The historic and devastating floods in Louisiana are the latest in a series of heavy deluges that some climate scientists warn will become even more common as the world continues to warm.
On Tuesday, the National Oceanic and Atmospheric Administration (Noaa) is set to classify the Louisiana disaster as the eighth flood considered to be a once in every 500-year event to have taken place in the US in little over 12 months.
Since May of last year, dozens of people have been killed and thousands of homes have been swamped with water in extreme events in Oklahoma, Texas, South Carolina, West Virginia and Maryland. Noaa considers these floods extreme because, based on historical rainfall records, they should be expected to occur only once every 500 years.
The Louisiana flooding has been so exceptional that some places in the state experienced storm conditions considered once-every-1,000-year events. Close to two feet of rain fell over a 48-hour period in parts of southern Louisiana, causing residents to scramble to safety from flooded homes and cars.
At least six people have died, with another 20,000 people having to be rescued. Even Louisiana governor John Bel Edwards had to evacuate after his governor’s mansion in Baton Rogue was swamped with chest-high water. A federal state of emergency has been declared, with 12,000 people crowding into shelters.
The National Weather Service balloon released in New Orleans on Friday showed near-record levels of atmospheric moisture, prompting the service to state that “we are in record territory”. Climate scientists have warned that the build-up of moisture in the atmosphere, driven by warming temperatures, is likely to cause a greater number of floods in the future.
“We have been on an upward trend in terms of heavy rainfall events over the past two decades, which is likely related to the amount of water vapor going up in the atmosphere,” said Dr Kenneth Kunkel, of the Cooperative Institute for Climate and Satellites.
“There’s a very tight loop – as surface temperatures of the oceans warm up, the immediate response is more water vapor in the atmosphere. We’re in a system inherently capable of producing more floods.”
The amount of heavy rainfall events in the US has risen well above the long-term average since the 1990s, with large regional variances. While the north-east, Midwest and upper Great Plains have experienced a 30% increase in heavy rainfall episodes – considered a once-in-every-five year downpours – parts of the west, particularly California, have been parched in drought.
Warmer air, influenced by heat-trapping gases released by human activity, can contain more water vapor than cooler air. With the extra heat helping nourish storms, scientists expect global warming to help produce more intense downpours.
“Assuming we don’t change our ways, warming is a virtual certainty and increased water vapor is virtual certainty,” Kunkel said. “That means increases in heavy rainfall is virtual certainty.”
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