(a) Temperature and greenness (above)

(b) Changes in growing season duration (above)

An 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. Ground-based temperature data and satellite-based vegetation data indicate 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. The greening trend is more pronounced in Eurasia than in North America, particularly throughout the forests and woodlands in central Europe, Siberia, and far-east Russia.

The study used a global data set from the period 1981-1999, based on measurements by the AVHRR sensor onboard a series of NOAA satellites, and the Normalized Difference Vegetation Index (NDVI), which is expressed on a scale from -- 1 to +1, with values increasing for increasing amounts of vegetation.

Temperature and greenness: The northern latitudes (poleward of 23.6o N) have warmed by about 0.8o C since the early 1970s, but not all areas have warmed uniformally. The warming rate in the U.S. is smaller than in most of the world. The figure shows a time series of NDVI and land temperature anomaly between 40o N-70o N. On both continents, NDVI is positively correlated with the temperature anomaly during the growing season at the 1% significance level. These results suggest that warmer temperatures may have promoted plant growth in the north during the 1980s and 1990s.