Posted by: JeffMasters, 2:45 PM GMT on June 15, 2010
Oklahoma City's rainiest day in history brought rampaging floods to the city and surrounding areas yesterday, as widespread rain amounts of 8 - 11 inches deluged the city.
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We've had an inordinate number of severe floods in the U.S. so far this year. The worst was the May Tennessee flood, which killed 31 people--the highest death toll from a non-tropical cyclone flooding event in the U.S. since 1994, and the most devastating disaster in Tennessee since the Civil War. The Tennessee floods were rated as a 1000-year flood for Middle Tennessee, West Tennessee, South Central and Western Kentucky and northern Mississippi. Two-day rain totals in some areas were greater than 19 inches.Last Friday's disastrous flash flood in Albert Pike Recreation Area, Arkansas, killed twenty people. That flood was triggered by 8+ inches of rain that fell in just a few hours over the rugged mountains west of Hot Springs. And in March, record rains from a slow-moving and extremely wet Nor'easter triggered historic flooding in Rhode Island and Southeastern Massachusetts, with several rivers exceeding their 100-year flood levels. The 16.32" of rain that fell on Providence, Rhode Island, made March that city's wettest month in recorded history.
All of these flooding events were associated with airmasses though [sp. probably "that"] brought record-breaking warm temperatures to surrounding regions of the country. For example, during the overnight hours when the June 11 flood in Arkansas occurred, fifty airports in the Southern and Midwestern U.S. had their highest minimum temperatures on record. During the 1000-year flood in Tennessee, 51 warm minimum temperatures records were set in the eastern half of the U.S. on May 1, and 97 records on May 2. Rhode Island's record wettest March also happened to be its record warmest March. And the air mass that spawned yesterday's Oklahoma City floods set record warm minimum temperatures at 22 airports across the central and Eastern portions of the U.S. on Monday. All this is not surprising, since more moisture can evaporate into warmer air, making record-setting rainfall events more likely when record warm temperatures are present. The total number of airports in the U.S. considered for these comparisons is around 500, so we're talking about significant portions of the U.S. being exposed to these record-breaking warm airmasses this year. For the spring months of March - May, it was the 21st warmest such period in the 116-year record, according to the National Climatic Data Center. At the 500 or so largest airports in the U.S., daily high temperature records outnumbered low temperature records by about a factor 2.5, 1200 to 508. Record high minimums this spring outnumbered record low maximums by 1163 to 568. So far in June, record daily highs have outpaced record lows by 176 to 13, and record high minimums have outpaced record low maximums, 419 to 62.
Groisman et al. (2004) found that in the U.S. during the 20th century, there was a 16% increase in cold season (October - April) "heavy" precipitation events (greater than 2 inches in one day), a 25% increase in "very heavy" precipitation events (greater than 4 inches in one day), and a 36% rise in "extreme" precipitation events (those in the 99.9% percentile--1 in 1000 events.) A sharp rise in extreme precipitation is what is predicted by global warming models in the scientific literature Hegerl et al. (2004). According the landmark 2009 U.S. Climate Impact Report from the U.S. Global Change Research Program, "the amount of rain falling in the heaviest downpours has increased approximately 20 percent on average in the past century, and this trend is very likely to continue, with the largest increases in the wettest places." Most of this increase came since 1970, due to the approximate 1°F increase in U.S. average temperature since 1970. That 1°F increase in temperature means that there is 4% more moisture in the atmosphere, on average.
Dr. Joe Romm over at climateprogress.org has an excellent interview with Dr. Kevin Trenberth of the National Center of Atmospheric Research on the subject of heavy precipitation events and global warming. Dr. Trenberth is the world's leading expert on water vapor in the atmosphere, and he comments that "since the 1970s, on average, there's about a 4% increase in water vapor over the Atlantic Ocean, and when that gets caught into a storm, it invigorates the storm so the storm itself changes, and that can easily double the influence of that water vapor and so you can get up to an 8% increase, straight from the amount of water vapor that's sort of hanging around in the atmosphere.
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We cannot say that any of this year's flooding disasters were definitely due to global warming, and part of the reason for this year's numerous U.S. flooding disasters is simply bad luck. However, higher temperatures do cause an increased chance of heavy precipitation events, and it is likely that the flooding in some of this year's U.S. flooding disasters were significantly enhanced by the presence of more water vapor in the air due to global warming. We can expect a large increase in flooding disasters in the U.S. and worldwide if the climate continues to warm as expected.
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Record precipitation events have also been happening in other countries around the world.
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