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| Trees take hold as permafrost thaws near the Altai Mountains in Russia. [PHOTO: Terry Callaghan, EU-Interact/Sergey Kirpotin, Tomsk State University] |
Washington: Vegetation growth at Earth's northern latitudes increasingly
resembles lusher latitudes to the south, according to a NASA-funded study based
on a 30-year record of land surface and newly improved satellite data sets.
An international team
of university and NASA scientists examined the relationship between changes in
surface temperature and vegetation growth from 45 degrees north latitude to the
Arctic Ocean. Results show temperature and vegetation growth at northern
latitudes now resemble those found 4 degrees to 6 degrees of latitude farther
south as recently as 1982.
"Higher northern
latitudes are getting warmer, Arctic sea ice and the duration of snow cover are
diminishing, the growing season is getting longer and plants are growing
more," said Ranga Myneni of Boston University's Department of Earth and
Environment. "In the north's Arctic and boreal areas, the characteristics
of the seasons are changing, leading to great disruptions for plants and
related ecosystems."
The study was
published Sunday in the journal Nature Climate Change.
Myneni and colleagues
used satellite data to quantify vegetation changes at different latitudes from
1982 to 2011. Data used in this study came from NOAA's Advanced Very High
Resolution Radiometers (AVHRR) onboard a series of polar-orbiting satellites
and NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on
the Terra and Aqua satellites.
As a result of
enhanced warming and a longer growing season, large patches of vigorously
productive vegetation now span a third of the northern landscape, or more than
3.5 million square miles (9 million square kilometers). That is an area about
equal to the contiguous United States. This landscape resembles what was found
250 to 430 miles (400 to 700 kilometers) to the south in 1982.
"It's like
Winnipeg, Manitoba, moving to Minneapolis-Saint Paul in only 30 years,"
said co-author Compton Tucker of NASA's Goddard Space Flight Center in
Greenbelt, Md.
The Arctic's
greenness is visible on the ground as an increasing abundance of tall shrubs
and trees in locations all over the circumpolar Arctic. Greening in the
adjacent boreal areas is more pronounced in Eurasia than in North America.
An amplified
greenhouse effect is driving the changes, according to Myneni. Increased
concentrations of heat-trapping gasses, such as water vapor, carbon dioxide and
methane, cause Earth's surface, ocean and lower atmosphere to warm. Warming
reduces the extent of polar sea ice and snow cover, and, in turn, the darker
ocean and land surfaces absorb more solar energy, thus further heating the air
above them.
"This sets in
motion a cycle of positive reinforcement between warming and loss of sea ice
and snow cover, which we call the amplified greenhouse effect," Myneni
said. "The greenhouse effect could be further amplified in the future as
soils in the north thaw, releasing potentially significant amounts of carbon
dioxide and methane."
To find out what is
in store for future decades, the team analyzed 17 climate models. These models
show that increased temperatures in Arctic and boreal regions would be the equivalent
of a 20-degree latitude shift by the end of this century relative to a period
of comparison from 1951-1980.
However, researchers
say plant growth in the north may not continue on its current trajectory. The
ramifications of an amplified greenhouse effect, such as frequent forest fires,
outbreak of pest infestations and summertime droughts, may slow plant growth.
Also, warmer
temperatures alone in the boreal zone do not guarantee more plant growth, which
also depends on the availability of water and sunlight.
"Satellite data
identify areas in the boreal zone that are warmer and dryer and ¬¬other areas
that are warmer and wetter," said co-author Ramakrishna Nemani of NASA's
Ames Research Center in Moffett Field, Calif. "Only the warmer and wetter
areas support more growth."
Researchers did find
found more plant growth in the boreal zone from 1982 to 1992 than from 1992 to
2011, because water limitations were encountered in the latter two decades.