"Greenhouse" Growing Greener on Patches of Earth, Study Finds

National Geographic News
September 6, 2001
Earth's "greenhouse" of plant life is growing more lush in some areas
of the Northern Hemisphere, according to satellite data from NASA
compiled over the past two decades.

Scientists attribute the
situation to longer growing seasons in North America and Eurasia
associated with rising temperatures and a buildup of greenhouse gases
such as carbon dioxide.

The finding may have a bearing on present interest in reducing excessive carbon dioxide in the atmosphere, which many people believe is helping to fuel a pattern of global warming. Plants and other vegetation act as "sinks" in absorbing carbon.

"This is an important finding because of possible implications to the global carbon cycle," said Ranga Myneni of Boston University, a member of the research team.

Under the Kyoto protocol of the international climate change agreement, developed countries may be able to meet some of their commitments to reduce greenhouse gas emissions by maintaining or expanding areas of vegetation.

Most developed countries lie in the Northern Hemisphere. If their forests are growing greener, Myneni said, they may already be siphoning off larger-than-expected amounts of carbon. "As to how much and for how long, that needs more research," he added.

The study was based on observations of every patch of land on Earth recorded continuously since 1981, acquired by a network of satellite sensors around the world. The images were recorded at least once a day.

These "greenness" images were correlated with temperature data from thousands of meteorological stations in North America and Eurasia, which experience different patterns and intensities of warming.

When scientists from NASA and Boston University analyzed the data, they found that while the total landmass of vegetation has not increased significantly, plants and trees have become more abundant in some areas above 40 degrees north latitude that are already green. That belt encompasses major cities such as New York, Madrid, Ankara, and Beijing.

The satellite images showed dramatic changes in the timing of when leaves first appeared in the spring and when they fell after the summer season ended.

The increased greening was most intense in Eurasia, where the growing season is now almost 18 days longer than it was two decades ago. Today, spring arrives a week earlier and autumn ten days later than typically occurred in the past.

Denser vegetation was especially evident across a broad swath of forests and woodlands extending from central Europe and through Siberia to far-east Russia.

In comparison, the pattern of change in North America's growth of vegetation was more fragmented, seen primarily in forests of the East and grasslands of the upper Midwest. On average, the growing season in North America was found to be 12 days longer.

"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 of Boston University, another member of the research team.

Zhou, Myneni, and their colleagues are reporting the findings in the September 16 issue of the Journal of Geophysical Research–Atmospheres, published by the American Geophysical Union.

The other co-authors were Robert Kaufmann and Nikolai Shabanov of Boston University and Daniel Slayback and Compton Tucker of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

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