One idea about climate change suggested that higher temperatures would boost plant growth and food production. That may have been a trend for awhile, where plant growth flourished with a longer growing season, but the latest analysis of satellite data shows that rising global temperatures has reached a tipping point where instead of being beneficial, higher temperatures are causing drought, which is now decreasing plant growth on a planetary scale. This could impact food security, biofuels, and the global carbon cycle. “This is a pretty serious warning that warmer temperatures are not going to endlessly improve plant growth,” said Steven Running from the University of Montana.
During the 1980s and 1990s global terrestrial plant productivity increased as much as six percent. Scientists say that happened because during that time, temperature, solar radiation and water availability — influenced by climate change — were favorable for growth.
During the past ten years, the decline in global plant growth is slight – just one percent. But it may signify a trend.
“These results are extraordinarily significant because they show that the global net effect of climatic warming on the productivity of terrestrial vegetation need not be positive — as was documented for the 1980’s and 1990’s,” said Diane Wickland, of NASA Headquarters and manager of NASA’s Terrestrial Ecology research program.
A 2003 paper in Science led by then University of Montana scientist Ramakrishna Nemani (now at NASA Ames Research Center, Moffett Field, Calif.) showed that land plant productivity was on the rise.
Running and co-author Maosheng Zhao originally set out to update Nemani’s analysis, expecing 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.
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 an algorithm 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,” Wickland said.
The researchers plan 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.
“Even if the declining trend of the past decade does not continue, managing forests and croplands 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.
The team published their findings Aug. 20 in Science.