"This is our Sandy"

Why storms like Helene are becoming more likely

It’s hard to describe 40 trillion gallons of rain. Imagine Manhattan’s streets covered by a pool 1.6 miles deep. Or the entire state of Massachusetts under 23 feet of water. That’s how much rain fell on the southeast US through Sunday. It’s apocalyptic, astronomical, unthinkable.

With ferociously accelerating climate change, more extremely wet storms like Helene are in our future. We need leaders with the capacity to plan ahead. We need people who can imagine living differently. At the least, we need to stop building infrastructure that can’t cope with extreme rainstorms.

We know the global climate has interrelated tipping points: melting in Greenland pushed beyond a threshold will trigger slowing of the global ocean currents that keep Europe temperate; slowing of these currents will allow even more water to slop over the East Coast of the US.

Well, regions of the globe have tipping points at which the dynamics of intense rainfall events become nonlinear. You have probably heard that warmer air has a higher water vapor carrying capacity: 7 percent more water for each one degree of warming (Celsius). That’s a steady, linear relationship

Rain happens when water vapor held in the air condenses—as that moist air cools—and falls to the ground. This 7 percent thermodynamic law (called the Clausius-Clapeyron, or CC, relationship after the two scientists who suggested it) is intense enough on its own, and helps account for heavier rainfall.

But the CC relationship covers a rising column of air undisturbed by the things around it. When air is rising in a context of increasing moisture carried by winds, or in a region to which water vapor leaving the land or ocean is being added, that can greatly amplify the amount of water vapor available to fall heavily on our heads and dwelling places.

Scientists have discovered that after a threshold of precipitation is reached and wind patterns change, even small deviations in the way moisture is distributed can have an enormous effect on the intensity and frequency of extreme precipitation events, going way beyond that 7 percent CC relationship. These thresholds vary regionally.

Under that threshold, on an ordinary rainy day, things are more predictable. But over that threshold, once it’s raining and the wind is blowing, the probability of an even more extreme rainfall event increases significantly. The “large event” tail, the probability of extreme events, stretches out and thickens.

As the climate warms, the likelihood of the biggest storms getting even bigger is rapidly increasing.

The trouble is that if climate models disagree on how moisture being drawn into a region affects precipitation, we may just average all the model predictions—and that may mean we are systematically underestimating the potential for extreme rainfall events. The actual, human-felt distribution may be skewed toward extremes, particularly in tropical areas.

Think of apocalyptic rain in Pakistan or Bangladesh in the rainy season. Now think of Asheville.

Remember Hurricane Harvey in 2017? Scientists found that climate change had made the storm 3.5 times more likely to happen, and had amped up the amount of rain that fell by nearly 20 percent—a much more substantial addition than the 7 percent predicted by the 1 degree of global warming we’d already experienced that year. Harvey’s rainbomb was fueled by moisture from the Gulf of Mexico near the Texas coastline.

This time massive Helene, having already smashed through the barrier islands off Florida’s west coast, Ft. Myers Beach, the Tampa Bay region, and the Big Bend, chewed through Georgia and upstate South Carolina. It combined with at least two other storm systems and a stalled low-pressure system that funneled warm water up from the Gulf. The region’s atmosphere was already warm and wet from hot evaporation coming off land below that had baked through an extreme summer.

All that moisture collected at 6,000 feet in the mountains of North Carolina, where it condensed and turned into calamitous amounts of rain. The same scientists who looked at Harvey are looking at Helene, and they see climate change effects accounting for 50 percent more rain in Georgia and some parts of the Carolinas last week.

“This is our Sandy,” says adaptation expert Rob Young. He’s talking about the Southeast. But rapid intensification and locally extreme rainfall are part of accelerating climate change, and the possibility of hugely destructive storms in many parts of the US is increasing as things get wetter and warmer.

It’s not just 7 percent more water vapor. It’s signaling the arrival of a tipping point. We need leaders and planners who are ready to put all the risk on the table.