The Northern Hemisphere is greener now than it was 20 years ago, possibly because the Earth’s temperatures are rising, according to a NASA-funded study. The scientists found that the growing season has lengthened and vegetation density has increased above 40 degrees north latitude. In North America, the growing season is now as much as 12 days longer; in Europe and Asia, the season has extended almost 18 days, with spring arriving earlier and fall later.