Warming Sparks Greener Pastures

A new green movement is under way. According to a series of studies presented this week at the joint Global Change and Terrestrial Ecosystems and Land Use/Cover Change Conference in Barcelona, Spain, a 17-year-long greening trend in northern regions continues unabated, and some warmer weather vegetation types are advancing north as well.

Ecologist Ranga Myneni of Boston University reported that satellite data show plant growth in the north increased 1% a year from 1991 to 1994, about the same rate it did during the 1980s. The greening appears to stem from a 1- to 2-week-longer growing season brought about by warmer springtime temperatures, says Myneni. The effect, he says, is greatest between 45 and 70 degrees north, a broad swath stretching from the Great Lakes to northern Alaska. A similar trend has not been seen in the Southern Hemisphere.

The same satellite data also reveal changes in the distribution of different vegetation patterns, or biomes, according to Nicolas Viovy of the Commissariat a l'Énergie Atomique in France. Viovy's team matched biomes, such as forests and tundra, with the characteristic frequencies of light they reflect into space and used satellites to track changes from 1982 to 1993. "The results show changes in high latitudes, particularly in Scandinavia and eastern Siberia, with a northward shift of biomes," indicating an expansion of forests further north, Viovy says. Both findings are consistent with recent climatological records: While mean global temperatures have risen only slightly over the last 30 years, winter and spring temperatures in middle to high northern latitudes have increased by as much as 4 degrees Celsius, resulting in earlier spring snowmelts.

Climate experts say this northern greening could have a significant impact on global climate, although for now the picture is too complex to sort out. For one thing, the amount and type of vegetation can alter how much greenhouse-warming carbon dioxide ends up in the atmosphere, says Anthony King, a climate modeler at Oak Ridge National Laboratory in Tennessee. According to King, a longer growing season raises the possibility that vegetation will pull CO2 out of the air as plants grow. But, King cautions, higher temperatures speed the decay of organic matter, causing CO2 to escape from the soil into the atmosphere.