Summer, 2013     #18
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A New Normal: Frequent Catastrophic Flooding

Downtown Calgary floods in June 2013

    When its two rivers flooded, Calgary, Alberta had to evacuate its entire downtown area on June 21 of this year. That flooding caused three deaths and the displacement of over 100,000 people in the Canadian province (from the Minneapolis Star-Tribune). (See story below, “A record wet spring of 2013.”)

    “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 book, “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 wet or dry patterns of rainfall change, hydrologists are revising predictions for how often a serious river flood of this magnitude 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 recur much more frequently later this century (from 2070 to 2100) in much of south and southeast Asia, wet equatorial Africa, eastern Siberia, the northern Andes, and the intensive farm belts of Argentina and Brazil. Their prediction of more frequent river flooding is robust for areas influenced by the Asian summer monsoon (southern India, Indochina and southern China), and for the farm belts in South America. Hirabayashi’s team calculated that a current “100-year flood” can be expected to recur every 10 to 20 years (about five times more frequently than now) in some regions.

    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-Austrian team finds that until 2020, there is a "window" of greenhouse gas emissions that would still limit global warming to 2°C over the long term. That window does not stay open for long.   Story here.

       We refer only to floods on rivers, not coastal flooding caused by storm surge or battering waves on 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 river's basin, as well as the local climate. In the years 2070 to 2100, under the most extreme of four cases of human climate alteration, a current 100-year flood would recur more often on 42% of the world's land. It would recur less often on another 18% of the land area. More than half of the climate models predicted this increase for places where flooding will become more frequent; that gave the research team more confidence that their prediction is valid.

     Rather than using a single global climate model linked to a single river runoff model, Hirabayashi's team assembled results from 11 different climate models, to gain more confidence in the overall result. For each model, they obtained simulations that assume four different pathways for future human emissions of greenhouse gases. In their summary, they only reported conclusions based on one pathway leading to the "most dangerous outcomes" (in their words) 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 now considered to be a 100-year event, the various models projected dramatically shorter return periods for a few rivers: as short as 5 years for the Nile River, and about 10 years for Africa's Congo, and the Mekong, Ganges, and Brahmaputra Rivers in Asia.

     Other students of natural disasters proposed the concept of flood exposure to measure the number of people living in inundation zones of floods large enough to have a return period of 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 changed, 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 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

A record wet spring of 2013 in central US

    “This is the worst spring I can remember in my 30 years farming. Just continuous rain, not having an opportunity to plant,” said Rob Korff of Missouri, who planted his corn a month late because of the weather.” (Minneapolis Star-Tribune, June 1, 2013).

    Iowa had its wettest spring ever in 2013 with 18 inches of precipitation, twice the 3-month normal amount. Very wet conditions were the rule in five other neighboring states (rainfall map, below).
   (From State of the Climate: synoptic discussion for May 2013, published online June 2013, NOAA/ National Climatic Data Center.

Credit: US National Climatic Data Center

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