Northern hemisphere is getting greener
Over the past 21 years, parts of the northern hemisphere have become much greener than they used to be.
Using satellite data, researchers from Boston University and NASA’s Goddard Space Flight Center have confirmed that plant life above 40 degrees north latitude, or above the cities of New York, Madrid, Ankara and Beijing, has been growing more vigorously since 1981 due to rising temperatures and buildup of greenhouse gases. They have also discovered that Eurasia seems to be greening more than North America, as existing vegetation is more lush for longer periods of time. “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 Liman Zhou, one of the researchers involved, adding that the area of vegetation has not extended, but the existing vegetation has increased in density.
The researchers also examined the differences in vegetation growth between North America and Eurasia, because the patterns and magnitudes of warming are different on the two continents. The greenness data from satellites were strongly correlated to temperature data from thousands of meteorological stations on both continents with the Eurasian greening being especially persistent over a broad contiguous swath of land from central Europe through Siberia to far-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.
Dramatic changes in the timing of both the appearance and fall of leaves are recorded in the two decades of satellite data with the researchers reporting a growing season that is now almost 18 days longer, on average, in Eurasia, with spring arriving a week early and autumn delayed by 10 days. In North America, the growing season appears to be as much as 12 days longer.
“This is an important finding because of possible implications to the global carbon cycle,” commented co-researcher, Ranga Myneni. Because under the Kyoto Protocol most of the developed countries in the north can use certain vegetation carbon sinks to meet their greenhouse gas emissions reduction commitments, more carbon dioxide may be being absorbed than was previously thought. “As to how much and for how long, that needs more research,” Myneni added.
In order to determine the ‘greening’ of plant life, researchers from the Goddard Space Flight Center developed the Normalised Difference Vegetation Index (NDVI) product, which uses red and near-infrared solar radiation reflected back to sensors of the advanced very high resolution radiometers aboard a series of polar-orbiting satellites. These data are records of sensor observations of every patch of land on Earth, at least once a day, continuously from July 1981, which were compiled from several thousand meteorological stations, and, in rural areas, by private observers, from around the world.
Processing of such massive amounts of data is a time consuming task, even on modern computers, and requires special methods to correct for atmospheric obscuration of Earth’s surface. The NDVI developed from processed data shows greening and browning of plants as they relate to seasonal changes and conditions such as drought or abundant rainfall.