Image: NASA

It's not your imagination. Spring is arriving earlier and fall, somewhat later¿at least if you live above 40 degrees north latitude, the line that intersects New York, Madrid and Bejing. A recent analysis of two decades of satellite data confirms that the growing season in the northern hemisphere is getting longer and plant life is becoming more lush as well. Scientists from Boston University and NASA's Goddard Space Flight Center will publish the study in the September 16th issue of Geophysical Research Letters.

The scientists believe the change may stem from rising greenhouse gases in the atmosphere and resulting higher temperatures. "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 says. For temperature records, the team used the Global Historical Climate Network, developed by Goddard researcher James Hansen using data from several thousand meteorological stations around the world.

Of interest, the satellite pictures revealed a difference between Northern America and Eurasia: the greening trend is more pronounced to the east, particularly throughout the forests and woodlands in central Europe, Siberia and far-east Russia (see image). In the U.S., the most dramatic changes appeared in the eastern forests and grasslands of the upper midwest. In keeping, the researchers found that since 1982 the growing season in Eurasia has gained nearly 18 days, whereas it has extended by only 12 days in the U.S..

"This is an important finding because of possible implications to the global carbon cycle," Ranga Myneni of Boston University notes. "However, more research is needed to determine how much carbon is being absorbed, and how much longer it will continue."