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March 19, 2007
NASA FINDS
SUN-CLIMATE CONNECTION IN OLD NILE
RECORDS
Long-term climate
records are a key to understanding how Earth's climate changed in the
past and
how it may change in the future. Direct measurements of light energy
emitted by
the sun, taken by satellites and other modern scientific techniques,
suggest
variations in the sun's activity influence Earth's long-term climate.
However,
there were no measured climate records of this type until the
relatively recent
scientific past.
Scientists have traditionally relied upon indirect data gathering
methods to
study climate in the Earth's past, such as drilling ice cores in
Greenland and Antarctica.
Such samples of accumulated snow and ice
drilled from deep within ice sheets or glaciers contain trapped air
bubbles
whose composition can provide a picture of past climate conditions.
Now,
however, a group of NASA and university scientists has found a
convincing link
between long-term solar and climate variability in a unique and
unexpected
source: directly measured ancient water level records of the Nile,
Earth's longest river.
Alexander Ruzmaikin and Joan Feynman of NASA's Jet Propulsion
Laboratory,
Pasadena, Calif., together with Dr. Yuk Yung of the California
Institute of
Technology, Pasadena, Calif., have analyzed Egyptian records of annual
Nile
water levels collected between 622 and 1470 A.D. at Rawdah Island in
Cairo.
These records were then compared to another well-documented human
record from
the same time period: observations of the number of auroras reported
per decade
in the Northern Hemisphere. Auroras
are bright glows in the night sky that happen when mass is rapidly
ejected from
the sun's corona, or following solar flares. They are an excellent
means of
tracking variations in the sun's activity.
Feynman said that while ancient Nile
and
auroral records are generally "spotty," that was not the case for the
particular 850-year period they studied.
"Since the time of the pharaohs, the water levels of the Nile were
accurately measured, since they were critically important for
agriculture and
the preservation of temples in Egypt,"
she said. "These records are highly accurate and were obtained
directly,
making them a rare and unique resource for climatologists to peer back
in
time."
A similarly accurate record exists for auroral activity during the same
time
period in northern Europe and the Far
East.
People there routinely and carefully observed and recorded auroral
activity,
because auroras were believed to portend future disasters, such as
droughts and
the deaths of kings.
"A great deal of modern scientific effort has gone into collecting
these
ancient auroral records, inter-comparing them and evaluating their
accuracy," Ruzmaikin said. "They have been successfully used by
aurora experts around the world to study longer time scale variations."
The researchers found some clear links between the sun's activity and
climate
variations. The Nile
water levels and aurora
records had two somewhat regularly occurring variations in common - one
with a
period of about 88 years and the second with a period of about 200
years.
The researchers said the findings have climate implications that extend
far
beyond the Nile River basin.
"Our results characterize not just a small region of the upper Nile,
but a
much more extended part of Africa,"
said
Ruzmaikin. "The Nile River
provides drainage
for approximately 10 percent of the African continent. Its two main
sources -
Lake Tana in Ethiopia
and
Lake Victoria in Tanzania,
Uganda
and Kenya
- are in equatorial Africa.
Since Africa's
climate is interrelated to climate variability in the Indian and Atlantic
Oceans,
these findings help us better
understand climate change on a global basis."
So what causes these cyclical links between solar variability and the Nile? The authors suggest that
variations in the sun's
ultraviolet energy cause adjustments in a climate pattern called the
Northern
Annular Mode, which affects climate in the atmosphere of the Northern
Hemisphere during the winter. At sea level, this mode becomes the North
Atlantic Oscillation, a large-scale seesaw in atmospheric mass that
affects how
air circulates over the Atlantic
Ocean. During
periods of high solar activity, the North Atlantic Oscillation's
influence
extends to the Indian Ocean.
These adjustments
may affect the distribution of air temperatures, which subsequently
influence
air circulation and rainfall at the Nile
River's
sources in eastern equatorial Africa.
When solar activity is high, conditions are
drier, and when it is low, conditions are wetter.
Study
findings were recently published in the Journal
of Geophysical Research.
For
more information and images, visit:
http://www.jpl.nasa.gov/news/features.cfm?feature=1319
http://www.nasa.gov/vision/earth/lookingatearth/nilef-20070319.html
##
Contact:
Alan Buis
NASA Jet Propulsion Laboratory
818-354-0474
This text is
derived from:
http://www.jpl.nasa.gov/news/features.cfm?feature=1319
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