Climate, Civilization, and Dynasties

———————– Key Quotes —————

“A lack of rainfall could have contributed to
social upheaval and the fall of dynasties.

“The researchers discovered that periods of weak
summer monsoons coincided with the last years of
the Tang, Yuan and Ming dynasties, which are
known to have been times of popular unrest.

“Conversely, the scientists found that a strong
summer monsoon prevailed during one of China’s
“golden ages,” the Northern Song Dynasty.”

” …  the study showed that the dry period at
the end of the Tang Dynasty coincided with a
previously identified drought halfway around the
world, in Meso-America, which has been linked to
the fall of the Mayan civilization.

“The study also showed that the ample summer
rains of the Northern Song Dynasty coincided with
the beginning of the well-known Medieval Warm
Period in Europe and Greenland.”
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National Science Foundation
Ñews Release: 6-Nov-2008

Dry spells spelled trouble in ancient China
Weakening of summer monsoons to blame

Chinese history is replete with the rise and fall
of dynasties, but researchers now have identified
a natural phenomenon that may have been the last
straw for some of them: a weakening of the summer
Asian Monsoons.

Such weakening accompanied the fall of three
dynasties and now could be lessening
precipitation in northern China.

Results of the study, led by researchers from the
University of Minnesota and Lanzhou University in
China, appear in this week’s issue of the journal
Science.

The work rests on climate records preserved in
the layers of stone in a 118-millimeter-long
stalagmite found in Wanxiang Cave in Gansu
Province, China.

By measuring amounts of the elements uranium and
thorium throughout the stalagmite, the
researchers could tell the date each layer was
formed. And by analyzing the “signatures” of two
forms of oxygen in the stalagmite, they could
match amounts of rainfall–a measure of summer
monsoon strength–to those dates.

The stalagmite was formed over 1,810 years; stone
at its base dates from A.D. 190, and stone at its
tip was laid down in A.D. 2003, the year the
stalagmite was collected.

“It was unexpected that a record of surface
weather would be preserved in underground cave
deposits,” said David Verardo, director of the
National Science Foundation (NSF)’s
Paleoclimatology Program, which funded the
research. “These results illustrate the promise
of paleoclimate science to look beyond the
obvious and see new possibilities.”

“Summer monsoon winds originate in the Indian
Ocean and sweep into China,” said Hai Cheng,
author of the paper and a scientist at the
University of Minnesota. “When the summer monsoon
is stronger, it pushes farther northwest into
China.”

These moisture-laden winds bring rain necessary
for cultivating rice. But when the monsoon is
weak, the rains stall farther south and east,
depriving northern and western parts of China of
summer rains.

A lack of rainfall could have contributed to
social upheaval and the fall of dynasties.

The researchers discovered that periods of weak
summer monsoons coincided with the last years of
the Tang, Yuan and Ming dynasties, which are
known to have been times of popular unrest.

Conversely, the scientists found that a strong
summer monsoon prevailed during one of China’s
“golden ages,” the Northern Song Dynasty.

The ample summer monsoon rains may have
contributed to the rapid expansion of rice
cultivation from southern China to the midsection
of the country. During the Northern Song Dynasty,
rice first became China’s main staple crop, and
China’s population doubled.

“The waxing and waning of summer monsoon rains
are just one piece of the puzzle of changing
climate and culture around the world,” said Larry
Edwards, geologist at the University of Minnesota
and a co-author of the paper.

For example, the study showed that the dry period
at the end of the Tang Dynasty coincided with a
previously identified drought halfway around the
world, in Meso-America, which has been linked to
the fall of the Mayan civilization.

The study also showed that the ample summer rains
of the Northern Song Dynasty coincided with the
beginning of the well-known Medieval Warm Period
in Europe and Greenland.

During this time–the late 10th century–Vikings
colonized southern Greenland. Centuries later, a
series of weak monsoons prevailed as Europe and
Greenland shivered through what geologists call
the Little Ice Age.

In the 14th and early 15th centuries, as the cold
of the Little Ice Age settled into Greenland, the
Vikings disappeared from there. At the same time,
on the other side of the world, the weak monsoons
of the 14th century coincided with the end of the
Yuan Dynasty.

A second major finding concerns the relationship
between temperature and the strength of the
monsoons. For most of the last 1,810 years, as
average temperatures rose, so, too, did the
strength of the summer monsoon.

That relationship flipped, however, around 1960,
a sign that the late 20th century weakening of
the monsoon and drying in northwestern China was
caused by human activity.

If carbon dioxide is the culprit, as some have
proposed, the drying trend may well continue in
Inner Mongolia, northern China and neighboring
areas on the fringes of the monsoon’s reach.

If, however, the culprit is man-made soot, as
others have proposed, the trend could be
reversed, the researchers said, by reduction of
soot emissions.

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The research also was supported by the National
Science Foundation of China and the Gary Comer
Science and Education Foundation.

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