In the days before a catastrophic flash flood swept through rural Texas over the July 4 weekend, the region was parched by drought.
Even as search crews trudged through mud and debris in search of victims — at least 135 people lost their lives — some parts of the county remained super dry.
The rapid swing between drought and heavy precipitation is known as weather whiplash.
Northeastern University researchers say to expect more of it in the future.
The term “weather whiplash,” or “climate whiplash,” is relatively new in scientific literature, according to Samuel Munoz, an associate professor of marine and environmental sciences at Northeastern.
“It’s a rapid transition from unusually dry conditions to unusually wet conditions, or the opposite, from unusually wet to unusually dry,” he says. “So a quick transition from dry to wet or wet to dry.”
The rapid cycle from wet to dry is also potentially destructive. In California it has created the perfect conditions for wildfires after heavy rains gave rise to an explosion of plant life that then acted as combustible fuel in hot, dry weather.
“The concern is these kinds of events could become more frequent in some places,” Munoz says.
“It is all driven by warmth,” says Auroop Ganguly, Northeastern distinguished professor of civil and environmental engineering.
The Earth’s average surface air temperature has warmed by one degree Celsius, or 1.8 degrees Fahrenheit, since 1900, with more than half of the increase coming since the mid-1970s.
“There have been reports that say that precipitation extremes intensify on an average by around 7% per degree warming, although that varies significantly regionally and seasonally,” Ganguly says.
He says the extra heat means water evaporates from the surface more quickly when there is less water, leading to intensification of droughts.
When there is more water, the Earth’s atmosphere can allow more of that water to be absorbed, leading to intensification in the extremes of rainfall or snow when conditions are right for precipitation to occur, Ganguly says.
“When the two happen, each can get more intense and/or much faster paced, he says. “This warming-related intensification of the hydrologic or hydroclimate cycle, as it is sometimes called, can be a leading cause of the whiplash.”
“Get used to the idea that we will have more of these whiplashes,” Ganguly says. “We have seen preliminary evidence from the literature that as the world warms we’ll see more of these.”
The World Meteorological Organization says there is an 80% chance that the annual global temperature will exceed pre-industrial era temperatures by 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, at least temporarily during one year between now and 2028.
The effect of warming on moisture retention in the atmosphere is almost exponential, Munoz says.
When there are more water molecules in the air, the air moves around faster and has the capacity to hold even more molecules, called the Clausius-Clapeyron Equation, he says.
“It’s not linear. It’s not that for every degree of warming you can hold 1% more water. A little bit of warming translates into a lot more water that the atmosphere can hold,” Munoz says.
Not every part of the world will experience weather whiplash in the same way, he says.
Michelle O’Donnell, a Northeastern Ph.D. candidate in civil engineering, studied the Mississippi River basin to understand how whiplash events might affect the volume of water moving through rivers, also known as streamflow, in the future.
Using observed data from the Mississippi and its tributaries and modeling data, she found that by the end of the century whiplash events will become more frequent in the western part of the basin, which includes the Arkansas and Missouri rivers and their tributaries.
The eastern part of the Mississippi River basin, which includes the Ohio River, is rainier and “is just going to get rainier,” O’Donnell says.
“But in the Arkansas River basin, where precipitation will get more variable, jumps in precipitation are likely to contribute to these whiplash events,” she says.
This is important information for water and river managers because flooding events could swamp agricultural fields and drier conditions contributing to low water levels can affect the shipment of goods on the Mississippi and its tributaries, O’Donnell says.
“For navigation on the Mississippi, it matters if water levels are too high or too low for barges,” she says.
“The variability is going to be there, and adapting to the variability is important,” O’Donnell says.
Adapting can include improving drainage on a large scale, using green infrastructure to absorb water and preventing residences and businesses from being built in flood plains, Ganguly says.
“We need to address the problems of the day,” he says.
“Think of another extreme weather phenomenon as an example. It’s one thing to say hurricanes are going to get more extreme. It’s quite another to be ready for that hurricane. The same with whiplash events,” Ganguly says.
“We have to spend more time and effort getting ready for those because they are already with us, and because preparedness is the key to saving human lives and ensuring prosperity,” he says.