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Severe Storms on the Increase


The number of severe storms is increasing, and so is their intensity.

Figure 1. Source: NOAA National Centers for Environmental Information.

In the previous post I noted that Hurricane Harvey was one in a series of storms that have devastated Houston, and indeed, the country as a whole. I asked what is going on, and whether it has always been this way.

The National Centers for Environmental Information tracks weather disasters that cause over $1 billion in damages. Figure 1 shows how many there have been each year going back to 1980. The number varies from year-to-year, but over time there has been a significant increase – there weren’t any in 1987, but in 2011 there were 16. Through July 7, 2017, roughly half the year, there have been 9.

(Click on chart for larger view.)

In the chart, the colors represent different types of weather disasters. Storms are divided into 3 categories: winter storms, which involve ice and snow, tropical cyclones (like Hurricane Harvey or Tropical Storm Irene), and severe storms. This last category includes thunderstorms and tornadoes, as well as severe rain events like the ones that caused flooding in Missouri in December 2015 and April 2017. You can see that the increased number of billion-dollar disasters has come from an increase in the number of severe storms. It has not come from tropical storms or winter storms.

Figure 2. Data source: NOAA National Centers for Environmental Information.

Figure 2 shows the damage cost from billion-dollar weather disasters each year. The damage cost is adjusted for inflation. The chart shows that there are many years when the total cost is below $25 billion. However, there are also years where the amount of damage spikes. The year with the largest damage was 2005, when Hurricane Katrina devastated New Orleans and a wide swath of the Gulf Coast, and damage topped $213 billion. That’s quite a chunk of change. The second highest cost occurred in 2012, when Hurricane Sandy came ashore in New York. This year, 2017, only includes damage up to July 7, so it doesn’t include Hurricane Harvey or Irma. I have seen news stories that the cost of damage from Hurricane Harvey may reach $150 billion, and Irma will add billions more. By the time the year is done, the damage cost is likely to be the highest in history.


Figure 3. Data source: NOAA, National Centers for Environmental Information.

Figure 3 shows the number of billion dollar weather disasters by type (through 7/7/2017). Since 1980, there have been a total of 212. Severe storms have accounted for 42% of the events.








Figure 4. Data source: NOAA National Centers for Environmental Information.

Figure 4 shows the total costs of billion dollar weather disasters by type (through 7/7/2017). Since 1980 costs have totaled $1.24 trillion dollars, and tropical cyclones have accounted for about 47% of the total cost. Though they constitute the largest number of events, severe storms account for only 16% of the cost of damages. That is because such storms, while severe, affect relatively small areas. Tropical storms and droughts, on the other hand, affect much larger areas.





Figure 5. Miami Beach in 1925. Source: Miami Design Preservation League.

All of the highest cost years have occurred since 2004. The data is inflation-adjusted, so that should not be the reason. One possible reason not related to the weather is that there are more people living in harms way – the population living along the coast has grown, and sprawl has caused more of the landscape to be covered with development, increasing the likelihood that a severe storm will hit something and damage it. For instance, in 1920 the population of Miami-Dade County (the location of the City of Miami) was 42,753 (that’s right, less than 50,000). But in 2010 it was 2,507,362. In 1992, when Hurricane Andrew devastated Homestead, a small community southwest of Miami, the area between Miami and Homestead was mostly open agricultural fields. Today, just 15 years later, it has filled-in, and is one continuous urban area. This story has been repeated all along the coasts of America, and in many inland areas as well. (See here.)





Figure 6. Intensity of Tropical Storms. Source: Kossin et al, 2013.

But I think that’s only part of the story. The number of tropical storms striking the U.S. may not have increased, but their intensity has. Figure 6 shows the intensity of tropical storms in different regions of the world over time. LMI stands for the lifetime maximum intensity of the wind in a storm, in meters per second. The lines represent quantiles. The 0.9 line (pinkish-purple) means that 90% of all storms that year were less intense than that value. The 0.8 line (light blue) means that 80% of all storms were less intense than that value, and so on. The authors dropped trend lines on the chart for each quantile. In the North Atlantic, storms have increased in intensity a lot. Those are the storms that strike the East Coast and Gulf Coast of the United States.


Figure 7. Source: GlobalChange.gov.

Other kinds of heavy precipitation events are also on the rise, as I reported here. Figure 7 repeats a chart from that post showing the trend over time.

Scientists project that climate change will cause an increase in storm intensity and in heavy rain events. It seems that this is not a prediction for the future, it is already happening. One cannot say that any individual storm is caused by climate change, but storms like Hurricane Harvey, Tropical Storm Irene, and the April storm in Missouri are already “more common,” and are likely to be even more “more common” in the future.


GlobalChange.gov. Broadcase_Trends-in-heavy-precip_V2. National Climate Assessment 2014. Downloads, Graphics (Broadcast). Downloaded 11/13/2016 from http://nca2014.globalchange.gov/downloads.

Kossin, James, Timothy Olander, and Kenneth Knapp. 2013. Trend Analysis with a New Global Record of Tropical Cyclone Intensity. Journal of Climate, 26, 9960-9976.

Miami Design Preservation League. Collins Ave. at 63rd Street in 1925.Downloaded 9/8/2017 from https://www.pinterest.com/pin/189714203027788727.

NOAA National Centers for Environmental Information (NCEI). U.S. Billion-Dollar Weather and Climate Disasters (2017). https://www.ncdc.noaa.gov/billions.

Wikipedia. Miami-Dade County, Florida. Viewed online 9/8/2017 at https://en.wikipedia.org/wiki/Miami-Dade_County,_Florida#2010_U.S._Census.


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