Update: Hurricane Barry made landfall in Louisiana as a Category 1 storm on Saturday, July 13, bringing dangerous storm surge and heavy rain.
The Gulf Coast is about to be pummelled by a three-punch combo: Flooding from heavy rains over the winter and spring has been sending record floodwaters coursing down the Mississippi River, pushing the river close to the top of its protective levees in Louisiana. Now a cyclone fueled by warm offshore waters is threatening to bring downpours to the same area and to push a storm surge up the bayous at the river's mouth.
Louisiana Gov. John Bel Edwards warned residents this week to be prepared for flooding from two sides—both the Gulf of Mexico and the Mississippi River.
It's the kind of compounding of risks that scientists have been warning about as the climate changes. Climate change has loaded the dice for this kind of coincidence, scientists say.
It was the water, rather than the wind, that was causing the most intense concern as Tropical Storm Barry gained strength and disaster declarations and evacuations began.
Like Hurricane Florence in North Carolina last year and the remnants of Hurricane Harvey, which sat over Houston for days in 2017, Barry was moving slowly, creating a threat of days of heavy rainfall and flooding on the coast and lower Mississippi Valley, the National Hurricane Center wrote on Friday.
In Louisiana's rivers, with water levels already high, Barry's expected storm surge of 3 feet or more would push the water even higher.
Climate scientists warn that as global warming trends persist, rising sea levels, coupled with more intense storms and heavy rainfall, will pummel coastal cities like New Orleans, making storm surge and rainfall flooding more frequent and recovery efforts more costly.
"Water is the biggest risk," said Kevin Trenberth, a scientist at the National Center for Atmospheric Research, noting that floods not only pose immediate danger, but also broader health risks by potentially spreading toxins and disease.
When Storm Surge Meets River Flooding
The last time Louisiana faced significantly high river levels along with a tropical storm was 2009 during Hurricane Ida, said Jeff Graschel with the The National Weather Service's Lower Mississippi River Forecast Center.
Graschel said that storm brought the Mississippi's water level—which typically sits between 6 and 8 feet this time of year at the New Orleans gauge—to 13 feet.
Currently, the water level there is around 16 feet, he said, largely from the Midwest's extreme rainfall and widespread flooding this year that is still draining down the Mississippi. Forecasts predict it could rise to 19 feet—the highest it has been since 1973, Graschel said. That part of the river reaches flood stage at 17 feet, with moderate flooding occurring at 19 feet and major flooding at 20 feet, according to the National Weather Service.
If the surge and rainfall predictions hold, some Mississippi levees in Louisiana could be overtopped, but officials with the Army Corps of Engineers told New Orleans' NOLA.com that they weren't worried about levee failures.
As the planet warms, the increasing frequency and intensity of heavy precipitation will continue, the latest National Climate Assessment warns.
Nationwide, the average annual precipitation is up 4 percent since the early 1900s, according to the assessment, but it's the extreme storms that are increasing even more. In the Midwest, the amount of precipitation falling during the heaviest 1 percent of storms was up 42 percent from 1958 to 2016.
Warmer air holds more moisture, increasing the intensity and frequency of heavy downpours, Trenberth explained. "That's the thing we've seen especially in the last three or four years is the moisture content in the air that's going into these storms has never been seen before," he said.
That's exactly why Louisiana is in the place it is now, Graschel said. For six months, heavy rainfall that caused widespread flooding in the Midwest drained into the Mississippi River, bringing the water level downstream to where it is today and increasing the risk of flooding during storms.
Flooding has been so bad, Graschel said, that the Bonnet Carre spillway, which helps manage flooding on the Mississippi, has been in operation nonstop for more than 60 days now. When a strong storm came through on Wednesday, New Orleans' streets flooded from 8 inches of rain.
"Clearly, there are several things this year that are pretty unprecedented," he said.
The Risk of Multiple Climate Hazards
The triple whammy of river flooding from the north meeting a tropical downpour and storm surge coming up the same river from the south is one example of the compounding risks when multiple climate hazards are triggered at the same time.
Heat waves that dry the land, potentially putting food security at risk while also creating conditions ripe for wildfires, are another.
Two studies last summer warned of a rising risk of simultaneous crop failures across the world's breadbaskets, and the potential for food shocks and malnutrition that would likely hit the poorest countries hardest. One study found that the likelihood of the four biggest corn-exporting regions—the U.S., Brazil, Argentina and Ukraine—all losing 10 percent or more of their crop at the same time would rise from a percentage in the single digits at 2 degrees Celsius ( warming to more than 85 percent at 4°C.
University of Hawaii scientist Camilo Mora and a team explored more interconnections in a study published last December, including the risks simultaneous climate hazards such as heat waves, droughts and storms would pose to the environment, food security, infrastructure and human health. By the end of the century, they found, populations could be facing three or more large-scale climate crises concurrently if the greenhouse gas emissions driving global warming aren't reduced.
The state of Louisiana recognizes many of the risks facing its residents as the planet warms and has started to take steps to prepare.
The low-lying state has lost nearly 2,000 square miles of land to flooding since the 1930s, and it could lose 4,000 square miles more over the next 50 years, according to a recently released state plan for adapting to climate change. The plan's strategies include the reality that many people may have to abandon their coastal homes.