Aug. 15, 2016 Richard Rossi and his 4-year-old great grandson Justice wade through water in search of higher ground after their home took in water in St. Amant, La. Jonathan Bachman/Reuters
Here we are again, with a flood event upending the lives of large numbers of Americans and making everybody wonder about the role of climate change...

In this case, it’s the stunning, multiday flooding in southern Louisiana that hit after a low pressure system combined with record amounts of atmospheric water vapor, dumping more than two feet of rainfall over three days in some places. At least six people were killed, and thousands have had to leave their homes.

On Monday, climate researchers and weather experts were in what’s by now a familiar posture — explaining that, no, this event wasn’t “caused” by climate change, but then again, it’s precisely the sort of event that you’d expect to see more of on a warming planet.

“Climate change has already been shown to increase the amounts of rain falling in the most intense events across many parts of the world, and extreme rainfall events like this week’s Louisiana storm are expected [to] grow increasingly common in the coming years,” wrote the Weather Underground’s Bob Henson and Jeff Masters.

“Louisiana is always at risk of floods, naturally, but climate change is exacerbating that risk, weighting the dice against us,” Katharine Hayhoe, a climate researcher at Texas Tech University, told The Washington Post. “How long will it be until we finally recognize that the dice are loaded?”

The easiest link between climate change and extreme weather events involves heat waves, a recent study by the National Academy of Sciences found. This makes sense: A warming planet overall breaks warm-temperature records more frequently than cold-temperature records and sets the stage for lengthier, or stronger, bouts of extreme heat.

But one of the other relatively simple links involves rainfall and heavy flooding. “Heavy rainfall is influenced by a moister atmosphere, which is a relatively direct consequence of human-induced warming, though not as direct as the increase in temperature itself,” the NAS report noted.

This is because as the atmosphere warms, its ability to retain water vapor increases. Thus, the climate influence on precipitation events runs from tropical cyclones to blizzards, and all of them should be able to produce more precipitation in extreme events than before.

Indeed, when it comes to heavy precipitation, a trend has already been documented in the United States and in the region currently experiencing disaster, notes the National Climate Assessment:

“Observations over the US and many other places around the world show that heavy rain events have been becoming heavier over the last several decades. Climate models very consistently predict that this should happen as the climate warms, and basic physics leads us to interpret this change  as, in large part, a consequence of increasing water vapor in the atmosphere,” explains Adam Sobel, a climate scientist at Columbia University. “On this basis we can say that climate change has most  likely increased the probability of an event like this. One still can’t say that  climate change ’caused’ this event, as each event has many causes and no event can be viewed solely as a consequence of long-term trends.”

To be sure, researchers could try to go further and more comprehensively prove a link between this event and climate change. To do so, they could perform what is called an “attribution study,” which determines whether an event like this one is more likely to occur in a world full of human greenhouse gas emissions than in one without it. So far, no such study appears to have been attempted — but it could be soon. These types of studies are growing increasingly common.

That still wouldn’t be a causal determination. Rather, it would likely result in a statement about a shifting of the odds in favor of an event like this one.

But not all researchers are convinced that this is the only way to tackle this problem.

Climate scientist Kevin Trenberth of the National Center for Atmospheric Research has argued that we should turn the tables and, rather than assuming that a climate link to a given extreme event remains to be proved, simply assume one exists for extreme events driven by thermodynamic factors — e.g., heat and moisture — like heat waves and major rain events.

“Because global warming is real and present, it is not a question as to whether it is playing a role but what that role is,” wrote Trenberth and two colleagues in a 2015 paper.

In the meantime, though, most scientists continue to talk about an increase in a particular type of event and shifting odds, which are no less consequential for our society.

“We design our infrastructure and plan our society looking backwards, assuming that the past is a reliable predictor for the future. And looking backwards does keep us safe, when climate is relatively stable, as it has been over much of the history of human civilization on this planet,” Hayhoe said.

“When climate is changing, though, relying on the past to predict the future will give us the wrong answer — and not just a wrong answer, but a potentially dangerous one,” she said. “We buy a house outside the 100-year flood zone, believing that means we’re safe; we expect our storm sewer drains and our levees to protect us from all but the rarest extremes, failing to account for how these extremes are rapidly becoming more frequent in a changing climate.”

August 15 at 4:45 PM

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