Compound Events Contributing To More Extreme Weather

The past few weeks have brought light to the stark increase in the frequency and intensity of extreme weather, including hurricanes, heat waves and wildfires. These are part of a new and alarming trend in which we see the compounding effect of multiple weather events occurring simultaneously, amplifying the collective impact and posing a challenge for communities and businesses across the U.S.

Understanding Compound Weather Events

Simply put, compound weather events are a combination of weather and climate drivers that lead to potentially high-impact events. For example, a heatwave followed by heavy rainfall leads to flooding, or a prolonged drought followed by an intense storm surge leads to the same disastrous results. These combined events can interact in ways that significantly heighten their impacts, resulting in cascading consequences that stretch beyond the sum of their parts. The ARC Centre of Excellence for Climate Extremes does an excellent job of explaining compound events by comparing them to going to a doctor’s visit.

While subsequent impacts from weather events are not new, the changing climate landscape can cause events that were not only once improbable but also are happening more frequently. Another factor driving this phenomenon is that compound weather events often trigger feedback loops, where one event exacerbates the conditions for another. For example, a wildfire may reduce vegetation cover, increasing the likelihood of soil erosion and mudslides during heavy rainfall. This creates a domino effect and amplifies the overall impact on communities, businesses and the environment.

Compound Weather Event Examples

The recent catastrophic wildfire event in Hawaii is a recent example of climate change driving compound events. Studies have shown that this is driving the increase in wildfire season length and the frequency of wildfires. The ingredients for high fire risk—drought, ignitable grass species and high temperatures—were already present in Hawaii, which is why the island state was under a red flag warning. The strong winds generated by Hurricane Dora, a Category 4 hurricane literally fanning the flames, made the fire worse. Individually, both events can cause structural damage and interrupt business continuity, but their occurrence is not unusual. However, the presence of both extreme events at the same time magnified the impact.

Wildfires also compound health impacts. This summer, air quality issues due to wildfires have been prevalent throughout much of the United States, causing heightened risks to public health. New York City experienced its worst-ever wildfire-related air quality when residents were exposed to pollution levels more than five times above the national standard, and on several occasions earlier this summer, several U.S. cities had some of the unhealthiest air anywhere on Earth.

Another example of compound weather events is within our oceans. Rising sea surface temperatures can intensify hurricanes, leading to more powerful storms and heavier rainfall, as we saw with Hurricane Ian last year and more recently with Hurricane Idalia, the strongest storm to make landfall in the Big Bend area of Florida in over 100 years. The reason Hurricane Idalia increased to Category 4 intensity was the warmer sea temperatures in the Gulf, adding energy or “fuel” for its formation. The compounding events with hurricanes are the extreme storm surge and rainfall leading to devastating flooding, even in towns hundreds of miles from landfall.

While we will continue to see compound weather events increase and cause unprecedented impacts, it’s important to know that scientists and researchers worldwide are collaborating to prioritize research and technology development that will help communities and businesses make informed, long-term financial and infrastructure choices. Over time, attribution science will be able to scientifically determine whether climate change contributed to extreme events that will further inform governments, businesses and communities on how to design more resilient plans for a changing climate.

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