A 1-in-100-year storm has a 1% chance of occurring in any given year
You’ve probably heard “storm of the century” used to describe a storm so extreme we only imagine we’ll experience it once in 100 years or once in a lifetime. Scientists use the term “1-in-100-year storm” to express the probability of such extreme precipitation events. Both describe the same thing: a rare, severe storm event.
However, “1-in-100-year storm” describes this event in a stable climate—one where the next 100 years could be expected to resemble the last 100 years. As our climate is changing, how is the definition of “1-in-100-year storm” changing?
What is a 1-in-100-year storm?
A 1-in-100-year storm is a storm with so much precipitation in a single day that it has only a 1% chance of occurring in any given year, or, on average, only once in 100 years. A 1% chance seems like a low probability, but since it applies to every year, the chance of such an event happening rises substantially over time.
The threshold that determines what qualifies as a 1-in-100-year storm is relative to local precipitation patterns, and, therefore, differs from place to place. In a location that experiences many days with heavy precipitation—like monsoon-prone Mumbai—the 1-in-100-year storm is a much wetter event than it would be somewhere like Paris, where the local precipitation trends are less extreme. A 1-in-100-year storm in Paris might feel like a typical day in the rainy season in Mumbai. Whether it’s worthy of “storm of the century” is all about what’s normal to you in your area and how prepared you are for the magnitude.
“1-in-100-year” might sound like it’s a storm that happens once every 100 years, but the phrase expresses probability or likelihood—not a window of time. Because “1-in-100-year” is an average measure, it doesn’t mean we should expect 100 years between storms. For example, a 1-in-100-year storm has a 26% chance of happening over 30 years. If the climate is stable over 500 years, we would expect a 1-in-100-year event to occur five times in that duration, but those five occurrences could fall within a short period of time, even four within the first 100 years.
1-in-100-year storms in a warming world
As global average temperature rises because of human emissions of greenhouse gasses, extreme precipitation events are occurring more frequently and are more severe, carrying much greater precipitation.
The increase in extreme precipitation is due to something called the Clausius-Clapeyron Relation, which describes the atmosphere’s ability to hold more water, up to a limit, as it warms. For each 1°C the atmosphere warms, it can hold 7% more water. This effect is cumulative, so air warmed by 6°C can hold 50% more water, and air warmed by 10°C can hold almost 100% more water.
The warmer the air is, the more moisture it can hold. The heavier the air is with moisture, the more intense the deluge of precipitation can be once it starts to fall.
As the planet continues to warm, storms that were previously considered to be 1-in-100-year events now have a higher probability of occurring than their name would suggest. Instead of being a 1% probability event, the same event might have a 2, 3, or even 4% chance of occurring in any given year—that’s two, three, or four times more likely.
These events could happen at any point over an average century, and there is no way to predict the interval of years that could separate occurrences. While the name 1-in-100-year storm was appropriate during a stable climate, these events are now higher probability outcomes and the same terminology does not describe what places are experiencing.
Probable Futures aims to increase our chances that the future is good by offering tools to visualize climate change along with stories and insights to help you understand what those changes mean. Want to see how 1-in-100-year storms could change in places you care about? Read our Water volume to learn more about the future of precipitation. Use our maps of 1-in-100-year storm frequency and precipitation to see how these storms could change at different warming scenarios.