What is Causing the Mississippi River Floods?
by Andrew Freedman, Climate Central, May 12, 2011
Two main factors are behind the record flooding. First is the heavy rain that fell during April in the Ohio Valley, where six states — Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and West Virginia — recorded their wettest April since instrument records began 117 years ago. Nine states recorded their wettest February through April period on record, according to a report released Monday by the National Oceanic and Atmospheric Administration (NOAA). In addition, snowmelt from the Midwest added more water to the Mississippi River and its tributaries.
Precipitation totals during April, showing a strip of magenta and purple hues across the Ohio River Valley, where more than 20 inches in some locations. Credit: NOAA.
Here’s how NOAA described the weather pattern that led to the repetitive doses of heavy rain in the Ohio Valley during April.
The storm track repeatedly tapped Gulf of Mexico moisture in a southerly surface airflow that generated storm systems week after week over the Midwest... Some areas received up to 20 inches of rain during the month, which is nearly half their normal annual precipitation.
What Does Climate Change Have to Do With the Flooding?
Climate change cannot be blamed for causing the flooding, but scientists have detected large-scale trends indicating that extreme precipitation events are becoming more likely as temperatures warm in response to increasing amounts of greenhouse gases in the air. This means that heavy rainfall events are more frequent than they used to be, in part because a warmer atmosphere holds more moisture that can be wrung out by storm systems.
Scientists are working to detect the “fingerprint” of global warming in specific extreme weather events, and their methods are still in their infancy. It will take many months for studies to be completed on whether climate change may have made April’s heavy rains more likely. For now, though, we can look at studies that have already been completed that offer some clues about the relationship between climate change and heavy precipitation events.
Two studies published this year in the journal Nature have tied climate change to precipitation trends. One studyfound that in most of the U.S., two measures of extreme rainfall — the highest one-day rainfall amount per year and the highest five-day amount per year — are increasing.
In other words, the study concluded that extreme rainfall events are becoming more common in the US, and indeed throughout much of the Northern Hemisphere. Importantly, the paper attributed these larger scale trends in part to climate change from increasing amounts of greenhouse gases in the atmosphere.
The other study analyzed one particular flood event that occurred in the U.K. in the fall of 2000. In this case, scientists simulated the atmospheric conditions when that flood took place, looking to see how various levels of greenhouse gases (2000 levels vs. preindustrial times) influenced the odds that the floods would happen. In order to accurately simulate the floods, they paired an atmospheric computer model with a hydrographic model that simulates river conditions.
The study found that the higher levels of greenhouse gases increased the risk of flooding by 20 percent, and in a smaller proportion of the simulations, the flood risk was increased by much more – 90 percent.
Peter Stott, who leads the Climate Monitoring and Attribution team at the U.K.’s Met Office and was a co-author of the U.K. rainfall study, told Climate Central’s Alyson Kenward that his team’s approach could pave the way for more analyses of particular flooding events. “We’re still trying to understand the robustness of these results,” he said, “But we would like to develop a situation where we can do more studies with this design so that we can better understand the risk of which extreme events we can attribute to climate change with confidence.”
Still, in general, climate scientists have a much more difficult time detecting precipitation changes on regional and local levels compared to the bigger picture, mainly because of the greater natural variability in climate conditions on local levels.
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