June 6, 2001 An analysis of vegetation growth in North America between 1982 and 1998 using satellite observations indicates a significant increase in the rate at which carbon is being taken up by plants, according to a new study.
University of Colorado at Boulder Research Associate Jeffrey Hicke, who led the study, said it is still unclear why North American vegetation growth has been increasing in the last two decades. "But we definitely are seeing an increase in carbon uptake that could generate a carbon sink similar to those observed by other researchers."
Carbon sinks, or storage areas, include the atmosphere, the oceans and the terrestrial environment, said Hicke, a research associate in CU-Boulder’s department of geological sciences. A 1995 study led by CU-Boulder indicated the equivalent of about half of the world’s fossil-fuel emissions was absorbed by terrestrial vegetation in the Northern Hemisphere in 1992 and 1993.
The results of a new study on the subject by Hicke, CU-Boulder geological sciences department Assistant Professor Greg Asner and the California Institute of Technology’s James Randerson were presented at the annual spring meeting of the American Geophysical Union held May 29 to June 2 in Boston.
Other study co-authors included Chris Field of the Carnegie Institute of Washington at Palo Alto, Calif. and Compton J. Tucker and Sietse Los of NASA’s Goddard Space Flight Center in Greenbelt, Md.
"There definitely is a limit to how much carbon dioxide plants can soak up," said Hicke. He said the amount of future uptake of carbon by North American vegetation will depend on the mechanisms that are driving the processes, which still need to be identified.
A study published in the May 24 issue of Nature by Duke University scientists indicated the ability of pine trees to absorb significant amounts of CO2 dropped markedly after three years in part because plant nutrients and water were depleted.
The levels of CO2 in Earth’s atmosphere have been rising since the Industrial Revolution began in the late 18th century, climbing from about 280 parts per million to 350 PPM today.
"Solid evidence that the increase in atmospheric CO2 has caused warming is only now appearing," said Hicke. But the vast majority of atmospheric scientists believe increasing C02 and other heat-trapping greenhouse gases released into the atmosphere as a result of human activity have contributed to climate warming over the past century, he said.
The U.S. regions where vegetation productivity showed the largest increases in the new study were managed forests in the Southeast and croplands and grasslands on the Central Plains.
Two other areas also showing increases were forests in southeast Canada -- which are recovering from insect damage -- and western Canada and Alaska. Recent warming in the northwest part of the continent appears to have triggered earlier annual snowmelt and an earlier beginning to the growing season, Hicke said.
The observations were made in the visible and near-infrared bands of instruments on board several generations of the National Oceanic and Atmospheric Administration’s Advanced Very High-Resolution Radiometer satellites. Tucker and Los from NASA’s Goddard Space Flight Center produced the satellite measurements.
"With the models and techniques we employed, we can’t assess the specific contribution of CO2 fertilization to the stimulation of plant growth," said Hicke. He said increased growth of Southeast forests could be due to advanced agricultural practices, including more efficient fertilizers and improved genetic stock.
The Central Plains grasslands increase could be due to increasing precipitation in recent years, he said. But better fertilization and irrigation practices also have triggered increases in cropland vegetation in this region, said Hicke.
"It is likely that changing land use practices, the stimulation of vegetation growth by increased atmospheric CO2 and climate change are the primary causes of the recent U.S. vegetation increases," he said.
"There has been a big thrust recently to understand the role and cycle of CO2," said Hicke. "But the science is so complex, I don’t think we will have any definitive answers any time soon."
The Bush Administration’s March abandonment of a campaign pledge to curtail CO2 emissions by U.S. industry played a pivotal role in the April failure of the world’s nations to ratify the Kyoto Protocol that would have reduced global CO2 emissions.
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