How climate change is pushing weather whiplash to new extremes

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Monday morning, the Dallas-Fort Worth area woke up in disaster. The rain was falling at a rate of 2 to 3 inches per hour. Highways became deadly lagoons, streams became pools, and thousands of people rushed to the heights.

Just a day earlier, the city had faced one of its worst droughts on record, with farmers forced to reduce their herds as reservoirs rapidly shrank. Twenty-nine percent of the Lone Star State was locked in an “exceptional” Level 4 of 4 drought. The very dry conditions weighed heavily on crops and imposed widespread water restrictions.

The extreme case of atmospheric whim has highlighted a growing problem plaguing communities across the United States and the world: meteorological boost.

This summer, several places in the United States experienced these wild and rapid fluctuations from one extreme climate to another. About half of the country has experienced at least moderate drought this summer. Parts of the West, Midwest and Texas have experienced exceptional and historic drought conditions.

Then the storms came. On July 26 in St. Louis, a shocking 8.65 inches of rain fell to mark the city’s wettest day on record. The following day in eastern Kentucky, rainfall rates exceeded 2 inches per hour and claimed the lives of 38 people. In August, eastern Illinois, Death Valley and Dallas also experienced significant or record rainfall. On Wednesday, flash flooding in central Mississippi washed out roads and prompted rescues.

Five 1,000-year rain events hit the United States in five weeks. Why?

“It’s unusual, especially on extreme rainfall [and] flash flood side,” said Daniel Swain, a climatologist at the University of California, Los Angeles. “They’re not just breaking an all-time record by a marginal amount, but they’re completely breaking it, and then some.”

Still, he’s not surprised: a warmer climate pushes rainfall to higher extremes, both in floods and droughts.

“The increase in both extreme precipitation events and these wild fluctuations between extreme precipitation and extreme aridity – this is how most people and most ecosystems on Earth are experiencing climate change,” said Swain said.

Two sides of the same coin

How can episodes of drought and heavy rains result from climate change? Simple.

Warmer air can hold more water. In fact, for every degree Fahrenheit the atmosphere warms, the air can hold about 4% more water. Where there is moisture available, such as along the Gulf or Eastern coasts, more moisture can be transported and released, leading to flooding and high rainfall totals.

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But where moisture is scarce, as in the West, warmer air sucks moisture out of the ground. This parched landscape reinforces the extreme heat, leading to drought and extreme wildfire behavior.

The United Nations Intergovernmental Panel on Climate Change highlighted this problem in a recent assessment report, writing that “aridification” and “extreme precipitation events that lead to severe flooding” are both by-products of global warming.

“Whiplash events have always happened, but now we see the shifts from one weather pattern to another becoming more violent and disruptive…yet another clear signal that the climate crisis is with us now,” said Jennifer Francis, a senior scientist. to the Woodwell Climate Research Center, in an email.

There has also been a tendency for the weather to become “stuck”, stagnant for longer. Perhaps that’s why Dallas faced drought for months and was 11 inches behind for the year before this week’s floods. However, such extreme precipitation is nothing more than feast or famine.

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This may be related to a more undulating jet stream, which is believed to move weather features slower from west to east (in the northern hemisphere). This allows systems to stall. If a heated dome roots in place, weeks of sunshine and dry weather can prevail. In all cases of millennial rain events in the West, Midwest and Texas this summer, a blocked frontal boundary was responsible for the downpours.

Don’t get me wrong – when it comes to weather, getting a perfectly average day is atypical. Averages are simply found by smoothing out the dips and peaks in a chaotic random system. But when thermal energy and entropy, or a little extra chaos, are added to this system, the dips and ridges become much more extreme.

Although events have become more extreme, Francis said recent studies suggest whiplash events have not yet become more common, but she added that computer models “paint a clear picture of more frequent events. if we continue to warm the globe by burning fossil fuels and destroying vast tracts of forest.

A flash flood won’t cure a drought

A record flood should fix a record drought, right? Not enough.

Water during a drought can help, but the speed and amount of water that falls is important.

During a drought, the soil dries out and becomes less permeable. Upper soils harden, which facilitates water runoff. Drought also kills plants and leaves the soil bare, further limiting the amount of water the soil can absorb. When it rains, much of the water runs off immediately and does not replenish the soils, aquifers, or river flow beyond the initial surge.

“You get more instantaneous runoff, higher flash flood rates on rivers and streams, but less of that water seeping into the ground,” Swain said. “So you get less soil moisture from the same amount of water.”

In fact, drought can actually lead to a greater risk of flooding. Dry soil hit by rapid rainfall can promote runoff and trigger widespread flooding.

For example, in Dallas, while rainfall was desperately needed, most of what fell did not benefit the greater metropolitan area. The event caused deadly flash floods, but also nearly all of the water that came down spilled into a watershed that drains into Lake Livingston and eventually into Houston. The Fort Worth National Weather Service summed it up, writing “heavy rain, but bad watershed.”

And last week, around 200 people were trapped for several hours in New Mexico’s Carlsbad Caverns National Park amid heavy flooding.

Swain said the weather boost also means there are more dry days between the few rainy days, which provides more opportunities for water to evaporate into the atmosphere. Even the water that remains can evaporate quickly, especially in a warming world.

“Soil moisture and vegetation will continue to react in the long term as if there were a severe drought, because in the long term there always is,” Swain said.