Climate change is not only causing droughts, wildfires, and extreme weather, but also creating contradictions, especially concerning what are known as “hundred-year” weather events.
Long-term weather predictions, which forecast conditions months, years, or even decades in advance, rely on probabilities, considering current conditions, trends, and historical data. Regions with past instances of unusually rainy springs or intense tropical storms are likely to experience them again under similar conditions, particularly when the atmosphere is moisture-rich and oceans are unusually warm.
Environmental scientists can now analyze weather history to estimate the likelihood of severe storms or floods occurring in a specific area, such as once every 100, 500, or 1,000 years. However, climate change-induced meteorological volatility is causing these rare events to occur more frequently. Since 1999, North Carolina’s coast has seen multiple storms classified as hundred- or thousand-year events. From 2015 to 2019, a St. Louis suburb experienced repeated severe flooding, with two events meeting the criteria for hundred-year occurrences. The carbon removal project Deep Sky estimates a 300% increase in deadly hurricanes, with 100-year storms now predicted to occur every 25 years.
Climate change is also altering the definition of these rare events. According to climate scientist Andrew Pershing, an extreme rainfall event in the Mississippi Valley in April was classified as a 100-year event under current climate conditions, but would have been considered a 500-year event without climate change.
These calculations require significant data modeling, as climate patterns evolve over millennia, while modern weather records only span about a century. Scientists analyze 30 to 50 years of data to determine event frequency. However, this data reflects a climate that was approximately two degrees cooler than today. When calculations are adjusted for the current climate, events expected to occur once every hundred years may now occur once every 20 years.
The math is relatively straightforward: a hundred-year storm has a 1% chance of occurring in any given year, while a 500-year storm has a 0.2% chance, and a thousand-year storm has a 0.1% chance. However, the probability resets each year. Even if a hundred-year storm occurs in a location like the Carolina coast in 2025, the area still has a 1% chance the following year, and climate change is increasing this likelihood. Therefore, these events cannot be precisely scheduled, according to Pershing.
The increased frequency is driven by a “thirstier” atmosphere that holds more moisture due to higher temperatures. This supercharged water cycle increases the likelihood of larger rain events, according to Pershing.
The National Oceanic and Atmospheric Administration (NOAA) anticipates a potentially severe 2025 Atlantic hurricane season, running from June 1 to November 30. While NOAA doesn’t predict specific 100-, 500-, or 1,000-year events, they foresee a 60% chance of an above-average season, a 30% chance of an average season, and only a 10% chance of a below-average season. They project 13 to 19 named storms, with winds of 39 mph or higher, and up to 10 developing into hurricanes with winds of 74 mph or more. Up to five could become major hurricanes (Category 3, 4, or 5, with winds of 111 mph or more). The impact could extend beyond coastal regions.
Acting NOAA administrator Laura Grimm noted that the impacts of hurricanes can extend far inland, as seen with significant flooding from hurricanes Helene and Debby last year. The Pacific region may also face challenges. Tropical Storm Adrian formed off the southwest coast of Mexico, two weeks before the start of the eastern Pacific hurricane season.
Besides hurricanes, floods, and storms, projections can be made for heat waves, droughts, and wildfires. Pershing explains that a hotter atmosphere holds more water, but when that moisture is released over mountain ranges, it results in drier air masses that draw moisture from the ground, intensifying droughts.
There is no simple solution for a warming atmosphere. Adaptation measures like dikes, levees, and relocation can offer short-term relief for flood-prone cities and eroding coasts. However, the most effective long-term strategy is to reduce greenhouse gas emissions to stabilize the climate and limit the frequency of extreme weather events. Pershing urges a rapid shift away from fossil fuels to allow the climate to stabilize and provide an opportunity for adjustment.
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