What you need to know
- This brief investigates whether climate change is affecting hurricane severity.
- Analysis by others confirms that the number of storms is not increasing.
- In this brief, we analyze the trend in major hurricanes.
In October 2024’s news coverage of Hurricanes Helene and Milton and their catastrophic damage, some stories argued that climate change has increased the severity of such storms. This brief evaluates this hypothesis: are severe storms becoming more common?
How is hurricane severity measured?
The Saffir-Simpson Scale measures hurricane severity. It divides hurricanes into five groups, from 1 (winds 74-95 miles per hour, with damage expected to roofs, tree limbs, and power lines) to 5 (130 - 156 MPH, widespread catastrophic damage expected). A major hurricane is category 3 (111-129 MPH) or higher.
In 2024, Hurricane Helene came ashore as a category 4 storm. Hurricane Milton reached category 5 over the Gulf of Mexico, weakening to category 3 over the Florida coast.
Another way to measure storm severity is to calculate the dollar value of damage. By this measure, storms today are much more severe than a generation ago – but this conclusion reflects that coastal regions have become much more developed, so more buildings and infrastructure are being damaged.
Why might global warming produce more severe hurricanes?
The central argument for a link between climate change and hurricane severity is that hurricanes form over oceans and strengthen given large temperature differences between ocean water and the upper atmosphere. One observed consequence of global warming is that oceans are warming, increasing these temperature disparities.
For example, one study published in 2013 stated, “We conclude that since 1975 there has been a substantial and observable regional and global increase in the proportion of Category 4-5 hurricanes of 25-30 percent per °C of anthropogenic (human-caused) global warming.”
The best-available information (including the study quoted here) is that climate change has not affected the number of hurricanes each year.
What is the trend in major hurricanes over time?
There are two primary aspects to consider with the study quoted here. One is that the study looks at a relatively short time span, from the 1970s to the early 2010s. Hurricane experts have identified the 1960s to the 1980s as a time when there was an abnormally low number of hurricanes. A potentially more accurate analysis of whether global warming has affected hurricane severity should likely use a longer time span.
Moving to longer time spans raises a new problem: before 1980, some major storms were unrecorded because they did not make landfall or because coastal regions were largely uninhabited. Now, weather satellites provide global coverage. One potential solution to address this limitation is to adjust historical data by adding storms recorded in ship’s logs and other sources. Our analysis references updated data compiled by scholars at Princeton University and the National Oceanographic and Atmospheric Agency.
The chart below shows the adjusted number of major Atlantic Ocean hurricanes from 1930 to the present. Each point shows the number of major hurricanes for a given year. There are two trend lines, one for the longer time and the other for 1975 - 2020 (the period cited in the earlier quote).
Examining data over the longer period (during which global temperatures have increased), there is little to no sign of any material increase in the number of major hurricanes. The red trend line does, however, show a significant increase over a shorter time, but this relationship goes away if we look at it over a longer time.
In addition, there have only been four category 5 storms since 1960: Camille in 1969, Andrew in 1992, Michael in 2019, and Milton in 2024. At present, there is no sign of an increase.
The Take-Away
That news coverage of hurricanes links global warming to hurricane severity is not surprising: hurricanes strengthen as they move through areas of warm ocean water, and ocean temperatures are increasing.
The most comprehensive data available on record reflecting the number of major hurricanes over time does not show any material increase, even though global temperatures have risen over the same time.
The perception that hurricanes today are more dangerous than a generation ago may reflect increased populations and development in America’s coastal communities.
Further reading
Weinkle, J., Landsea, C., Collins, D., Musulin, R., Crompton, R. P., Klotzbach, P. J., & Pielke Jr, R. (2018). Normalized hurricane damage in the continental United States 1900–2017. Nature sustainability, 1(12), 808-813.
Wehner, M. F., & Kossin, J. P. (2024). The growing inadequacy of an open-ended Saffir–Simpson hurricane wind scale in a warming world. Proceedings of the National Academy of Sciences, 121(7): 100-103.
Sources
How do we measure the severity of hurricanes?
National Weather Service (2024). Saffir-Simpson Hurricane Scale. https://tinyurl.com/mryj5hb8, accessed 10/11/24
Helderop, E., & Grubesic, T. H. (2022). Hurricane storm surge: toward a normalized damage index for coastal regions. Natural Hazards, 110(2), 1179-1197.
Why would global warming produce more severe hurricanes?
Holland, G., & Bruyère, C. L. (2014). Recent intense hurricane response to global climate change. Climate Dynamics, 42, 617-627.
What is the trend in major hurricanes over time?
Vecchi, G. A., Landsea, C., Zhang, W., Villarini, G., & Knutson, T. (2021). Changes in Atlantic major hurricane frequency since the late-19th century. Nature communications, 12(1), 4054.
Vecchi, G. A., & Knutson, T. R. (2011). Estimating annual numbers of Atlantic hurricanes missing from the HURDAT database (1878–1965) using ship track density. Journal of Climate, 24(6), 1736-1746.
Tropical Meteorology Project (2024) North Atlantic Ocean Historical Tropical Cyclone Statistics. https://tinyurl.com/2z2jjte7, accessed 10/11/24.
Hurricane Research Division (2024) Continental United States Hurricane Impacts/Landfalls, https://www.aoml.noaa.gov/hrd/hurdat/All_U.S._Hurricanes.html, accessed 10/15/24.
Contributors
Allison Cooper (Senior Intern) is a Political Science and Philosophy (Law and Policy) student at Washington University in St. Louis. She will graduate in May 2027 and plans to attend law school post-graduation.
Olivia DiPietro (Senior Intern) is a junior at Fordham University pursuing a double major in French and journalism.
Dr. William Bianco (Research Director) is Professor of Political Science and Director of the Indiana Political Analytics Workshop at Indiana University. His current research is on representation, political identities, and the politics of scientific research.