On the heels of a once-in-a-hundred-year total solar eclipse, Hurricane Harvey brought what many called a “500-year flood” to the Gulf of Mexico. But in the wake of a predictable darkness streaking across the United States, this Category 4 hurricane unleashed a shocking deluge– stalling over a region including my hometown of Houston, which has already witnessed three such “500-year hurricanes” since 2014.
But how can 500-year floods occur consecutively? Have meteorologists’ predictions gone wonky? Is Harvey’s intensity–and now Irma’s– symptomatic of a rapidly warming planet? Or, has God doomed Houston to a Great Flood as retribution for electing a lesbian mayor?
The answer is not straightforward, but I think the best explanation is part-meteorological and part-etymological (though unlikely biblical). A “500-year” disaster is actually a statistical phrase used as shorthand by actuaries, weather experts, and governmental agencies. And it has confused the public.
What does a 500-year event mean? 500-year events do not refer to the time intervals between them. Rather, a 500-year event refers to the probability of that some action will occur. A 10-year hurricane has odds of occurring once in ten years, a 100-year hurricane once in one hundred, and a 500-year hurricane once in five hundred, and so on.
The phrasing “500-year” helps us wrap our minds around the rarity of the event, but it also lends itself to misinterpretation. An easier way to think about a 500-year event is as an event that has 1 in 500 odds (0.2%) of occurring in any given year and in any given area. But, many folks, particularly those with lower numeracy, do not intuitively process this concept of statistical independence when they hear the phrase. For example, some might believe that once a 500-year hurricane occurs, another cannot occur again for another 499 more years.
Beyond being slightly confusing, the term is also a bit of a misnomer. A flood’s classification as “500-year” varies regionally. It does not describe the US as a whole. Above is an example of the distribution of floodplains in a small area in Harris County near my home. You can see how widely one’s risk of flooding varies, just across the tollways.
So, the likelihood of the greater Houston area getting hit by three 500-year disasters three years in a row is low, but non-zero. In fact, the entire US has experienced 25 500-year precipitation events in the last 7 years.
Are Houston and America just unlucky?
Perhaps. But it turns out that representing flooding as “500-year” is also a scientific inaccuracy, because most robust existing data on hurricanes goes back only about thirty years. In fact, many meteorologists are uncomfortable with extrapolating beyond a 100-year prediction. Precipitation science is a moving target, far less predictable than the eclipse of the moon and the sun. Instead, weather predictions like those we use for estimating Harvey depend on a continuously updating database, which derives from an obviously dynamic planet.
In light of these facts, misunderstanding about the frequency of precipitation events can adversely impact dialogue about city planning and climate change as well as public preparedness for future catastrophes like Harvey, Irma, and Jose.
First, people living in higher-risk flood areas like Houston, New Orleans, and Miami make decisions about housing and flood insurance amidst the uncertainty that a natural disaster will occur. To protect these people, the Federal Emergency Management Agency (FEMA) mandates that homes in 100-year floodplains or lower purchase flood insurance.
But perhaps these thresholds are too generous or based on outdated information. City planners and FEMA could update guidelines to accommodate the potentially growing risk of flooding. At the very least, Houston’s underpreparedness for Harvey reflects a pressing need for greater public caution; even without an official change from FEMA, understanding the mounting risks associated with floodplain housing can help homeowners make better, more informed decisions when preparing for rare but possible events.
Second, media usage of this term could also affect people's safety. As one model predicts, storm intensity and media coverage are some primary drivers for household evacuation decisions. How does reporting on storm frequency affect these outcomes? And, can precise reporting encourage individuals to make choices that are best for their wellbeing?
Finally, the back-to-back hurricanes this month have intensified dialogue on climate change. Understanding the nuances of atmospheric sciences will be particularly important for each of us as news consumers. While some attribute frequent, intense hurricanes like Harvey and Irma solely to climate change and others might choose to explain them away as rare events, the truth is likely somewhere in the middle; the connection between our warming planet and natural disasters is more abstract and indirect.
So, particularly as the American public searches for someone to blame for these consecutive, devastating events, be on the lookout for superficial analyses and unwarranted finger-pointing. Often, the way we frame science drives the narrative more than the science itself.
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Sarina Madhavan is a Trinity senior. Her column runs on alternate Thursdays.