More rain, more often —

Was August deluge in Louisiana worsened by climate change?

Rapid analysis says: probably.

A Coast Guard boat carries several Baton Rouge residents to safety.
Enlarge / A Coast Guard boat carries several Baton Rouge residents to safety.

Southern Louisiana was hit hard by relentless rains a few weeks ago. Some locations received a ridiculous soaking: two feet of rain within three days. More than a dozen lives were lost in the resulting flooding, and tens of thousands of homes were inundated.

The culprit was a low pressure system that was strong enough to have become a tropical depression if it had moved out into the Gulf of Mexico. It slowly crawled across the Gulf Coast instead, wringing out an incredible amount of moisture from air pulled in from over the Gulf—about triple the total volume dumped by Hurricane Katrina.

Our warming climate is predicted to intensify storms. But what can we say about how climate change may have affected this particular deluge?

One group of climate scientists has been hammering out a process to provide a rapid look at that question for events like this while they're still fresh in the minds of those outside the affected area. In their work, they analyze weather datasets and an archive of previously run climate model simulations to see if those models show changes in that sort of weather as the climate warms. They work fast—they’ve already submitted their paper on the Louisiana floods for peer review, and they released their analysis last week.

In this case, the team went hunting for extended extreme precipitation events using three-day totals. They widened their search from southern Louisiana to include the rest of the US Gulf Coast to avoid, figuratively, missing changes in the forest for focusing on one tree. Intense precipitation here can come from several different types of weather systems, which makes this a little tricky. There are tropical low pressure systems, but also fronts when the jet stream dips far enough south, and other conditions that cause moist surface air to rise and cool.

The researchers analyzed simulations from two climate models that run with exceptionally fine spatial resolution, as those do a better job with this kind of weather.­ They looked at simulations run with climate “forcings” like CO2 and solar activity matching the historical changes since 1860, as well as baseline simulations with forcings permanently stuck in 1860 (or another year).

If you pick any individual weather station, the probability of experiencing an extreme event like the one southern Louisiana saw a few weeks ago in a given year is in the neighborhood of 1-in-550. The probability of seeing an event like that somewhere in the Gulf Coast region in a given year comes down to about 1-in-30. But all of the weather station data show that these probabilities are increasing over time as extreme events have gotten stronger.

According to the model simulations, that’s what we would expect to see in the region as the climate warms. They show that a rainfall event like the recent one is at least 1.4 times as likely (and probably closer to double) as it was in 1860. That means a 100-year rain event drops to at least a 70-year event—with a new, stronger 100-year event taking its place.

So the verdict is that we can expect to see more extreme rain more frequently than we would have 150 years ago in this region. It’s fair to say climate change is influencing extreme weather like this.

However, this study is only one way of looking at it; you might also search for the precise series of atmospheric events that produced the Louisiana storm and try to determine the contribution of climate change from those conditions. That would speak to climate's contribution to this storm, rather than to storms of this magnitude. The researchers were looking at three-day rainfall totals, regardless of the type of rainstorm responsible—what they describe as the “climatic context.” It’s not an inferior approach; it’s just not the only possible approach.

Future studies might tackle the Louisiana floods from different angles, but that might take more than a couple of weeks.

Channel Ars Technica