Vegetation is becoming denser across northern regions of the northern hemisphere as growing seasons lengthen. Chalk up another consequence of global warming.
Two decades worth of satellite data now show "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," says Liming Zhou of Boston University. His university colleague Ranga Myneni says that "we've seen an enormous amount of warming [in those regions] in the past 25 years." It amounts to 1.4 degrees Fahrenheit on average.
Generally, vegetation has grown denser, not increased in area, for land north of the 40-degree-latitude circle that runs roughly through Ankara, Beijing, New York, and Madrid.
The average growing season has increased by almost 18 days across northern Eurasia since 1981.
Most of the vegetation there is in forests and woodlands. It has lengthened by as much as 12 days in parts of North America, where vegetation change is significant mainly in Eastern forests and Midwestern grasslands. Dr. Myneni calls this "an important finding because of possible implications to the global carbon cycle."
Some of the heat-trapping carbon dioxide (CO2) humans put into the atmosphere is soaked up by soils and growing vegetation. Northern industrial nations that sign the Kyoto protocol to limit CO2 emissions can use the estimated amount of carbon such "sinks" sequester to help meet their CO2 emission-reduction goals.
The greening of northern forests may indicate that trees already are soaking up more of the emitted CO2. "As to how much [is being absorbed] and for how long, that needs more research," Myneni says.
Meanwhile, Britain's Royal Society has warned against relying on vegetation carbon sinks. Its report last July explains that these may soon saturate. Two ongoing experiments underscore that point.
Duke University is studying Loblolly pine grown in an enhanced CO2 atmosphere in North Carolina. Oak Ridge National Laboratory in Tennessee is conducting similar experiments with sweetgum. In both cases, the trees' ability to absorb CO2 maxed out after a few years. "The forest impact on carbon dioxide may not materialize in any important way," Duke ecologist Ram Oren told The Associated Press.
It's also hard to judge how much of the northern greening is human driven. Matthew Sturm and colleagues at the US Army Cold Regions Research and Engineering Laboratory in Fort Wainwright, Alaska, have found significant increase in shrub cover in the Alaskan Arctic.
Meanwhile, Feng Sheng Hu at the University of Illinois at Urbana-Champaign and colleagues have found two earlier Alaskan warming episodes in lime deposits in lakes. These occurred during the years 0 to 300 and 850 to 1200 of the present calendar. This offers what Professor Hu calls "baseline information on natural climatic variability" to help scientists separate natural variation from any human-induced climate change today.
Myneni notes that, while there's no smoking gun, there is a strong scent of burnt gun powder. Even the most advanced computer simulations can't explain all the current warming as natural.
In a paper published in the current issue of the Journal of Geophysical Research - Atmospheres, the scientists explain how they matched satellite data, which measures plant growth, with temperature data gathered by thousands of meteorological stations around the world.