Summer 2014     #20
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A New Normal: Frequent Catastrophic Flooding

Downtown Calgary floods in June 2013

    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).

    “FDR dispatched 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 see.

    “The overflow 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 other illnesses.
            (From “The Thousand-Year Flood” by David Welky (1).) more

    A 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.

Climate News

A new normal: Frequent catastrophic floods:
Feature article

What would it take to limit global warming to 2°C ?

    In 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

Is it still possible to limit warming to 2°?

    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.
2. “ 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,
DOI: 10.1007/s10584-011-0148-z.

   next story                                 ( News headlines)

Mekong River in a typical flood

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