In Kerrville, Texas, it only rained five times in June, and July started off with just a couple of showers. In fact, the surrounding county was 100% in drought at the start of July.
Ironically, that drought helped beget the deadly floods that swept through the region on Friday. Its one of a number of factors, including the abnormally hot Gulf of Mexico, that fueled a storm that killed 27 campers and counselors at Camp Mystic as well as dozens more across Texas.
More than a foot of rain fell on Friday, sending the Guadalupe River and other waterways surging over their banks. While researchers havent analyzed the storm that spawned the floods, extreme precipitation is becoming increasingly common as the planet warms.
One of the clearest fingerprints of the climate crisis is the uptick in heavy rain events, like the one responsible for the tragedy in Texas this week, said Jennifer Francis, a climate scientist at the Woodwell Climate Research Center. Texas is particularly flood-prone because the fever-hot Gulf of Mexico is right next door, providing plenty of tropical moisture to fuel storms when they come along.
As climate change warms the world, the atmosphere can hold more moisture. For every 1.8F (1C) increase in temperature, the air can carry about 7% more moisture.
The mechanics are so well-studied, the formula for it has a name: the Clausius-Clapeyron equation, used to calculate the saturation of water vapor pressure to temperature, said Ryan Truchelut, president of commercial forecaster WeatherTiger.
The carrying capacity increases faster and faster as the temperature increases, he said.
But that isnt the only issue fueling the mechanics of drought and flood. Warmer temperatures lead to more evaporation, particularly over the ocean.
Human-caused increases in heat-trapping greenhouse gases have warmed oceans, which evaporate more moisture into the warmer air, Francis said. Not only does this moisture increase rainfall, but it also fuels stronger storms.
The floods also got a boost from moisture flowing north from the remnants of Tropical Storm Barry, which had made landfall on Mexico's east coast a week ago, according to the National Weather Service.
In Texas, the situation was also made worse by the drought because dry soils are less able to absorb water when it falls as rain, Truchelut said. Nearly 90% of Kerr County was in either extreme or exceptional drought the two highest categories on the Drought Monitors five-step scale prior to the storm.
Nothing is going into the parched dirt, Truchelut said.
Soil in that area of Texas isnt known for its water-absorbing qualities even in the best of times, said Tyler Roys, a meteorologist with AccuWeather Inc. When multiple inches of rain fall in an hour, as it did during the storm, the ground is going to absorb less, he said.
While the atmosphere is able to carry 7% more water vapor per 1.8F degree, that translates to a 2% to 3% increase in global average rain and snow, according to a review paper co-authored earlier this year by Daniel Swain, a climate scientist at the University of California at Los Angeles.
This means a decrease in the number of light-to-moderate rainy days and an uptick in the overall number of dry days, Swain wrote. But at the other end of the spectrum, days with the heaviest rainfall have increased.
In other words: theres growing evidence not only that precipitation extremes will increase (in general) due to climate change but also that the most intense, rarest & most dangerous rain events will increase faster than more moderate extremes, Swain wrote in a post on BlueSky.
While there are signs that climate change may have contributed to the extreme rainfall in Texas, larger weather patterns that are typical for summer appear to have added to the volatility.
High pressure across the U.S. West and Great Plains led to a dearth of winds aloft to move thunderstorms across Texas, Roys said. That essentially allowed storms to park over the central part of the state, unloading rain over a relatively small geographic area.
The storms which continued into Monday were part of a larger pattern that started to draw in moisture from the Gulf as well as all around the region, Truchelut said. This became a large rotating system of storms called a mesoscale convective storm complex, which fed off the warm, moist air.
In the hours before the flood struck, the U.S. Weather Prediction Center sent a series of mesoscale alerts warning rain could fall at rates of 3 inches (7.6 centimeters) per hour or more in the regions west of Interstate 35, which cuts through the heart of Austin.
While natural patterns added to the dangers, Francis noted that cutting emissions would at least lower the risk of human-caused climate change.
The horrific flooding in Texas is yet another glimpse into our future of more extreme weather, unless we kick our addiction to fossil fuels and stop deforestation, she said.
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