02 October 2001

Researchers Discover Greener
Vegetation Growth in Northern Hemisphere

By Jim Fuller
Washington File Science Writer

Washington -- Researchers from Boston University and the National Aeronautics and Space Administration (NASA) report that a gradual greening of the Earth's Northern Hemisphere has occurred over the past two decades -- believed to be caused by rising temperatures linked to the buildup of greenhouse gases in the atmosphere.

The researchers used satellite data in correlation with temperature data from thousands of meteorological stations in North America and Eurasia to confirm that plant life seen above 40 degrees north latitude -- representing a line stretching from New York to Madrid to Beijing -- has been growing more vigorously since 1981.

The scientists, whose report appeared in the September 16 issue of the "Journal of Geophysical Research - Atmospheres," also said that the growing season in parts of the Northern Hemisphere has increased by several days over the same time period. Further, Eurasia appears to be greening more than North America, with more lush vegetation flourishing for longer periods of time.

"This is an important finding because of possible implications to the global carbon cycle," said Ranga Myneni, associate professor of geography at Boston University.

Carbon dioxide -- from the burning of fossil fuels -- is a main greenhouse gas and is thought to play a major role in rising global temperatures. The gas also promotes the growth of vegetation. Scientists believe that a warming of the Earth could also result in significant impacts such as sea level rise, changes in weather patterns and health effects.

Under the 1997 Kyoto Protocol, developed countries can use so-called carbon sinks, such as forests and croplands that absorb carbon from the atmosphere, to meet their commitments for reducing greenhouse gas emissions.

"If the northern forests are greening, they may already be absorbing more carbon than expected," Myneni said. "As to how much and for how long, that needs more research."

But the researchers believe that the gradual greening of the northern latitudes could help solve the mystery of the so-called "missing carbon." During the 1980s, fossil fuel use, deforestation and other land-use changes should have accounted for a total annual carbon dioxide emission rate estimated at about 6,000 million tons of carbon. Monitoring stations indicate, however, that only about 3,000 million additional tons of carbon are accumulating in the atmosphere each year.

Some scientists have recently suggested that the "carbon dioxide fertilization effect" may be causing land vegetation to absorb much more of the excess carbon dioxide than was previously thought. The idea is that plants -- especially those in the Northern Hemisphere where most man-made carbon dioxide is produced -- will grow faster, and so absorb more carbon dioxide.

"Basically scientists are not able to balance the global carbon cycle -- there's less carbon in the atmosphere than we thought," said Robert Kaufmann, a researcher at Boston University, in a interview. "Our estimate, based upon the satellite images of increased biomass in the Northern Hemisphere, is roughly consistent with what scientists think is missing from the atmosphere."

The researcher said the latest findings "may be the first step toward some kind of monitoring system where you could keep track of how much carbon is being absorbed by biomass in different countries."

Looking at the Northern Hemisphere, researchers found that plants have been growing more vigorously in Eurasia compared to North America.

They found that the Eurasian greening was especially persistent over a broad contiguous swath of land stretching from central Europe through Siberia to eastern 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.

The data showed that only about 30 percent of the vegetated areas between 40 and 70 degrees north latitude in North America show a high amount of greening. In contrast, more than 61 percent of the same region in Eurasia showed high greening. The area of vegetation has not extended, but the existing vegetation has increased in density.

The researchers also report a growing season that is now almost 18 days longer, on average, in Eurasia, compared to two decades ago -- with spring arriving a week early and autumn delayed by 10 days. In North America, the growing season appears to have become 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.

The researchers reported that differences in temperature change could account for the differences in vegetation between Eurasia and North America. While the Earth's northern latitudes have warmed by about 0.8 degrees Celsius since the early 1970s, not all areas have warmed uniformly.

The warming rate in the United States is smaller than in most of the world, while data for Eurasia suggests an increasingly greener region as temperatures continue to warm. Kaufmann says the difference may also be due to forest regrowth. "In large parts of Russia and former Eastern Europe you have people abandoning plots of land, and these plots are growing into forests," he said.

If the global average climate continues to warm, scientists predict that growing seasons will continue to extend. Northern areas that are now sparsely vegetated could become much more lush, and capable of absorbing far more carbon dioxide.

"The implications are that the further north you go, the longer the growing season is becoming," said Compton Tucker, senior Earth scientist as Goddard's Laboratory for Terrestrial Physics, in an interview. "So, for example, places like Canada, Sweden and the northern part of Russia should be able, if this continues, to have a better growing season. Whereas before their growing season was more limited by temperature."

Tucker developed the Normalized Difference Vegetation Index (NDVI) to help determine the "greening" of plant life. This index uses red and near-infrared solar radiation reflected back to sensors aboard polar-orbiting satellites. The sensors observed every patch of land on the Earth, at least once a day, continuously from July 1981. The vegetation index shows the greening and browning of plants as they relate to seasonal changes and conditions such as drought and abundant rainfall.