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Oct.
3 , 2006: In
June 1912, Novarupta—one of a chain of volcanoes on the Alaska
Peninsula—erupted in what turned out to be the largest blast
of the twentieth century. It was so powerful that it drained
magma from under another volcano, Mount Katmai, six miles
east, causing the summit of Katmai to collapse to form a caldera
half a mile deep. Novarupta also expelled three cubic miles
of magma and ash into the air, which fell to cover an area
of 3,000 square miles more than a foot deep.
 Despite
the fact that the eruption was comparable to that of the far
more famous eruption of Krakatau in Indonesia in 1883 and
so near the continental United States, it was hardly known
at the time because the area was so remote from English-speaking
people.
Right:
An aerial view of the Novarupta Dome in Alaska. USGS photo
by Gene Iwatsubo, July 29, 1987. [More]
Almost
a hundred years later, researchers are paying attention. Novarupta
is near the Arctic Circle and its impact on climate appears
to be quite different from that of "ordinary" tropical
volcanoes, according to recent research by climatologists
using a NASA computer model.
When
a volcano anywhere erupts, it does more than spew clouds of
ash, which can shadow a region from sunlight and cool it for
a few days. It also spews sulfur dioxide. If the eruption
is strongly vertical, it shoots that sulfur dioxide high into
the stratosphere more than 10 miles above Earth.
Up
in the stratosphere, sulfur dioxide reacts with water vapor
to form sulfate aerosols. Because these aerosols float above
the altitude of rain, they don't get washed out. They linger,
reflecting sunlight and cooling Earth's surface.
This
can create a kind of nuclear winter (a.k.a. "volcanic
winter") for a year or more after an eruption. In April
1815, for instance, the Tambora volcano in Indonesia erupted.
The following year, 1816, was called "the year without
a summer," with snow falling across the United States
in July. Even the smaller June 1991 eruption of Pinatubo in
the Philippines cooled the average temperature of the northern
hemisphere summer of 1992 to well below average.
But
both those volcanoes as well as Krakatau were in the tropics.
Novarupta
is just south of the Arctic Circle.
Using
a NASA computer model at the the Goddard Institute for Space
Studies (GISS), Prof. Alan Robock of Rutgers University and
colleagues found that Novarupta's effects on the world's climate
would have been different. (Their research was funded by the
National Science Foundation.)
 Robock
explains: "The stratosphere's average circulation is
from the equator to the poles, so aerosols from tropical volcanoes
tend to spread across all latitudes both north and south of
the Equator." Aerosols would quickly circulate to all
parts of the globe.
But
the NASA GISS climate model showed that aerosols from an arctic
eruption such as Novarupta tend to stay north of 30ºN—that
is, no further south than the continental United States or
Europe. Indeed, they would mix with the rest of Earth's atmosphere
only very slowly.
Right:
The inner workings of "volcanic winter," from Robock,
Alan, 2000: Volcanic eruptions and climate. Rev. Geophys.,
38, 191-219. Copyright 2000 AGU. [More]
[Larger image]
This
bottling up of Novarupta's aerosols in the north would make
itself felt, strangely enough, in India. According to the
computer model, the Novarupta blast would have weakened India's
summer monsoon, producing "an abnormally warm and dry
summer over northern India," says Robock.
Why
India? Cooling of the northern hemisphere by Novarupta would
set in motion a chain of events involving land and sea surface
temperatures, the flow of air over the Himalayan mountains
and, finally, clouds and rain over India. It's devilishly
complex, which is why supercomputers are needed to do the
calculations.
To
check the results, Robock and colleagues are examining weather
and river flow data from Asia, India, and Africa in 1913,
the year after Novarupta. They are also investigating the
consequences of other high-latitude eruptions in the last
few centuries.
Do
Indians need to keep an eye on Arctic volcanoes? The GISS
computer says so. Stay tuned to Science@NASA for updates.
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Author: Trudy E.
Bell | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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