Earth has done an ecological about-face: Plant growth that once flourished under warming temperatures and a lengthened growing season is now declining – struck by the stress of drought.
University of Montana researchers Maosheng Zhao and Steve Running discovered the global shift during a routine analysis of NASA satellite data. The 10-year decline is slight – just 1 percent – but the shift could impact food security, biofuels and the carbon cycle.
The team published their findings Aug. 20 in Science. The article is online at http://www.sciencemag.org/cgi/content/full/329/5994/940.
“We see this as a bit of a surprise, as well as potentially significant on a policy level because previous interpretations suggested that global warming might actually help plant growth,” Running said.
Conventional wisdom held that global land plant growth was on the rise. A 2003 paper in the journal Science led by then UM scientist Ramakrishna Nemani, now at NASA Ames Research Center in Moffett Field, Calif., showed that growth increased between 1982 and 1999 by as much as 6 percent. That’s because for nearly two decades, climate change eased the constraints on growth: temperature, solar radiation and water.
Setting out to update the analysis, Zhao and Running expected to see similar results as temperatures have continued climbing.
Instead, they found that between 2000 and 2009 the impact of drought overwhelmed the positive influence of a longer growing season, driving down global plant growth. “This is a pretty serious warning that warmer temperatures are not going to endlessly improve plant growth,” Running said.
The discovery comes from a growing-season analysis that combines plant growth data from the Moderate Resolution Imaging Spectroradiometer on NASA’s TERRA satellite with climate constraints, including temperature, solar radiation and water. The plant and climate data drive an algorithm that weighs the constraints at geographical regions and measures their relative impact.
For example, it might seem straightforward that growth is limited in northern areas by temperature and in deserts by water. But the relationship can be complex, with each constraint having a varying degree of impact at different times in the growing season. Scientists can apply this thought process – mixing temperature, solar radiation and water availability parameters – for anywhere on Earth.
Zhao and Running’s global analysis showed that high-latitude northern hemisphere forests have continued to benefit from warmer temperatures and a longer growing season since 2000. But it was not enough to overcome warming-associated drought that limited growth in the southern hemisphere, resulting in a global net loss of land plants.
Researchers are keen to continue maintaining a record of the trends into the future, as a continued decline could have consequences affecting society.
“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.
Running directs the Numerical Terradynamic Simulation Group, which has written software for NASA environmental satellites. Zhao is a researcher in NTSG, which is part of UM’s College of Forestry and Conservation. For more information, visit http://www.ntsg.umt.edu. For more information about NASA and its programs, visit http://www.nasa.gov.