NASA satellite data suggest that for more than two decades there's been a gradual greening of the northern latitudes of Earth.

Researchers confirm that plant life seen above 40 degrees north latitude, which represents a line stretching from New York to Madrid to Beijing, has been growing more vigorously since 1981. One suspected cause is rising temperatures possibly linked to the buildup of greenhouse gases in our atmosphere.

Over this same time period, parts of the Northern Hemisphere have become much greener and the growing season has increased by several days. Further, Eurasia appears to be greening more than North America, with more lush vegetation for longer periods of time.

This view of the top of the world shows enhanced plant growth over the last 20 years, from the north pole, stretching southward to the 30 degree North latitude line (the circle). The color key represents changes in vegetation lushness. It ranges from a low increase in the heartiness of vegetation as denoted by the yellow color, to the highest increase in the lushness of plants as denoted by the purple color. Photo: NASA

The results of this NASA-funded research will appear in the September 16 issue of the Journal of Geophysical Research - Atmospheres.

"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," Liming Zhou of Boston University said. 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 NorthAmerica and Eurasia, since the patterns and magnitudes of warming are different on the two continents.

The greenness data from satellites were strongly correlated with temperature data from thousands of meteorological stations on both continents. The Eurasian greening was especially persistent over a broad area 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 in Eurasia that is now nearly 18 days longer. Spring arrives a week early and autumn is 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. Dr. James Hansen, of NASA's Goddard Institute for Space Studies in New York, developed this data set and 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."

Scientists believe the results indicate a greener greenhouse. "This is an important finding because of possible implications to the global carbon cycle," said Ranga Myneni of Boston University. "However, more research is needed to determine how much carbon is being absorbed, and how much longer it will continue."

Carbon dioxide is a main greenhouse gas and is suspected of playing a role in rising global temperatures. If the northern forests are greening, they may already be absorbing carbon -- a process that can impact global temperature changes.

Researchers used the Normalized Difference Vegetation Index (NDVI) to help determine the "greening" of plant life. Dr. Compton Tucker, of NASA's Goddard Space Flight Center, Greenbelt, Md., is a co-author of the report and developed NDVI to analyze spectral data collected by orbiting weather satellites.

This work was made possible through funding by NASA Headquarters' Earth Science Enterprise, a long-term research program dedicated to understanding how human-induced and natural changes affect our global environment.