Scientists recently looked at temperature data collected by monitoring stations around the world from 1981 through 1999 and images of plant activity captured by the National Oceanic & Atmospheric Administration's earth-sensing satellites over the same period. "Changes in growth and duration of the growing season of northern vegetation are tightly linked to changes in temperature," concludes Liming Zhou, a Boston University geographer who headed the research team. Many scientists believe that the changes in temperature may be caused by increases in greenhouse gases such as carbon dioxide.
Zhou and his colleagues found that the effect on plant growth is most pronounced in a band north of 40 degrees latitude -- a line that stretches from New York to Beijing to Madrid. But the scientists, who reported their findings in a recent issue of the Journal of Geophysical Research, found that the changes, while significant, vary between the continents.
Global Warming's Footprint:
View of the top of the world shows changes in vegetation since 1981. Purple indicates the highest change in the heartiness of plants, while yellow is the lowest
DRY SPELL. The biggest gains in lushness are occurring in Eurasia, along a broad swath that covers the grasslands and croplands of southern Russia as well as the wild forests and tundra of Siberia. North America presents a fragmented picture, with the most increases in the density of vegetation in the forests of the East and the grasslands of the upper Midwest.
Overall, the researchers found that 61% of vegetated land in Eurasia showed elevated levels of photosynthetic activity vs. only 30% in North America. The reason: Warming in much of North America has not been accompanied by increases in rainfall. In some places, including Alaska, a temperature-induced drought actually resulted in declines in plant growth.
The satellite data also revealed continental differences in the growing season. In North America, spring -- characterized by sprouting buds -- now arrives about 8 days earlier than it did in 1981, and autumn, marked by falling leaves, occurs 4 days later than two decades ago -- adding 12 days to the growing season. Eurasian spring arrives 6 days earlier, while fall there is delayed by nearly 12 days, tacking 18 days to the growing season.
Is this good news for farmers? Only up to a point, says Robert K. Kaufmann of Boston University, a coauthor of the report. "Individual plant species have an 'optimal temperature' beyond which further increases in temperature reduce their growth," he says.
HOT DEBATE. Nor will plants simply extend their ranges into the climates they prefer. "Plants cannot migrate fast enough to keep up with the changing temperature," says Kaufmann. One possible result: Temperature-sensitive plant species will begin to decline while those adapted to warmer regions will not move in to replace them.
This study, as well as others now under way, is likely to find its way into the negotiations for a global climate treaty. Some scientists and policymakers argue that plants, especially forests, are a vast "carbon sink" that will be goaded into more rapid growth by warmer temperatures and sop up the carbon dioxide spewed into the atmosphere by cars and factories.
The data indicate that, at least in North America, absorption capacity may be smaller than some previous estimates. "More research is needed to determine how much carbon is being absorbed and how much longer the absorption will continue," says Ranga Myneni of Boston University.
While the extent to which humans are contributing to global warming continues to be heatedly debated, it's clear that the "warming pulse" from 1981 to 1999 has had an immediate effect. The changing climate has also been noted in altered northward migration patterns of birds and butterflies. The earth isn't just a warmer place -- it's rapidly becoming a different place, probably in more ways than we realize. By Alan Hall in New York