A Greener Planetary Greenhouse

Image of northern forest
Along with other northern vegetation, high-latitude forests have been growing more vigorously since 1981.
Credit: USDA Forest Service.

For more than two decades, northern hemisphere vegetation has become gradually more lush, according to new research based on NASA satellite data.

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 possible cause is rising temperatures, linked perhaps to the buildup of greenhouse gases in our atmosphere.

The area of northern vegetation has not actually expanded, but it has increased in density. The growing season has also increased by several days. Furthermore, Eurasia appears to be greening more than North America, with more lush vegetation for longer periods of time.

"When we looked at temperature and satellite vegetation data, we saw that year-to-year changes in growth and the duration of the growing season were tightly linked to year-to-year changes in temperature," said Liming Zhou of Boston University.

Zhou and colleagues also examined the differences in vegetation growth between North America and Eurasia, because the patterns and magnitudes of warming on the two continents are different.

Chart displaying tempature vs. vegetation
In North America, temperature anomalies (red bars) averaged during the months April through October are correlated with the greenness of vegetationd. Click on the image for full-size graphs showing data for North America and Eurasia.

The greenness data from satellites were strongly correlated with temperature data from thousands of meteorological stations on both sides of the world. 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.

Global image of Earth showing
Researchers used the Normalized Difference Vegetation Index (NDVI) to measure the "greening" of plant life. The algorithm relies on spectral data collected by orbiting weather satellites, such as the vegetation data shown on this map. [more information]

Dramatic changes in the timing of both the appearance and fall of leaves are recorded in these two decades of satellite data. The researchers reported a growing season in Eurasia that is now nearly 18 days longer than it was before. 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 planetary 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 scientists suspect it plays an important 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.

The greening trend revealed by this research provides an important piece of the puzzle of global climate change, and will help scientists produce more accurate predictions of how greenhouse gases will affect our climate in the decades to come.