GLOBAL WARMING: Satellite images taken over decades reveal startling trend, researchers say.
The tundra of Alaska and northern Canada has been "greening" dramatically as the Arctic warms, with more plant growth and longer growing seasons, according to a new study that analyzed thousands of satellite images taken over two decades.
But in the vast boreal forests that stretch from Alaska's Interior into northeastern Canada, the satellites uncovered a far different outcome.
After an initial growth spurt triggered by rising temperatures and carbon dioxide concentrations, the greening of Northern forests had actually declined by 2003 -- surprising the scientists conducting the survey and suggesting one factor in the surge of summer wildfires over the past few years.
The causes remain complex, but the forests appear to be drying out as the air warms up, said Scott Goetz, a senior scientist at Woods Hole Research Center in Massachusetts and one of four scientists working over the satellite data. Insect outbreaks and lack of nutrients are some other factors.
"Everyone was assuming that these forests were going to continue to green, and it turns out that there may be other factors that are causing unexpected results," Goetz said in a telephone interview. "Alaska's pretty dramatic. There's quite a gradient in the changes that we're seeing from the Interior forests of Alaska to the North Slope."
A different study examined a patch of tundra near Nome and found that all this new growth may be making things warmer and triggering even more growth.
Bushes that grow above the snow make the surface darker, trapping more and more of the late winter sun, accelerating heating by as much as 70 percent, said longtime Alaska snow researcher Matthew Sturm.
The two studies reached similar conclusions about tundra growth using very different tools: One spied on a continent with weather satellites, while the other intercepted sunlight a few feet above the ground in the hills northeast of Nome.
Both glimpses, from space and Earth, are part of a new research trend that monitors vegetation for evidence of climate change in the Arctic, said Sturm, with the U.S. Army Cold Regions Research and Engineering Laboratory at Fort Wainwright.
Global air temperature has risen at least 1 degree Fahrenheit over the past century, and many scientists agree that the rise of human-produced greenhouse gases such as carbon dioxide is a major cause. The Arctic, where average temperatures have risen more than 3 degrees in a half century, has been hit hardest so far, with disintegrating permafrost, melting glaciers and a reduction in summer sea ice.
Shrinking ice "is very tangible and a relatively easy thing to track," Sturm said. "I think we're just starting to get a handle on land surface changes."
That rising temperatures cause problems for Alaska's Interior spruce forests has already been shown in studies by University of Alaska Fairbanks ecologist Glenn Juday, a lead author on last year's Arctic Climate Impact Assessment. Changing climate would shift the zones where certain plants can grow, Juday and his co-authors found. Shrubs and trees might advance north into tundra, but existing forests would face stress from buggy, parched summers.
Combined with the vast forest fires that swept through Alaska in 2004 and 2005, the new satellite study confirms predictions that growth will fall in the dry areas of the Northern forests, Juday said Wednesday in an e-mail message.
It "clarifies and advances our confidence in the findings quite a bit," he said. "There is no doubt about it now."
The new satellite study examined seasonal "greening" seen from space over time. The report, by Goetz and three other researchers, was published online in the Proceedings of the National Academy of Sciences.
The scientists gathered thousands of images of 5-mile-square grids taken every 15 days over 22 years -- from 1982 to 2003 -- and then calculated the total amount of photosynthesis (greening) taking place in each grid as time passed. The scientists also included the type of land cover -- forest or tundra or farm -- air temperatures and forest fires in the analysis.
"It was very computer intensive," Goetz said.
The results were complicated. Plant action rose in the 1980s, fell for a few years after the 1991 eruption of Mount Pinatubo blocked sunlight and cooled the Northern Hemisphere, then rose again in the mid-1990s.
At least 15 percent of Alaska and northern Canada showed big changes. Almost all the boost appeared in tundra areas, with some of the most dramatic examples north of the Brooks Range in Alaska and along the coast, Goetz said.
This increase in photosynthesis in the tundra may be moving faster and faster, according to Sturm's study of shrubs and snow on the Seward Peninsula. The paper was published in the Journal of Geophysical Research.
The scientists used cables to suspend instruments measuring how much sunlight was reflected or absorbed over tundra at five locations in the hills near Council, Sturm said. The spots ranged from tundra with no bushes to places where brush had taken over.
"Out on the tundra everything is covered in snow and everything is white, and an awful lot of that energy is going back into space," Sturm explained. But when shrubs grow enough to stick above the snow, things begin to change.
"All those dark shrubs sticking up will be absorbing solar energy, and they will make it warmer," he said.
"It's one more feedback loop," he said. Winter warmth ends up amplifying summer growth.
But in contrast to the greening tundra and its new shrubs, Interior trees suffered from the middle 1990s on. After the satellite researchers subtracted land burned by forest fires, they found vast reaches of boreal forest where photosynthesis dropped over 22 years. The length of the growing season was unchanged.
Interior forests around Fairbanks and along the Yukon River Valley showed some of the most intense declines, Goetz said.
Alaskans "are probably already aware that they're right at the heart of the changes that are occurring," he said. "These satellite observations confirm what people are seeing with their own eyes."Anchorage Daily News
