Plant growth declines as warming causes drought

Randolph E. Schmid, Associated Press Yahoo Groups 20 Aug 10;

WASHINGTON – Plant growth that had been spurred by global warming has reversed, despite temperatures that continue to rise.

Researchers say the change could affect food security and development of biofuels.

The amount of carbon taken up by growing plants increased from 1982 through 1999 as temperatures rose and the amount of carbon dioxide in the atmosphere increased.

But a new study in Friday's edition of the journal Science found a drought-related decline in such plant growth from 2000 to 2009, even though temperatures continued to climb.

As drought caused by warming reduces the land's ability to take up carbon, the result could be more carbon dioxide left in the atmosphere, and thus more warming, Maosheng Zhao of the University of Montana explained in a telephone interview.

"This is a pretty serious warning that warmer temperatures are not going to endlessly improve plant growth," co-author Steven W. Running, also of the University of Montana, said in a statement.

"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," he said.

Instead, he and Zhao found a small but measurable decline of about 1 percent, compared to a 6 percent increase in the 1980s and '90s.

Their study, based on data collected by NASA satellites, found that northerly areas continued to increase plant growth, thanks to warmer temperatures and a longer growing season.

But that was more than offset by warming-associated drought in the Southern Hemisphere.

"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," commented Diane Wickland, program manager of the Terrestrial Ecology research program at NASA Headquarters in Washington.

The research was supported by NASA and the National Oceanic and Atmospheric Administration.

World's Plants Growing Less Thanks to Warming, Drought
LiveScience.com Yahoo News 20 Aug 10;

The world's plants are growing less than they did in recent decades, thanks to the stress of droughts, a new study finds.

NASA satellites were used to measure global plant productivity over the last 10 years and found that growth was on the decline, after plants flourished under warming temperatures and a lengthened growing season in prior years.

The decrease was relatively minor - compared with a 6-percent increase spanning two earlier decades, the recent 10-year decline was just 1 percent - but it could impact food security, biofuels, and the global carbon cycle.

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

Conventional wisdom based on previous research held that land plant productivity was on the rise. A 2003 paper in Science led by then University of Montana scientist Ramakrishna Nemani (now at NASA Ames Research Center in Moffett Field, Calif.) showed that global terrestrial plant productivity increased by as much as 6 percent between 1982 and 1999. That's because for nearly two decades, temperature, solar radiation and water availability - influenced by climate change - were favorable for growth.

Setting out to update that analysis, Running and his UM colleague Maosheng Zhao expected to see similar results as global average temperatures have continued to climb. Instead, they found that the impact of regional drought overwhelmed the positive influence of a longer growing season, driving down global plant productivity between 2000 and 2009.

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

The discovery comes from an analysis of plant productivity data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite, combined with growing season climate variables, including temperature, solar radiation and water. The plant and climate data are factored into a formula that describes constraints on plant growth at different geographical locations.

For example, growth is generally limited in high latitudes by temperature and in deserts by water. But regional limitations can vary in their degree of impact on growth throughout the growing season.

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, of NASA Headquarters in Washington, D.C., and manager of NASA's Terrestrial Ecology research program.

Researchers are keen on maintaining a record of the trends into the future. For one reason, plants act as a carbon dioxide "sink," and shifting plant productivity is linked to shifting levels of the greenhouse gas in the atmosphere. Also, stresses on plant growth could challenge food production.

"The potential that future warming would cause additional declines does not bode well for the ability of the biosphere to support multiple societal demands for agricultural production, fiber needs, and increasingly, biofuel production," Zhao said in a statement.