Over the past 21 years, parts of the northern hemisphere have become much greener than they used to be. Researchers using satellite data have confirmed that plant life above 40 degrees north latitude (New York, Madrid, Ankara, Beijing) has been growing more vigorously since 1981.
One suspected cause is rising temperatures, possibly linked to the buildup of greenhouse gases in the atmosphere. Eurasia seems to be greening more than North America, with more lush vegetation for longer periods of time.
These results will appear in the September 16 issue of the Journal of Geophysical Research - Atmospheres, published by the American Geophysical Union. The authors are Liming Zhou, Robert Kaufmann, Nikolai Shabanov and Ranga Myneni of Boston University, and Daniel Slayback and Compton Tucker of NASA's Goddard Space Flight Center, Greenbelt, Maryland.
"When we looked at temperature and satellite vegetation data, we saw that year to year changes in growth and duration of the growing season of northern vegetation are tightly linked to year to year changes in temperature," said Liming Zhou from Boston University. The area of vegetation has not extended, but the existing vegetation has increased in density.
The authors also looked at the differences in vegetation growth between North America and Eurasia, because the patterns and magnitudes of warming are different on the two continents. The greenness data from satellites were strongly correlated to temperature data from thousands of meteorological stations on both continents.
The Eurasian greening was especially persistent over a broad contiguous swath of land from central Europe through Siberia to far-east Russia, where most of the vegetation is forests and woodlands. North America, in comparison, shows a fragmented pattern of change notable only in the forests of the east and grasslands of the upper Midwest.
Dramatic changes in the timing of both the appearance and fall of leaves are recorded in these two decades of satellite data. The authors report a growing season that is now almost 18 days longer, on average, in Eurasia, with spring arriving a week early and autumn delayed by 10 days. In North America, the growing season appears to be as much as 12 days longer.
The researchers used a temperature data set developed from the Global Historical Climate Network (HCN). Dr. James Hansen of NASA's Goddard Institute for Space Studies in New York, who developed this data set, said, "The data were compiled from several thousand meteorological stations in the United States and around the world. The stations also include many rural sites where the data are collected by cooperative private observers."
Myneni suggested that these results are indicative of a greener greenhouse. "This is an important finding because of possible implications to the global carbon cycle," he said. Carbon dioxide is a main greenhouse gas and is thought to play a major role in rising global temperatures.
Further, Myneni said, under the Kyoto protocol, most of the developed countries in the north can use certain vegetation carbon sinks to meet their greenhouse gas emissions reduction commitments. If the northern forests are greening, they may already be absorbing carbon. Myneni said, "As to how much and for how long, that needs more research."
Tucker developed the Normalized Difference Vegetation Index (NDVI) product to help determine the "greening" of plant life. The NDVI uses red and near-infrared solar radiation reflected back to sensors of the Advanced Very High Resolution Radiometers (AVHRR) aboard the National Oceanic and Atmospheric Administration (NOAA) series of polar-orbiting satellites.
These data are records of sensor observations of every patch of land on Earth, at least once a day, continuously from July 1981.
Processing of such massive amounts of data is a time consuming task, even on modern computers, and requires special methods to correct for atmospheric obscuration of Earth's surface.
The NDVI developed from processed data shows greening and browning of plants as they relate to seasonal changes and conditions such as drought or abundant rainfall.
The paper, Liming Zhou, Compton J. Tucker, Robert K. Kaufmann, Daniel Slayback, Nikolay V. Shabanov, and Ranga B. Myneni, "Variations in Northern Vegetation Activity Inferred from Satellite Data of Vegetation Index During 1981 to 1999," appears in the Journal of Geophysical Research - Atmospheres, Volume 106, number D17 (September 16, 2001), pages 20,069-20,083.
Boston University's Climate and Vegetation Research Group
American Geophysical Union
NASA's Goddard Space Flight Center
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