Explainer: Is Climate Change Playing a Role In Mississippi River Floods?

By Andrew Freedman

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. (Story continues below.)

“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.

Update, Thursday, May 12: According to the National Weather Service, the Mississippi River level at Memphis, Tenn., is currently at 47.2 feet, having come down only slightly from its crest height on Tuesday.

Downriver, the Mississippi has reached its highest-ever level at Natchez, Miss., at 59.2 ft., breaking the previous record set in 1937. The water there will continue to rise in the coming days, and is forecast to reach 64 feet on May 21.

At Baton Rouge, La., water levels continue to climb, and the current water level is 43.2 feet (the flood stage is 35 feet).

The New York Times reported today that at Helena, AR., the river crested at 56.5 feet, nearly 9 feet above the flood stage. And at Vicksburg, Miss., the river is now forecast to crest at 57.5 feet, which is 14 feet above the town’s flood stage.

To relieve looming flood pressure in New Orleans, the Corps has continued to open bays on the Bonnet Carre Spillway, and officials say that by the end of Thursday, 113 more bays will be opened, bringing the total to 223 open bays.

Update, Friday, May 13: As the massive bulge of water rolls inexorably downwriver along the Mississippi, the Army Corps of Engineers is getting closer to opening the Morganza Floodway to divert some of the flow into the Atchafalaya River Basin. If that happens, reports the New York Times, 2,500 people and thousands of acres of farmland will be in harms’ way (here's a potential flood inundation map from the Army Corps). But if it doesn’t, many thousands more could be at risk of floodwaters overtopping levees all the way down to New Orleans, some 200 miles south — and stressing them badly in any case, running the risk of a catastrophic rupture.

The Corps had already opened 223 of the 350 bays on the Bonnet Carre Spillway by Friday morning, and, says the New Orleans Times-Picayune, the rest will be opened shortly, diverting water into Lake Ponchartrain and raising water levels there by up to 2.5 ft. As of Friday morning, the Mississippi River at New Orleans was slightly lower than it was yesterday, at 16.8 feet, but it is predicted to rise again throughout the weekend, peaking at around 19.5 feet on May 23rd, depending on whether the Morganza is opened.

Upriver, meanwhile, the latest National Weather Service reports show that river levels are still dropping slowly in Memphis, down less than half a foot since yesterday. At Natchez, Mississippi, the current stage is 59.7 ft. heading for a projected 64 ft. a week from tomorrow (the record, set in 1937, is 58.4 feet).

Update Saturday, May 14: The Army Corps will begin opening the Morganza Spillway today, flooding large portions of the sparseley populated Atchafalaya River Basin in order to relieve pressure on more heavily populated downstream areas, including New Orleans. The New Orleans Times-Picayune has the details. The only other time the Morganza was opened was between April 19 and June 13, 1973, and the effects this had on surrounding lands can be seen in this NASA satellite image comparison from that time period.

Update Sunday, May 15: The opening of the Morganza Spillway yesterday means that the Mississippi River will crest earlier and lower from Baton Rouge through New Orleans. In Baton Rouge, the river is expected to reach 45 feet on Monday, which is 10 feet above flood stage. In New Orleans, the river is already near its crest of 17 feet, and is expected to hover around that level for many days, as the Morganza and Bonnet Carre Spillways are operated.

Update Monday, May 16: Water continues to flow out of the Morganza Spillway, slowly inundating large portions of the Atchafalaya River Basin, forcing thousands to evacuate to higher ground. Water levels are expected to remain high — at or below major flood stage — in Baton Rouge and New Orleans, Louisiana. The Mississippi River set a new record at Vicksburg, Mississippi, where the waters rose to 56.7 feet, 13 feet above flood stage and about a foot below the predicted crest on Thursday. The Weather Service has developed a prototype website where you can view the latest flood levels and forecasts.

Update Tuesday, May 17: According to the National Weather Service, Mississippi River water levels in Kentucky, Tennessee, and Arkansas have continued to decline this week, as the surge of floodwaters heads further downstream towards New Orleans. At Vicksburg and Natchez, Miss., however, the water is still at record high levels, and is expected to continue rising, to 57 feet and 63 feet, respectively, by this weekend. The Army Corps of Engineers has opened two more gates along the Morganza Spillway today (now 11 out of 125 gates are open), and ABC News reports that water is flowing into the spillway at a greater rate than over Niagara Falls — at more than 100,000 cubic feet per second. 

Update Wednesday, May 18: At almost all points along the Mississippi River, water levels are now either declining or leveling off. In several areas, however, severe flooding continues, with the river height at many locations still well above major flood stage. At Vicksburg, Miss., the river is still climbing slowly but at just over 57 feet, the National Weather Service predicts the river is near its crest point for the area.

This morning, the Huffington Post reported on the differential flooding in Vidalia, La., and Natchez, Miss., where water levels are currently at 62 feet and are expected to keep climbing through this weekend. On the west side of the river, in the City of Vidalia, high water has been threatening houses, a riverfront hotel, and the town's hospital. Yet in Natchez, on the east bank, the town is built up on a bluff and has so far been largely protected from the floods. 

Update Thursday May 19:Officials reported the first flood-related death this morning, at Vicksburg, Miss.. At Natchez, Miss., the water level has nearly crested, and the National Weather Service forecasts that over the next few days, water levels will remain high but begin to fall over the weekend. Farther towards the Gulf of Mexico and along the Morganza Spillway, flood levels are still climbing. Officials expect that by early next week, floods could reach record levels at Morgan City, along the Atchafalaya River. The Atlantic's In Focus blog has a photo gallery of dramatic images from the flooding. 

With waters still at record levels along the Mississippi River, new stories are emerging daily about some unexpected consequences of the flooding. Yesterday, PBS NewsHour reported on how runoff from flooded farmland is drawing fertilizer into the river and downstream into the Gulf of Mexico. Scientists are predicting that the high concentrations of fertilizer, and potentially pesticides, will be devestating for fish and shellfish in the Gulf this year. On the other hand, a new story in the Christian Science Monitor explains how the floodwater coming through the partially-opened Morganza Spillway is giving scientists a rare opportunity to study how river water filled with silt and natural debris might help replenish marshlands along the coast.

Typically, levees and spillways prevent silt from reaching the lower regions of the Mississippi River, and this has led to deterioration of the coastal wetlands.

Update Friday May 20: According to the National Weather Serivce's latest outlook, water levels in New Orleans remain on the cusp of flood stage, as the U.S. Army Corps of Engineers continue to divert water through the Morganza Spillway. Rerouted water has led to flooding along the Atchafalaya River, and NOAA still anticipates that next week water will reach record high levels at Morgan City, towards the base of the river. 

The U.S. Geological Survey (USGS) issued a press release yesterday explaining how the waters moving through the Morganza Spillway are expected to change as much as 800,000 acres of Bayou landscape. The USGS says that the floodwaters will carry new sediment into the Atchafalaya River and down into the coastal wetland areas, which comprises the nation's largest swamp. 

Yesterday, The Atlantic's Alexis Madrigal offered a detailed explanation of where the waters of the Mississippi originate, what the environment is like alongside the river, and how human intervention has driven change along the Mississippi. NASA's Terra satellite has captured a glimpse of the flood from above, with a false-color perspective showing how the floods have spread into the Atchafalaya in the few days since the Morganza Spillway was first opened. 

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.

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 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.”

Another study, to be published in the Journal of Climate, found an increase in an indicator of the strength of the water cycle. Using a measure of precipitation intensity and the length of dry spells, researchers from the U.S., Italy and China found that both precipitation intensity and length of dry spells are increasing and should continue to do so as the world warms. This is because of increases in both evaporation and the amount of water vapor in the atmosphere. The study implies that this could have a significant influence on extreme events, including floods and droughts, stating: “We stress that the intensification of the hydrologic cycle manifesting itself with less frequent and more intense events is not a trivial fact, and might thus provide some guidance into a deeper understanding of the mechanisms determining the response of the Earth’s water cycle to global warming.”

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.

Online Resources:

US Climate Change Science Program report: Weather and Climate Extremes in a Changing Climate

What's with the Weather? Is Climate Change to Blame?, YaleE360

CNN Explainer on flood terminology.

Mississippi River flooding information from the National Weather Service.

US Army Corps of Engineers

New Orleans Times-Picayune flooding page

Mississippi River flood history since 1543, NWS