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Conventional wisdom holds that plants should thrive when temperatures warm and the growing season lengthens under a changing climate. A 2003 study showed that plant growth indeed increased as temperatures warmed between 1982 and 1999. But when researchers at the University of Montana updated the study for the last decade, 2000–2009, they discovered that even though the decade was the warmest since instrumental recordkeeping began, plant growth slowed.
The study is significant because plants soak up carbon from the atmosphere as they grow. As growth slows, the amount of carbon plants take up slows as well, leaving more carbon in the atmosphere to contribute to climate change. It is also a warning that a warming climate could bring a decrease in food and biofuel production.
Researchers Maozheng Zhou and Steven Running published the work in Science on August 20, 2010. They used data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite to monitor plant growth and calculate the total amount of carbon plants store as they grow, a value called net primary productivity. Their results are illustrated in this image, which shows the change in the amount of carbon consumed by plants between 2000 and 2009. Green areas show where plants grew more (and consumed more carbon), and brown areas show where they grew less. The deeper the color, the stronger the trend.
In general plant growth and consequently the amount of carbon plants store increased in the Northern Hemisphere and decreased in the Southern Hemisphere. The difference, say Zhou and Running, is water. Three things limit plant growth: light, water, and temperature. In the Northern Hemisphere, warmer temperatures and a longer growing season allowed plants to grow more. But in the Southern Hemisphere, drought dominated the decade. Higher temperatures dried plants and soil, generally slowing growth. The attached graphs illustrate the direct relationship between drought and the amount of carbon plants absorbed between 2000 and 2009.
During the decade, large-scale droughts, primarily in the Southern Hemisphere, were strong enough to overwhelm increased plant growth in other places, resulting in decreased plant growth overall. While 65 percent of the Northern Hemisphere’s vegetated land had increasing growth, 70 percent of the Southern Hemisphere’s vegetated land had decreasing growth. Added together, global plant growth decreased slightly in the decade, reducing carbon uptake by an estimated 0.55 petagrams, a one percent decrease.
It is too early to tell if the recent decade marks the beginning of a longer term trend in plant growth related to long-term climate change. Climate is defined by patterns seen over longer periods of time, at least 20-30 years. In the previous 20 years (1982–1999), plant growth and the related carbon uptake increased as much as six percent. Regardless, the observed decrease in plant growth between 2000 and 2009 helps researchers better understand how plants might respond to a changing climate.
“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,” Running said. “This is a pretty serious warning that warmer temperatures are not going to endlessly improve plant growth.”
Drought Drives Decade-Long Decline in Plant Growth
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A snapshot of Earth’s plant productivity in 2003 shows regions of increased productivity (green) and decreased productivity (red). Tracking productivity between 2000 and 2009, researchers found a global net decrease due to regional drought. Credit: NASA Goddard Space Flight Center Scientific Visualization Studio
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Interannual shifts in plant productivity (green line) fluctuated in step with shifts in atmospheric carbon dioxide (red line) between 2000 through 2009. Credit: Maosheng Zhao and Steven Running Earth has done an ecological about-face: Global plant productivity that once flourished under warming temperatures and a lengthened growing season is now on the decline, struck by the stress of drought.
NASA-funded researchers Maosheng Zhao and Steven Running, of the University of Montana in Missoula, discovered the global shift during an analysis of NASA satellite data. Compared with a six-percent increase spanning two earlier decades, the recent ten-year decline is slight — just one percent. The shift, however, 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,” Running said.
“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.
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, Moffett Field, Calif.) showed that global terrestrial plant productivity increased as much as six 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, Zhao and Running 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. The team published their findings Aug. 20 in Science.
“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 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 very 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.
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This narrated video gives an overview of net primary production and the carbon cycle. High-resolution data from the Moderate Resolution Imaging Spectroradiometer, or MODIS, indicate a net decrease in NPP from 2000-2009, as compared to the previous two decades. Credit: NASA/Michelle Williams
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.
“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.