Climate

How one weather extreme can make the next one even more dangerous

A construction truck driving through floodwater.

A worker drives through standing water while creating sand berms to protect beachfront homes from flooding on February 20, 2024 in Long Beach, California.

Last year was the hottest year humans have ever recorded. That heat fueled a number of events — heat waves, downpours, fires — reaching unprecedented scales. And their effects are still reverberating.

Canada’s record-breaking fires last year continued to smolder over the winter and have begun to regain strength. Hurricane activity in the Atlantic Ocean is likely to be well above normal due in part to the extraordinary amount of heat the ocean absorbed last year. Heat waves and warm winters in many parts of the world are laying the foundations for even more severe heat this year, raising the possibility that this year may be even hotter than the last one. Already places like Finland, Mexico, and India are experiencing heat waves well ahead of their respective summers.

Disasters of different types can compound too. Wildfires can strip trees and burn ground cover, denuding the landscape and leaving it vulnerable to landslides. Years of drought can dry out soil, compressing it and rendering it impermeable so when heavy rains set in, they cause more expansive flooding.

There is natural variability in the weather, and that includes extremes. But humanity’s output of greenhouse gasses is raising global average temperatures and thus increasing the likelihood and strength of many types of extreme weather events.

The increasing frequency and intensity of weather extremes is creating a regime where many disasters are not discrete, stand-alone events but built on a foundation of prior heat waves, droughts, wildfires, and downpours.

It’s not just disasters occurring one after another; it is extreme events that push subsequent calamities to a greater destructive potential. It creates a cumulative effect that doesn’t necessarily reset year to year. And more people are facing these combined effects. By one estimate, more than 90 percent of the world’s population is facing greater risks of combined heat waves and droughts, driven by changes in the climate system as well as shifts in water and land use. Add to that a growing population that continues to build in vulnerable areas.

The result is more frequent, dangerous, and costly disasters.

Understanding how extremes can beget extremes can help people anticipate and prepare for future threats. But right now, scientists are still struggling to gather the basic information they need to figure out how extremes influence each other.

How past extremes can influence current extremes, explained

There are a number of ways disasters can influence each other. One example is how extreme droughts can lead to more extreme floods. The Western United States is still reeling from a multi-decade drought, the worst dry spell in at least 1,200 years. The lack of rain coupled with rising average temperatures dried out soil and vegetation since warmer air can draw out more moisture.

Less moisture in the ground leads the soil to compact, making it less able to hold onto water. The withered grasses, shrubs, and trees also do little to capture rainfall with their roots.

So when torrential downpours occur, like with California’s atmospheric rivers earlier this year, the storms dish out more water over land that is less able to absorb it, leading to more flooding. The cycling back and forth between drought and flooding creates a phenomenon some scientists have described as weather whiplash.

Wildfires can then exacerbate this. Infernos take down the trees and other plants that anchor the soil in place with their roots, so when heavy rains fall, they trigger dangerous landslides. That was vividly illustrated in 2018 in Montecito, California. The preceding year, a wet winter led to an ample early-season bloom of vegetation that soon desiccated in the hot, dry spring and summer months that followed. Those grasses and shrubs ignited and fueled the Thomas Fire, which burned through 280,000 acres, driven by fast seasonal winds. At the time, it was California’s largest blaze on record. Then on January 9, 2018, after an intense rainstorm, residents of Montecito awoke to a wave of mud upward of 15 feet high sliding toward them. The mudslide killed 23 people and destroyed more than 400 homes.

Cal Firefighter Alex Jimenez walks out after marking a spot with a stick where he found a body under the mud at a house along Glen Oaks Drive in Montecito after a major storm hit the burn area Wednesday on January 10, 2018 in Montecito, California.

Another example we’re seeing play out this year is how hot ocean temperatures can build up over time and create the raw ingredients for hurricanes.

In 2023, Atlantic Ocean temperatures reached levels humans have never measured before. Hurricanes require surface water to be at least 80 degrees Fahrenheit, but they also require the air above the sea to have minimal wind shear. El Niño, the warm phase of the Pacific Ocean’s temperature cycle, was in full swing last year. It tends to raise global temperatures, but it also increases wind shear above the Atlantic, which limits hurricane formation. That allowed the Atlantic Ocean to charge up with more heat, leading water temperatures to remain elevated this year. With El Niño giving way to La Niña this year, the air over the Atlantic is poised to settle down, creating the conditions for this year’s above-normal hurricane season. Buckle up, as NOAA is forecasting upward of 25 named storms and seven major hurricanes.

The impacts of compounding extremes can be far-reaching and complicated

These intersecting extreme events have real-world consequences. Beyond the direct impact on people’s lives and property, one of the most consequential effects is on food security.

Extreme heat, drought, and excess moisture are all factors that can eat into harvests, but when they compound, these events can take a bigger bite out of crop yields. Of particular concern is a “hot drought,” where high temperatures converge on top of below-normal precipitation levels, a particularly stressful combination for plants. In places like India, Ethiopia, and the United States, these compounding extremes can diminish yields by up to 30 percent.

“Because it’s been getting warmer on average, droughts tend to be hotter on average,” explained Corey Lesk, a researcher at Dartmouth College studying climate impacts. Meanwhile, severe rainfall events are becoming more common. Rather than spreading out evenly through a growing season, precipitation is falling in fewer, more intense events. That not only causes flooding but it means plants don’t get the water they need in between downpours.

But Lesk noted that there are factors pushing in the other direction too. Climate change can mean a longer growing season for crops in some parts of the world, leading to more bountiful harvests. More carbon dioxide in the atmosphere can help enhance plant growth to an extent. And farming techniques are improving with more widespread tools like hardier crop varieties, precision fertilization, and early disease detection. “The strongest trend pushing yields up in general is the technological trend,” Lesk said.

Compounding weather extremes are not just coming for our stomachs; they are also rifling through our wallets. Insurance provides a vivid example. In the US, insurers are dropping customers or leaving states entirely as their risk portfolios prove greater than they can bear under the coverage and price regulations they face in some states. Losing insurance can then affect even more foundational decisions like where people should live and the places they may need to abandon.

For insurers, one challenge is that it’s getting more difficult to anticipate how different types of extremes will intersect and play out. According to the Intergovernmental Panel on Climate Change’s special report on the ocean and cryosphere (the frozen parts of the planet), compound disasters “are examples of deep uncertainty because data deficiency often prevents the assessment of probabilities and consequences of the risks from compound events.”

While epochal storms and raging infernos are major events, they aren’t that common compared to ordinary natural phenomena, so there aren’t a lot of robust measurements over time that illustrate how they are changing. Much of the best data about hurricanes, for instance, only dates back to the 1970s, when new high-resolution satellite instruments entered orbit. Insurance companies use historical records to gauge risks, but without good numbers, it’s harder for them to build a baseline understanding of how a drought last year will influence a landslide this year. As average temperatures rise, the way these variables will align in the future is even less certain.

The added complication is that climate change could create unexpected fundamental shifts in the basic mechanics of extreme weather patterns. For instance, further warming could cause the West Antarctic Ice Sheet to enter a self-propagating cycle of collapse. “[C]limate drivers that contribute to compound events could cross tipping points in the future,” according to the report.

Facing these unknowns, it’s prudent then to invest in measures that could save lives. That includes upgrading infrastructure to be more resilient, developing more robust forecasting models, and deploying early warning systems to get people out of harm’s way. But to halt the planet’s relentless warming, humanity must zero out its emissions of greenhouse gasses. Otherwise even more disasters will converge ahead.

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