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High Waters: A Climate Connection to the Mississippi River Floods?

A very rainy April set the stage for the cresting Mississippi.

From swarms of deadly tornadoes to record flooding and tinderbox drought conditions, it seems that Mother Nature threw everything in her arsenal against the Lower-48 States during April, with impacts continuing into May. The combination of a weather pattern associated with La Niña, a natural climate cycle involving ocean currents in the tropical Pacific Ocean, as well as other factors helped fuel what was, by any measure, an extremely unusual month.

In a series of explainers, Climate Central is breaking down some of the causes of all this extreme weather, in order to clearly lay out what scientists do and do not know about the relationship between climate change and extreme weather events. First up is what’s going on right now with the record-breaking flooding along the Mississippi River.

With input from our staff scientists, we tackle a key question on many peoples’ minds -- is this a sign of things to come as the climate continues to warm?

What’s the Status of Mississippi River Flooding?

A bulge of water is now making its way down the Mississippi River towards Louisiana, where levees protecting New Orleans will be put to the test. The Mississippi River crested at 47.79 feet on Tuesday, at Memphis, Tennessee, the second-highest level on record. That's about a foot below the level set in the great flood of 1937, but well above flood stage, which is 34 feet. River levels remain very high there, and will stay that way for a few more days, according to National Weather Service forecasts.

The Mississippi River is not expected to crest in Louisiana until next week, and Gov. Bobby Jindal is warning that up to 3 million acres could be affected by floodwaters. The National Weather Service is forecasting a 19.5 foot crest in New Orleans on May 23. The city’s levees only protect it against floodwaters of up to 20 feet.

This narrow margin has many residents of the Big Easy on edge considering the flooding that devastated portions of the city following Hurricane Katrina in 2005.

"After hurricanes Katrina, Rita, Gustav and Ike -- as well as the oil spill -- Louisiana can ill-afford another large-scale disaster," Sen. Mary Landrieu, a Louisiana Democrat, told CNN. "Billions of dollars in property is at stake, not to mention the threat to human life."

In Baton Rouge, upstream from New Orleans, the Mississippi River is expected to crest at 47.5 feet on May 22, which would be slightly above the record of 47.3 feet set in 1927.

The Army Corps of Engineers has been taking steps to relieve some of the pressure on the levees, including opening gates to the Bonnet Carre Spillway by draining some floodwaters into Lake Pontchartrain. They may be forced to open up the Morganza Spillway, upstream from Baton Rouge, to divert some of the water away from points downstream.

The Morganza may be key to protecting New Orleans from major flooding, according to an article in today’s New Orleans Times Picayune. The paper reports that a risk assessment completed by the Army Corps indicates the possibility that multiple levee failures will occur if the Morganza Floodway is not opened.

Opening the Morganza Spillway will cause flooding of its own in the Atchafalaya River Basin in Louisiana.

What is Causing the Mississippi River Floods?

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.

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 climate change 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 study found 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.

This story originally appeared at OnEarth partner Climate Central.

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Andrew Freedman is the managing editor of online content and climate policy analyst for Climate Central. Prior to working with Climate Central, Freedman was a reporter for Congressional Quarterly and Greenwire/E&E Daily. His work has also appe... READ MORE >
What a crock. There is no actual scientific evidence whatsoever connecting human activity with these weather patterns. Those "studies" were conducted by ecofreaks who started with the comclusion they wanted to reach and worked backwards. There is not now, nor has there ever been, an objective "smoking gun" linking human actvity with global warming. Creating a sense of crisis over inconclusive data is how your organization and others of your ilk stoke your sense of self importance and soak up funds from the gullible.
Climate change causes everything, might as well look for a "fingerprint" in the sun rising and setting. I'm sure they could find one there too.