraft over Colorado's Front Range this May and again in July to measure how much carbon dioxide mountain forests remove from the air. The project is funded by the National Science Foundation (NSF).
The researchers are developing new methods for assessing carbon uptake over complex terrain on regional scales. Accurate assessments could help show to what extent carbon dioxide storage in Western mountain forests--a potentially important "sink" for the greenhouse gas--may be slowing down as the ongoing drought affects tree growth.
"We now believe that complex interactions of organisms, human activity and chemical cycles control carbon fluxes in mountains," says Rachael Craig, program director in NSF's division of earth sciences, which funded the experiment. "The ACME project has the potential to clarify this key component of the carbon cycle."
International pressure is mounting to limit carbon emissions because of their role in global climate change. Better understanding of natural processes involved in forest-air carbon exchange may lead to more accurate monitoring methods and new ways to enhance carbon uptake. High carbon-emitting nations and industries are interested in devising strategies for meeting quotas and trading carbon credits.
ACME (short for the Airborne Carbon in the Mountains Experiment) gives scientists an opportunity for the first time to combine airborne data with ground-based measurements to paint a more accurate picture of carbon exchanges in rolling hills and mountain ranges. Results from the field program will also be used in testing computer models of forest ecosystem function. The models will help scientists understand the response of forests to drought, fire, insects, and climate change.
"Wildfires play a big role in controlling vegetation and carbon exchange in the Rockies," says NCAR scientist Dave Schimel, "but most burn areas are too small to assess from an aircraft. For the first time we have a chance to get airborne measurements of carbon directly over a large, disturbed area."
Forest losses during the 2002 wildfire season in Colo. reversed years of carbon uptake. The amount of carbon dioxide released from trees during the fires equaled an entire year's emissions from statewide transportation activities.
As the research plane samples air aloft, a dense network of instruments will gather data over a half-square mile on Niwot Ridge near Nederland, Colo. Perched atop three steel towers provided by NCAR, each between 100 and 200 feet tall, carbon dioxide sensors and sonic anemometers will measure changes in carbon levels and winds high above the tree tops.
"Today, we usually look for carbon in all the wrong places," says Schimel, "focusing on where it's easy to measure rather than where fluxes are largest." Most current studies are in flat areas, but most western forests are in the mountains, he explains. Schimel and colleagues have estimated that 25-50 percent of U.S. carbon uptake occurs in mountainous terrain.
In the northern mid-latitudes, significant carbon uptake occurs in forests, which are typically left to grow undisturbed in mountainous regions. Ground-based sensors work well in flat land: there are 200 such sites around the world. But in mountain ranges special conditions, such as turbulent airflow, snow pack, vegetation patterns, and contrasts in sunshine and shade, complicate data gathering.
Along with NCAR, the Universities of Colorado, Florida, and Utah, Colorado State University, and Scripps Institution of Oceanography are also participating in the project.
Human activities put 8 billion tons of carbon into the atmosphere each year--6 billion from fossil fuel burning and another two billion from forest destruction. But monitors show only half that much building up in the planet's atmosphere. The other 4 billion tons are removed each year by growing vegetation and ocean waters.
The United States alone is responsible for some 20 percent, more than 1.5 billion tons, of the global total carbon output. But U.S. forests may remove as much as 1 billion tons per year naturally as trees grow back on abandoned farmland, spread over historical grasslands, or grow thick with underbrush protected from wildfires. Understanding these removal processes better, along with more accurate monitoring, could lead to new methods for enhancing carbon uptake, says Craig.
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