New Normal: Frequent Catastrophic Flooding
When its two rivers
flooded last year (2013), Calgary, Alberta had to evacuate its entire downtown
area. The flooding caused three deaths
and the displacement of over 100,000 people in the Canadian
province (from the Minneapolis Star-Tribune).
thousands of relief workers. In a landscape fraught with dangers—from
unmoored gas tanks that became floating bombs to powerful
currents of filthy floodwaters that swept away whole towns—people
hastily raised sandbag barricades, piled into overloaded rowboats,
and marveled at water that stretched as far as the eye could
surpassed all estimations of the Ohio's potential strength.
At one point the river's entire 981-mile run stood above flood
stage. Buildings, farms, and cities . . . disappeared beneath
the waves. Water covered 15,000 miles of highway. . . . .
. . . Hundreds of people drowned or died from pneumonia and
“The Thousand-Year Flood”
by David Welky (1).) more
100-year flood is expected once in a century, by definition —
but that is true for the 20th century when climate statistics
were compiled. As the climate warms and the locations of wet or dry regimes of
rainfall change, hydrologists are revising predictions for
how often a serious river flood of this scale might recur
in the 21st century. A new estimate (2) published
in Nature Climate Change by Y. Hirabayashi and his team
at the University of Tokyo predicts that a current 100-year flood
will return much more frequently later this century (2070
to 2100) in much of south and southeast Asia, wet equatorial Africa,
eastern Siberia, the northern Andes, and the intensively farmed regions
of Argentina and Brazil. Their prediction of frequent river
flooding is robust for Asian areas with a summer monsoon
(southern India, Indochina and southern China), and the farm
belts of South America. Hirabayashi’s team estimated that
a current “100-year flood” can be expected to return
every 10 to 20 years (five times more frequently than now)
in some places.
A new normal: Frequent catastrophic floods:
would it take to limit global warming to 2°C ?
contrast, they predict a drier climate and less frequent flooding
in most of Europe, the central states and southern Rocky Mountains
of the USA, central Asia from Turkey to Pakistan, and far southern
South America. For example, the Danube River basin in Europe
is projected to dry out: a 100-year flood would recur only about
every 250 years in the new climate. more
A Swiss and Austrian
team finds that there is a "window" of time until 2020
in which emissions of greenhouse gases could still rise before
they would have to be reduced, in order to limit global
warming to 2°C over the long term. That window does
not stay open for long: about six years! Story here.
We refer only to river floods, not coastal flooding caused
by storm surge or waves battering a seashore. A once-in-a-century
river flood is said to have a "return period" of 100 years,
and its magnitude depends on the shape of the river basin, as well as
the local climate. In the years 2070 to 2100, under the most
extreme of four possibilities of human climate alteration, a current
100-year flood would recur more often on 42% of the world's
land, and less often on 18% of the land.
The research team is confident that their prediction
is valid, because most of the climate models they used predicted this
kind of increase wherever flooding is expected to become more frequent.
Rather than using
a single global climate model linked to a single river runoff
model, Hirabayashi's team used 11 different
climate models, to gain more confidence in the overall result.
With each model, they simulated four
different pathways for future human emissions of greenhouse
gases. But in their summary, they reported conclusions based
only on the one pathway leading to the "most dangerous outcomes"
for climate: a pathway (3)
in which industrial societies do not limit greenhouse gases
at all before the year 2100.
When the team projected
future return periods for a flood that is now considered to be a 100-year
event, various models projected dramatically short return
periods for a few rivers: as short as 5 years for the Nile
River, and 10 years for the Congo River in Africa and the Mekong,
Ganges, and Brahmaputra Rivers in Asia.
Other emergency managers talk about the concept
of "flood exposure" to measure the number
of people living in the inundation zones of floods large enough
to recur every 100 years in the present climate.
The recent flood exposure of the world's people was 15 million
for the years 1980-1999. The flood exposure started to rise
in 1996 after remaining flat for many decades.
Keeping the population constant at
what it was in 2005, but allowing climate to change and rivers
to flood more or less often as rainfall patterns change, Hirabayashi's
team estimated that 27 million people would be exposed to floods
of this size by the year 2100, if the global climate warms by
2°C above the average for 1980-1999. (Two degrees of warming
is the maximum that might avoid "dangerous interference in the
climate," according to many national scientific bodies.) And if it should
warm by 6°C, or three times as much, then 93 million people
would be living in the way of such catastrophic floods.
1. David Welky,
“The Thousand-Year Flood: The Ohio-Mississippi Disaster
of 1937”, book, 384 pp., © 2011. University
of Chicago Press.
Global flood risk under climate change,” by Y. Hirabayashi
and colleagues, Nature Climate Change, published
online 9 June 2013, Doi: 10.1038/NCLIMATE1911.
3. These pathways
are termed “representative concentration pathways”
(RCP) and are summarized in: “The
representative concentration pathways: an overview”
by D.P. van Vuuren and 14 colleagues, Climatic Change,
vol. 109, 5-31, Dec. 2011,
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