What is the Role of Climate Change in Rising Severe Storm Costs in the US? Part 1

This is the first part of a two-part series. Part two will be posted June 19, 2024.

Springtime means severe thunderstorm season—tornadoes, derechos, and hail, among other hazards—for much of the eastern two-thirds of the US.

Damage caused by severe thunderstorms is immense and represents the second most frequent type of billion-dollar disaster in NOAA’s disaster database for the US. Severe thunderstorms are also the weather category responsible for the second most insured losses globally, according to SwissRe (in both datasets, tropical cyclones or hurricanes are responsible for the most damage). Both US billion-dollar disasters and global insured disaster losses are increasing, and a large fraction of the overall increase seems to be driven by increases in losses from severe thunderstorms.

For many, especially in the media, the natural inclination is to see these rising trends in disaster losses from severe thunderstorms and reflexively attribute them to climate change.

Take, for example, the New York Times’ The Daily podcast (consistently a top 5 podcast) from May 15th, 2024, titled “The Possible Collapse of the U.S. Home Insurance System.” The episode is about how these increases in insured losses are straining the books of insurers. The reporter, Christopher Flavelle, had this to say:

“One of the really striking things about this data was it showed the contagion [climate change-driven disaster losses] had spread to places that I wouldn't have thought of as especially prone to climate shocks. For example, a lot of the Midwest, a lot of the Southeast.”

“Formerly unimportant weather events like hailstorms or windstorms didn't used to be the kind of thing that would scare insurance companies… But those are becoming so frequent and so much more intense that they can cause existential threats for insurance companies.”

“This market is starting to buckle under the cost of climate change, and this is all happening really fast.”

However, as I emphasized in the previous series on floods, disaster impacts result from the combination of natural hazards (e.g., a physical event like hail or a tornado), exposure (the presence of people, resources, or infrastructure in harm’s way), and vulnerability (the propensity of people, resources, or infrastructure to be adversely affected).

Economic loss trends can be driven by changes in exposure and vulnerability even if the hazard is not changing. That’s why Roger Pielke often says, “Climate data should be the basis for claims of detection and attribution of changes in climate variables, not economic loss data.”

So, what does climate science say about historical and expected changes in severe thunderstorms and their associated hazards of tornadoes and hail?

As I have mentioned in previous posts, it is clarifying to divide scientific evidence into 1) Historical trends, 2) Fundamental theory, and 3) Mathematical modeling.

In this post, I’ll examine historical trends and then discuss fundamental theory and mathematical modeling in Part 2.

Historical trends

Since thunderstorms are on a continuum of severity, it is not necessarily clear how to track changes in severe thunderstorms over time, and there are many different variables you could look at. An obvious place to start would be to just look at the annual number of tornadoes per year and how they are changing, but it turns out this is not as easy as it sounds.

You might think we have good data on phenomena as conspicuous as tornadoes, but unfortunately, not all tornadoes are reported, and there is a strong bias towards increased reporting in recent years due to increases in population and infrastructure that can be damaged by tornadoes (since damage itself is often used to report a tornado especially if it occurred at night). Thus, in order to study long-term trends in tornadoes, researchers need to account for these reporting biases. When studies have done this, they get results like those below.

The red line in the top figure shows no long-term trend in the number of US tornadoes, and the study concludes that “long-term climate change has not substantially affected the domain-wide tornado frequency during the 1975–2018 analysis period.” Interestingly, the study found a statistically significant decreasing trend in the strongest tornadoes (bottom right panel).

Other studies have confirmed no long-term trends in tornado occurrence but have noted changes in other statistics, such as a decrease in the number of days per year with tornadoes combined with an increase in the number of tornadoes on those days. Other studies have shown shifts in geography and seasonality of where tornadoes occur, but it is unclear how much of this might be due to natural decadal variability as opposed to being driven by long-term warming.

Hail is another major component of severe convective storms and is responsible for a great deal of economic losses from both property and crop damage. These losses are roughly $10 billion per year in the US, and some single events reach multiple billions, like a 2012 hailstorm in Phoenix, Arizona, which caused $4 billion in damage.

Hail data suffers from the same issues as tornado data in that it is heavily influenced by reporting biases towards areas with higher population density. However, since at least around 2004, we have radar estimates of hail that don't suffer from these reporting bias issues (but come with their own laundry list of uncertainties and caveats). Nevertheless, this data paints a complicated picture of change that depends on location and season. For the spring, we see increases in severe hail that cause the most damage (greater than 2 inches in diameter) over Texas and Oklahoma but decreases over most of the rest of the country, and for the summer, we see increases in severe hail over the northern high plains and Minnesota but decreases over much of the rest of the country.

The authors note that natural variability (associated with phenomena like El Nino) is responsible for the bulk of this pattern and that it is unclear what, if any, effect long-term warming has had thus far. A recent review titled “The effects of climate change on hailstorms” concluded that “Overall, no clear overarching national climatological hail trend has been found for the USA”

We see that there are large increases in the economic losses associated with severe thunderstorms, but there are not well-documented increases in the physical hazards responsible for those losses. When reporting biases are accounted for, we see no increase in total tornado counts and perhaps a decrease in the occurrence of the strongest tornadoes. For hail, there is documented natural variability but no established long-term trend associated with climate change.

This discrepancy suggests that changes in exposure—such as population growth, and increased value of properties—may play a major role in these rising losses. By simultaneously considering anticipated changes in severe thunderstorm hazards and exposure to those hazards, we gain more direct insight into the causes of and thus potential solutions for increasing damages from these weather events. In Part 2, we will delve deeper into these aspects.