Much of the Earth is getting greener, researchers say, but that's not necessarily good news for the environment.
A study of satellite data covering a wide swath of the Northern Hemisphere found that most of the area specially in Europe and Asia has become more densely packed with vegetation over the past 20 years.
But the growth could be the result of warmer temperatures, researchers said.
``It's yet another bit of evidence of the human impact on climate,'' said Ranga Myneni, one of the study's authors and an associate professor of geography at Boston University.
Increasing human production of carbon dioxide and other so-called greenhouse gases is widely blamed for rising global temperatures. In the northern latitudes, the focus of the study, temperatures have risen about 0.8 degrees Celsius since the 1970s.

Myneni and colleagues at Boston University and NASA's Goddard Space Flight Center in Greenbelt, analysed data from weather satellites from 1981 to 1999 for Europe, Canada and most of the United States and Asia.
They found that plant life north of 40 degrees north latitude — roughly that of New York, Madrid, Spain and Beijing — has been growing more vigorously since 1981.
``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 said in a statement.
The area of vegetation has not extended, but the existing vegetation has increased in density, he said.
In an area from central Europe to Siberia, researchers found that more than 60 per cent of the vegetated area has been growing more vigorously over the last two decades. In addition, the growing season in Eurasia lengthened by about 18 days, the researchers found.
The changes were less pronounced in North America, which has seen average temperatures fall in some eastern areas. There was a fragmented pattern of change notable only in the forests of the east and grasslands of the upper Midwest. Researchers also found dramatic changes in the timing of both the appearance and fall of leaves over the two decades of satellite data.
The increased vegetation found in the study might help cut greenhouse gases by drawing carbon dioxide out of the atmosphere, but that depends on what type of vegetation is gaining ground, Myneni said.
``What's good for the plants is not necessarily good for the planet,'' he said.
Michael E Mann, a professor of environmental science at the University of Virginia who was not involved in the study, said the study is important because it uses satellite data to bolster evidence for human-caused climate change.
The study is to appear in the September 16 issue of the Journal of Geophysical Research-Atmospheres, published by the American Geophysical Union. Bureau Report