The Old Testament
may seem like an unlikely source from which to draw inspiration for a modern-day
climatology study. But a story from the book of Genesis — in which Joseph
predicts seven years of abundant crops, followed by seven years of famine for
Egypt — drove researchers to scour centuries of water-level data for the
Nile River to determine if such a cycle actually exists, and if so, what causes
it.
This Byzantine-period mosaic from northern
Israel shows a man carving on a “nilometer” the highest level the
Nile reached in that year. The picture inspired geoscientists to revisit historical
river-level data and fill in the gaps, to reveal a seven-year cycle. Photograph
by Yigal Feliks, by permission of the Israel Nature and Parks Protection Authority.
Climatologists have already combed the abundant Nile River data and revealed
a connection between patterns in the water-level cycles and Indo-Pacific Ocean
patterns. But large gaps in the data, especially after A.D. 1470, have left
this analysis incomplete. Now, Michael Ghil, a geophysicist at the University
of California, Los Angeles, developed with colleagues what he calls an advanced
technique to fill the gaps. Analysis of the data, published in the May 24 Geophysical
Research Letters, turns up evidence for a seven-year cycle that researchers
say may be influenced by the North Atlantic ocean.
Ghil, working with Dmitri Kondrashov, the lead author, and Yizhak Feliks, attributes
credit for the idea to Feliks, who thought of the possible biblical connection
after visiting a Byzantine-period mosaic that was created a century or two before
the start of the Nile River flood records in A.D. 622. It shows a man clambering
to etch onto a column — called a “nilometer” — the highest
level reached by the Nile that year.
Locals routinely monitored water levels for the following 1,300 years because
the Nile directly affected their agricultural livelihood. Annual summer floods
covered the region and then receded, leaving behind arable soil for winter crops.
The resulting data, among the longest climatic records available, have been
of great interest to climatologists. “People have analyzed such records
for a long time, going back to the early 20th century,” Ghil says.
The dataset, however, as extensive as it appeared, was not without problems.
Typically when working with historical climate records, more data gaps exist
in the beginning. But for the Nile records, the early dataset between A.D. 622
and 1470 seems to be fairly complete. After 1470, however, full decades of data
went missing, and the record ended in 1922. Previously, researchers replaced
the missing data with the mean value, which was not always accurate. “The
resulting records were analyzed using fairly traditional methods,” Ghil
says.
To better fill in the gaps, Ghil and colleagues used an advanced form of the
Singular Spectrum Analysis, which helps to separate data into statistically
independent components that the researchers can classify as a trend, oscillations
or noise. “Using data adaptive methods, we are actually interrogating data
in a much more intensive manner,” Ghil says.
“They used a more advanced, sophisticated statistical method than previous
researchers,” says Matthew Lachniet, a paleoclimatoligist at the University
of Nevada, Las Vegas. What struck him about the study was the decision to include
in the models not only the high-water levels, but also the difference between
high and low measurements. “It’s not as common as only looking at
high levels,” Lachniet says. “I think it’s an interesting contribution.”
With the gaps filled in, Ghil and colleagues discovered multiple cycles in water
levels, ranging on scales from two to 256 years, which they say can be attributed
to several known influences, including that of the El Niño Southern Oscillation
(ENSO) from the Pacific Ocean. But the most striking cycle had a seven-year
period, which Ghil thinks is due to an additional influence from the North Atlantic
Oscillation (NAO), the Atlantic’s version of the Pacific’s ENSO.
Models of the North Atlantic’s variability and of the NAO — characterized
by changes in air pressure and storm tracks — show that they contribute
to weather patterns in Europe, the Middle East, central Asia and North Africa.
But according to Ghil, the influence likely extends farther south, all the way
to the East African sources of the Nile River. He and his colleagues found that
the Nile’s seven-year cycle parallels a known seven- to eight-year peak
in recorded NAO data. “Making that link to the North Atlantic doesn’t
surprise me,” Lachniet says. “The North Atlantic has dominant control
on climate globally.”
Ghil says that short periodicities of ENSO have been used to predict future
Nile River water levels, but the newly detected NAO cycles have not yet been
used as such. The first step would be to contact the hydrological services in
Egypt, Sudan and Ethiopia, and gather at least 30 to 40 years of recent Nile
data, Ghil says. “That would be fun.”
Kathryn Hansen
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