June
11, 2007
THE
WOES OF KILIMANJARO: DON'T BLAME
GLOBAL WARMING
The "snows" of Africa's Mount Kilimanjaro inspired the title of an iconic
American short story, but
now its dwindling icecap is being cited as proof for human-induced
global
warming.
However, two researchers writing in
the July-August edition
of American Scientist magazine say global warming has nothing to do
with the
decline of Kilimanjaro's ice, and using the mountain in northern Tanzania as a "poster child" for climate
change is simply
inaccurate.
"There are dozens, if not hundreds, of photos of
midlatitude glaciers you could show where there is absolutely no
question that
they are declining in response to the warming atmosphere," said
climatologist Philip Mote, a University of Washington research scientist.
But in the tropics –
particularly on Kilimanjaro –
processes are at work that are far different from those that have
diminished
glacial ice in temperate regions closer to the poles, he said.
Mote and Georg Kaser, a glaciologist
at the University of Innsbruck in Austria, write in American Scientist that the
decline in
Kilimanjaro's ice has been going on for more than a century and that
most of it
occurred before 1953, while evidence of atmospheric warming there
before 1970
is inconclusive.
They attribute the ice decline
primarily to complex
interacting factors, including the vertical shape of the ice's edge,
which
allows it to shrink but not expand. They also cite decreased snowfall,
which
reduces ice buildup and determines how much energy the ice absorbs
– because the
whiteness of new snow reflects more sunlight, the lack of new snow
allows the
ice to absorb more of the sun's energy.
Unlike midlatitude glaciers, which are
warmed and melted by
surrounding air in the summer, the ice loss on Kilimanjaro is driven
strictly
by solar radiation. Since air near the mountain's ice almost always is
well
below freezing, there typically is no melting. Instead ice loss is
mainly
through a process called sublimation, which requires more than eight
times as
much energy as melting. Sublimation occurs at below-freezing
temperatures and
converts ice directly to water vapor without going through the liquid
phase.
Mote likens it to moisture-sapping conditions that cause food to suffer
freezer
burn.
Fluctuating weather patterns related
to the Indian Ocean
also could affect the shifting balance between the ice's increase,
which might
have occurred for decades before the first explorers reached
Kilimanjaro's
summit in 1889, and the shrinking that has been going on since.
Glaciers in more temperate latitudes
have declined sharply
as the troposphere around them has warmed (the troposphere is the
atmospheric
layer from the Earth's surface to about 10 miles in altitude). The best
example
of a glacier declining because of atmospheric warming might be the
South
Cascade Glacier in Washington state, perhaps the most-studied
glacier in North
America.
Photographs by government scientists in 1928 and in 2000, along with
detailed
surveys, showed that the glacier lost half its mass during that time.
Similar
evidence exists for a number of other glaciers, Mote said.
But in their analysis of already
published research, Kaser
and Mote say the same factors do not apply to Kilimanjaro's icecap,
even though
its decline has been cited in forums such as the Academy Award-winning
documentary film "An Inconvenient Truth."
"There is no evidence to support that
assertion,"
Mote said. "It's not that it is impossible, but rather the decline is
most
likely associated with processes dominated by sublimation and with an
energy
balance dominated by solar radiation, rather than by a warmer
troposphere."
The volcano Kibo is the highest point
on Kilimanjaro, about
19,340 feet above sea level. A rough survey in 1889 suggested that
Kibo's
icecap occupied about 12.5 square miles. By 1912, more than two decades
before
Ernest Hemingway wrote his masterpiece short story "The Snows of
Kilimanjaro," it had dwindled to about 7.5 square miles. By 1953 it had
shrunk to about 4.3 square miles and by 2003 it was at a little more
than 1.5 square
miles.
The level of nearby Lake Victoria,
the world's largest tropical freshwater lake, also declined in the late
19th
century, when the decline of Kibo's icecap began. The lake and the
icecap
likely suffer from a precipitation decline caused by Indian Ocean
variability, which also could also have caused the icecap to vary in
size and
shape over millennia, Mote said.
"It is certainly possible that the
icecap has come and
gone many times over hundreds of thousands of years," he said. "But
for temperate glaciers there is ample evidence that they are shrinking,
in part
because of warming from greenhouse gases."
##
Contact:
Vince
Stricherz
vinces@u.washington.edu
206-543-2580
University of
Washington
This
text derived from:
http://uwnews.washington.edu/ni/article.asp?articleID=34106
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