Climate change helped fuel heavy rains that caused Hill Country floods, experts say

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Heavy rains over the weekend that pushed the Guadalupe River in Texas’ Hill Country to its second-highest height on record had by Tuesday resulted in more than 100 reported deaths, including 27 children and counselors from Camp Mystic. But as search and rescue teams and volunteers sweep the banks of the river for missing people, the number of confirmed deaths is expected to grow.

Climate scientists said the torrential downpours on July 4 exemplify the devastating outcomes of weather intensified by a warming atmosphere. These disasters, they said, will become more frequent as people around the world continue to burn fossil fuels and heat the planet.

“This is not a one-off anymore,” said Claudia Benitez-Nelson, a climate scientist at the University of South Carolina. Extreme rainfall events are increasing across the U.S. as temperatures rise, she said.

Warmer temperatures allow for the atmosphere to hold more water vapor, producing heavier rainfalls, she and other climate scientists said. This coupled with old infrastructure and ineffective warning systems can be disastrous.

“It is an established fact that human-induced greenhouse gas emissions have led to an increased frequency and/or intensity of some weather and climate extremes since pre-industrial time, in particular for temperature extremes,” the United Nations’ Intergovernmental Panel on Climate Change reported in 2021. “At the global scale, the intensification of heavy precipitation will follow the rate of increase in the maximum amount of moisture that the atmosphere can hold as it warms about 7% per 1°C of global warming.”

The U.S. government’s fifth National Climate Assessment, released in November 2023, says that “the number of days with extreme precipitation will continue to increase as the climate warms” and that “these changes in precipitation patterns can lead to increased flood hazards, impacting infrastructure, ecosystems, and communities.”

Central Texas is infamous for its flash flooding and arid soil, hard-packed ground into which water does not easily infiltrate. So when rain hits the ground, it runs off the region’s hilly terrain and canyons and accumulates into creeks and rivers rapidly, overwhelming them, causing them to rise quickly.

The flash flooding wasn’t a result of a full-strength storm, Benitez-Nelson said, but a remnant of a tropical storm. “That, to me, is really sad and deeply alarming,” Benitez-Nelson said. “Climate change is turning ordinary weather into these disasters.”

Damp remnants of Tropical Storm Barry moved up from eastern Mexico as humid air also moved north from Mexico’s southwestern coast, stalling over Texas’ Hill Country. The warm air in both the low and high levels of the atmosphere is a recipe for intense rainfall, said John Nielsen-Gammon, the state’s appointed climatologist for more than 20 years.

He and his colleagues compiled a list of all the rainfall events in Texas that produced more than 20 inches of rain a few years ago. One common feature the climatologists found was when wind blew from south to north, or when moisture was brought northward from the tropics, he said. “That sets up the possibility of very heavy rainfall,” Nielsen-Gammon said. He concluded in a report last year that extreme rain in Texas could increase 10% by 2036.

Increased moisture from the tropics is driven by warming oceans.

The oceans absorb over 90% of excess heat in the atmosphere produced by greenhouse gas emissions, warming ocean temperatures down to depths of 2,000 meters. Tropical storms gain strength from heat and evaporate more quickly at higher temperatures, adding more water vapor to the atmosphere, Nielsen-Gammon said.

A study released Monday by ClimaMeter, a project funded by the European Union and the French National Center for Scientific Research, found that meteorological conditions leading up to Friday morning’s floods were warmer and 7% wetter than similar events of the past. Natural variability alone can’t explain the changes in rain associated with the exceptional weather, the report said, and points to human-caused climate change as one of the main drivers of the event.

ClimaMeter’s analysis shows the difference in surface temperature, precipitation and wind speed between the present climate from 1987 and earlier decades, from 1950 to 1986.

“Climate change loads the dice toward more frequent and more intense floods,” said Davide Faranda, one of the report’s authors who is research director of climate physics in the Laboratoire de Science du Climat et de l’Environnement, part of the French National Center for Scientific Research. “The flash flood that tore through Camp Mystic at night, when people were most vulnerable, shows the deadly cost of underestimating this shift.”

He added: “A 7% increase of rain is a lot, but doesn’t really make the tragedy. If you have a good alert system, if the population knows the risk related to climate change for this weather phenomena and can take them into account, not minimize them, then you can save lives, because it’s not double the amount of precipitation, it’s not three times. It’s something that we can handle if we are prepared.”

Other factors in the flooding death toll such as land use change, urban sprawl and warning system failures weren’t analyzed and may have further amplified the disaster, the report said.

“We are in a more extreme climate,” Faranda said. “And every year, year after year, we make it more extreme by burning more fossil fuels … These extremes now start to touch the limits of what is normal life on this planet, in terms of humans, in terms of infrastructure that we built with the old climate, in terms of resilience of the ecosystem.”

Initial estimates for the damage and economic loss of this disaster will reach beyond $18 billion, according to AccuWeather.

Staff writer Bob Berwyn contributed to this report.