In Louisiana, the record rainfall that killed at least 11 people and flooded tens of thousands of buildings has been called a "1,000-year" event in some areas, meaning something like this only happens once a millennium. In other parts of the state, experts have called it a 500-year flood. But those names probably don’t make sense in an era of climate change.
Just like climate change makes wildfires and drought more likely in California—and may make it more likely for viruses like Zika to spread—it also makes extreme rainfall and devastating flooding more common.
"Warm air holds more water vapor than cold air, and we're warming up both the air temperature and we're warming up the oceans," says David Titley, a meteorology professor and the director of the Center for Solutions to Weather and Climate Risk at Penn State University. "Welcome to the future."
Extreme weather is becoming more common overall, and it's also possible to link specific events to climate change. In a National Academies of Sciences report published earlier this year, Titley and other researchers explained how individual weather events can be attributed to climate change.
If a region has a long record of data—and it's clear that numbers such as temperature are way out of range—researchers can run computer models to see if climate was a factor in a particular storm or drought. "You can use computer models in a variety of different ways to basically try to create the world without climate change and see if it was different than the one we observed," says Titley. Researchers also study whether the facts of a particular storm match up with what would be expected.
In an analysis of flooding in Europe earlier this year, scientists were able to determine—just days after the storm—that climate change made it 80%-90% more likely.
Though the flooding in Louisiana hasn't been analyzed yet, the facts are consistent with climate change. The moisture in the atmosphere was close to record highs. The storm meandered along the Gulf Coast—not an uncommon type of storm in the area—but the high levels of moisture made a difference.
"We've combined a fairly common weather feature now with very uncommon amounts of moisture," says Titley. "And the result is what we saw over Louisiana over this weekend."
The floods in Louisiana follow other recent deadly floods in Maryland, Oklahoma, Texas, South Carolina, and West Virginia. In Louisiana, which is particularly vulnerable, it raises questions about how parts of the state can stay habitable—and how to build cities if historic storms start to become more normal.
"The question becomes, 'How much are we going to protect ourselves?'" says Titley. "We do understand from a changing climate that these types of events are probably going to occur more often, not less often." Louisiana is already home to the first climate refugees in the lower 48 states.
Flooding is even more likely in places like California that are still suffering from drought; when it does rain, there's a bigger chance of heavier storms. "It seems like a paradox that you can increase both drought and flooding," says Titley. "But that's the world we're going to."