In the presence of water and sunlight, plants take in carbon dioxide (CO2) during photosynthesis to create fuel, glucose and other sugars, for building plant structures. Better understanding of biological and physical processes that contribute to carbon uptake by plants will help scientists predict climate change and future levels of CO2, a heat-trapping gas in the atmosphere.

"The changes in the hydrologic cycle is one of the mechanisms that is often overlooked in the recent debate over carbon sequestration in the United States," said Ramakrishna Nemani, a researcher at the University of Montana's School of Forestry, and lead author of the study that appears in an issue of Geophysical Research Letters later this month.

Scientists have noticed that the U.S. terrestrial carbon sink, an effect where carbon is drained from the air and stored in the land, has been increasing since the latter part of the 20th century. Previous research has claimed this rise may be due to an observed greening of the U.S. as a result of forest re-growth, as well as greater concentrations of atmospheric CO2 and warming temperatures.

For the first time, however, this study suggests that changing rainfall patterns may play a bigger role in plant growth and carbon absorption. Computer model results showed that on average from 1950 to 1993 higher humidity combined with an eight percent increase in precipitation has led to a 14 percent increase in plant growth in the U.S. The data over that time period also show increases in cloud cover, minimum temperatures, soil moisture and stream flows, which are all signs of a changing hydrologic cycle.

Whether or not shifting rainfall patterns result in a positive uptake of carbon by land ecosystems depends o
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Contact: Cynthia O'Carroll
Cynthia.M.OCarroll.1@gsfc.nasa.gov
301-614-5563
NASA/Goddard Space Flight Center
16-May-2002