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(Boston, Mass.) — Large regions of Earth have been “greening” in recent decades because of global climate changes that have brought more rainfall, better growing temperatures, and more sunlight, according to findings reported by Ranga Myneni, associate professor of geography at Boston University, and co-investigators in the June 6 issue of Science. The researchers indicate the increase, however, falls far short of the food, shelter, and heating demands of a burgeoning global population.
The increase in plant growth has been especially evident in areas such as northern Canada, the upper Great Lakes states of the U.S., Northern Europe, and the Amazon basin in South America, the researchers found. Their analyses may improve the way we assess how long-term changes in patterns of temperature, humidity, cloud cover, and rainfall will affect the environments of plants and humans that inhabit Earth.
Building on earlier studies by Myneni that showed an increase in the length of growing seasons and biomass in Northern Hemisphere forests, the current study was designed to provide a globally comprehensive analysis of how climate changes affect the growth of vegetation — from wildflowers to soybeans and cacti to seaweed — throughout the world.
Using a measure known as the net primary production (NPP), the researchers reanalyzed satellite observations of plant growth and global climate change data that had been gathered between 1982 and 1999. Climate changes during this period had many unwelcome outcomes such as record-setting temperatures, three intense El Niño events, and major changes in tropical cloudiness and in monsoon activity. The increase in plant growth found in this study was one welcome effect.
NPP quantifies annual plant growth by showing how much carbon, the basic chemical of life on Earth, remains stored in plants. Using measures of a basic life activity — the exchange of carbon dioxide gas — NPP is calculated by subtracting the amount of carbon dioxide removed from the atmosphere by photosynthesis from that released into the atmosphere by the respiration of plants. The difference is NPP, the source of most food, fiber, and fuel used by humans.
For this study, the researchers mapped the relative contributions that changes in water availability, temperature, and sunlight had on global plant growth. When they looked at how climate changes between 1982 and 1999 had affected growth in the regions of Earth that can sustain plant life, they found that water availability had the biggest effect on whether plants would flourish: changes in rainfall amounts strongly limited growth in more than 40 percent of Earth’s plant-growing area. Extremes in temperatures were the next biggest factor, limiting plant growth on 33 percent of Earth’s vegetated surface. Insufficient solar radiation was the limiting factor for 27 percent of Earth.
The researchers also found these climate factors were co-limiting, that is, they act in combination. Cold winters and dry summers, for example, limit vegetation in western North America while cold winters and cloudy summers limit plant growth in the northern regions of Europe and Asia. Tropical regions, usually unaffected by low temperatures, could show less plant growth because of long periods of dryness or of cloud cover that affects solar radiation.
Globally, the investigators found NPP increased by a little more than 6 percent during the 18-year period they analyzed. Because humans consume about 50 percent of the world’s NPP, this increase effectively made another 3 percent NPP available to humans. Human population, however, increased about 35 percent between 1982 and 1999; NPP fell short of demand by about 32 percent.
The researchers attributed 80 percent of the global NPP increase to plant growth in tropical regions and in northern Europe and Asia, Canada, and other regions in the high latitudes of the Northern Hemisphere. A single tropical region — that surrounding the Amazon River — registered a 1 percent increase during each of the years studied, making this region responsible for 42 percent of the world’s increase in plant production. The researchers speculate this increase was the result of increased solar radiation resulting from a decline in cloud cover over the region’s forests.
This research is reported in “Climate-Driven Increases in Global Terrestrial Net Primary Production from 1982 to 1999.” Its authors are Myneni of Boston University; Charles D. Keeling and Stephen C. Piper of the Scripps Institution of Oceanography at the University of California in San Diego; Compton J. Tucker of NASA/Goddard Space Flight Center in Greenbelt, Maryland; and Hirofumi Hashimoto, William M. Jolly, Steven W. Running, and lead author Ramakrishna R. Nemani of the School of Forestry at the University of Montana in Missoula.
Boston University, with an enrollment of more than 29,000 in its 17 schools and colleges, is the fourth-largest independent university in the United States.
Additional information may be found at http://cybele.bu.edu/globalgarden/globalgarden.html and at http://www.gsfc.nasa.gov/topstory/2003/0530earthgreen.html