Global plant productivity that once was on the rise with warming temperatures and a lengthened growing season is now on the decline because of regional drought, according to a new study of NASA satellite data.

Plant productivity is a measure of the rate of the photosynthesis process that green plants use to convert solar energy, carbon dioxide and water to sugar, oxygen and eventually plant tissue. Compared with a 6 percent increase in plant productivity during the 1980s and 1990s, the decline observed over the last decade is only 1 percent. The shift, however, could impact food security, biofuels and the global carbon cycle.

Researchers Maosheng Zhao and Steven Running of the University of Montana in Missoula discovered the global shift from an analysis of NASA satellite data. The discovery comes from an analysis of plant productivity data from the Moderate Resolution Imaging Spectroradiometer on NASA's Terra satellite, combined with other growing season climate data, including temperature, solar radiation and water.

"We see this as a bit of a surprise, and potentially significant on a policy level because previous interpretations suggested global warming might actually help plant growth around the world," Running said.

Previous research found land plant productivity was on the rise. A 2003 paper in the journal Science led by scientist Ramakrishna Nemani, now a researcher at NASA's Ames Research Center in Moffett Field, Calif., showed the 6 percent increase in global terrestrial plant productivity between 1982 and 1999. The increase was traced to nearly two decades of temperature, solar radiation and water availability conditions, influenced by climate change, that were favorable for plant growth.

Setting out to update that analysis, Zhao and Running expected to see similar results as global average temperatures continued to climb. Instead, they found the negative impact of regional drought overwhelmed the positive influence of a longer growing season, driving down global plant productivity between 2000 and 2009. The team published its findings August 19 in Science.

"This is a pretty serious warning that warmer temperatures are not going to endlessly improve plant growth," Running said.

Zhao and Running's analysis showed that since 2000, high-latitude Northern Hemisphere ecosystems have continued to benefit from warmer temperatures and a longer growing season. But that effect was offset by warming-associated drought that limited growth in the Southern Hemisphere, resulting in a net global loss of land productivity.

"This past decade's net decline in terrestrial productivity illustrates that a complex interplay between temperature, rainfall, cloudiness, and carbon dioxide, probably in combination with other factors such as nutrients and land management, will determine future patterns and trends in productivity," said Diane Wickland, program manager of the Terrestrial Ecology research program at NASA Headquarters in Washington.

Researchers want to continue monitoring these trends in the future because plant productivity is linked to shifting levels of greenhouse gas carbon dioxide in the atmosphere and stresses on plant growth that could challenge food production.

"Even if the declining trend of the past decade does not continue, managing forests and crop lands for multiple benefits to include food production, biofuel harvest, and carbon storage may become exceedingly challenging in light of the possible impacts of such decadal-scale changes," Wickland said.