Earth is not only getting warmer; it's getting greener as well, says a group of U.S. researchers in tomorrow's issue of Nature. Their analysis of satellite data shows that there has been a huge increase in vegetation around the world during the 1980s, and they pin this luxuriance on global warming. Both the analysis and the explanation are sure to spark controversy, however.
The data come from measurements of light reflected from Earth's surface by a series of satellites launched for the National Oceanic and Atmospheric Administration (NOAA). To drive photosynthesis, plants absorb a lot of light in the visible area of the spectrum, so more vegetation cover decreases the amount of visible light reflected back into space. By analyzing the changing ratio of infrared to visible light coming from Earth's surface, researchers can extrapolate the amount of vegetation needed to produce such ratios.
The analysis, done by researchers from Boston University; the Scripps Institution of Oceanography in La Jolla, California; NASA; and the University of Montana, showed that plant growth increased significantly around the world between 1981 and 1991. "There's quite a lot of change--10% in some areas," says Ranga Myneni, a geographer at Boston University and a member of the team. The changes are greatest in the upper northern latitudes. Armed with that information, the researchers propose that the increase is due to global warming. They point to higher temperatures and a longer growing season as possible mechanisms.
"It's a marvelously provocative article," says Inez Fung, a global change scientist at the University of Victoria in British Columbia. "They convinced me that there's a positive trend, but I'm not sure about the magnitude ... 10% over a decade--that's a whopping number." Fung notes that the data come from three satellites--NOAA-7, NOAA-9, and NOAA-11--the instruments of which may be calibrated differently, and that the authors extract their large change in vegetation from a small change in the reflected-light signal. Adds William Rossow, a climatologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who has learned that remote-sensing satellites can be fickle, "Ten percent over a decade--that's [produced by] a small enough signal that I'd be concerned."
Still, Fung thinks the results warrant further consideration: "I'm trained to be skeptical, but none of us are in a position to prove them wrong."