The Northern Hemisphere is greening up, according to an analysis of 18 years of satellite data. Spring is coming earlier, fall later, and a measure of chlorophyll visible from orbit shows that plants are responding to the changed conditions. Researchers blame greenhouse gases and global warming.
To track Earth's greening, geographer Liming Zhou of Boston University and colleagues at NASA used a measure called the normalized difference vegetation index (NDVI), which relies on satellite-based spectral measurements that gauge chlorophyll levels in grass, shrubs, and trees. The phenomenon is especially pronounced in Siberia and eastern Russia, they found, where almost two-thirds of the vegetated areas above 40° north latitude (the level of New York City) show substantial increases in vegetation density over the 18-year period, compared with about 30% of such areas in North America, which has been warming up at a lower rate. Growing seasons in some parts of Eurasia, the analysis showed, have lengthened by as much as 18 days in the past 2 decades, the team reports in the 16 September issue of the Journal of Geophysical Research-Atmospheres.
Environmental scientist William Schlesinger of Duke University says the study is "a very solid piece of work, very intriguing ... it's exactly what you'd expect of a planet that's undergoing global warming." Indeed, the new measures correlate well with data from meteorological stations all over the Northern Hemisphere, which indicate an average rise of 0.8°C in surface temperature since the early 1970s.