December 2012, Week 1


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Sat, 1 Dec 2012 00:40:42 -0500
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Megastorms Could Drown Massive Portions of California

     Huge flows of vapor in the atmosphere, dubbed
     "atmospheric rivers," have unleashed massive
     floods every 200 years, and climate change could
     bring more of them

By Michael D. Dettinger and B. Lynn Ingram

Editor's note (11/30/12): The article will appear in
the January 2013 issue of Scientific American. We are
making it freely available now because of the flooding
underway in California.

Geologic evidence shows that truly massive floods,
caused by rainfall alone, have occurred in California
about every 200 years. The most recent was in 1861, and
it bankrupted the state.

Such floods were most likely caused by atmospheric
rivers: narrow bands of water vapor about a mile above
the ocean that extend for thousands of miles. Much
smaller forms of these rivers regularly hit California,
as well as the western coasts of other countries.

Scientists who created a simulated megastorm, called
ARkStorm, that was patterned after the 1861 flood but
was less severe, found that such a torrent could force
more than a million people to evacuate and cause $400
billion in losses if it happened in California today.

Forecasters are getting better at predicting the
arrival of atmospheric rivers, which will improve
warnings about flooding from the common storms and
about the potential for catastrophe from a megastorm.

More In This Article


Hurricane Sandy: An Unprecedented Disaster


The Future of Climate Change


Extreme Weather and Climate Change

Editor's note (11/30/12): The article will appear in
the January 2013 issue of Scientific American. We are
making it freely available now because of the flooding
underway in California.

The intense rainstorms sweeping in from the Pacific
Ocean began to pound central California on Christmas
Eve in 1861 and continued virtually unabated for 43
days. The deluges quickly transformed rivers running
down from the Sierra Nevada mountains along the state’s
eastern border into raging torrents that swept away
entire communities and mining settlements. The rivers
and rains poured into the state’s vast Central Valley,
turning it into an inland sea 300 miles long and 20
miles wide. Thousands of people died, and one quarter
of the state’s estimated 800,000 cattle drowned.
Downtown Sacramento was submerged under 10 feet of
brown water filled with debris from countless mudslides
on the region’s steep slopes. California’s legislature,
unable to function, moved to San Francisco until
Sacramento dried out—six months later. By then, the
state was bankrupt.

A comparable episode today would be incredibly more
devastating. The Central Valley is home to more than
six million people, 1.4 million of them in Sacramento.
The land produces about $20 billion in crops annually,
including 70 percent of the world’s almonds—and
portions of it have dropped 30 feet in elevation
because of extensive groundwater pumping, making those
areas even more prone to flooding. Scientists who
recently modeled a similarly relentless storm that
lasted only 23 days concluded that this smaller
visitation would cause $400 billion in property damage
and agricultural losses. Thousands of people could die
unless preparations and evacuations worked very well

Was the 1861–62 flood a freak event? It appears not.
New studies of sediment deposits in widespread
locations indicate that cataclysmic floods of this
magnitude have inundated California every two centuries
or so for at least the past two millennia. The 1861–62
storms also pummeled the coastline from northern Mexico
and southern California up to British Columbia,
creating the worst floods in recorded history. Climate
scientists now hypothesize that these floods, and
others like them in several regions of the world, were
caused by atmospheric rivers, a phenomenon you may have
never heard of. And they think California, at least, is
overdue for another one.

Ten Mississippi Rivers, One Mile High

Atmospheric rivers are long streams of water vapor that
form at about one mile up in the atmosphere. They are
only 250 miles across but extend for thousands of
miles—sometimes across an entire ocean basin such as
the Pacific. These conveyor belts of vapor carry as
much water as 10 to 15 Mississippi Rivers from the
tropics and across the middle latitudes. When one
reaches the U.S. West Coast and hits inland mountain
ranges, such as the Sierra Nevada, it is forced up,
cools off and condenses into vast quantities of

People on the West Coast of North America have long
known about storms called “pineapple expresses,” which
pour in from the tropics near Hawaii and dump heavy
rain and snow for three to five days. It turns out that
they are just one configuration of an atmospheric
river. As many as nine atmospheric rivers hit
California every year, according to recent
investigations. Few of them end up being strong enough
to yield true megafloods, but even the “normal” storms
are about as intense as rainstorms get in the rest of
the U.S., so they challenge emergency personnel as well
as flood-control authorities and water managers.

Atmospheric rivers also bring rains to the west coasts
of other continents and can occasionally form in
unlikely places. For example, the catastrophic flooding
in and around Nashville in May 2010—which caused some
30 deaths and more than $2 billion in damages—was fed
by an unusual atmospheric river that brought heavy rain
for two relentless days up into Tennessee from the Gulf
of Mexico. In 2009 substantial flooding in southern
England and in various parts of Spain was also caused
by atmospheric rivers. But the phenomenon is best
understood along the Pacific Coast, and the latest
studies suggest that these rivers of vapor may become
even larger in the future as the climate warms.

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