Humid heat extremes on the rise

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Heat is the deadliest form of extreme weather, and it’s especially dangerous when paired with high humidity. If it’s too damp to cool off by sweating, the consequences can be fatal. Most days are far less serious, but the worst conditions are occurring more often as the climate warms—according to recent research from Columbia University.

The study focused on extreme wet bulb temperatures, which measure how heat, moisture, and other factors affect the body in direct sunlight. (You can see how extremes vary globally in the researchers’ interactive). Outdoor activities are all but impossible when the wet bulb reaches 90°F (equivalent to a 132°F heat index), but much lower values are still dangerous. For this release, the researchers analyzed the most extreme 5% of days in a locality, using hourly data from over 450 U.S. stations.

In most states and regions, these humid heat extremes have already doubled in frequency—when comparing 2000-2019 to the previous two decades. In other words, conditions on the muggiest 18 days of the year (on average in 1980-99) may now occur on 36 days or more. All but one region has seen these frequencies double, and all but one state has risen by 50%. Parts of New England and the inland West have increased by 2.5 times.

Why such a big jump? Consider the bell curve below; even small shifts in averages lead to large changes in extremes. As temperatures rise in a warming climate, humid heat extremes are persisting—a trend that will worsen unless we reduce greenhouse gas emissions.

Impacts are already visible. Humid heat led to several deaths last year, and recent temperatures have broken records from Phoenix to Florida. But not everyone is affected equally. Disproportionate impacts fall on senior citizens and communities of color, as well as outdoor workers in agriculture and the military. Continued climate change would lead to declines in labor productivity, while worsening social and economic inequities.

And then there’s the pandemic. Many cooling centers remain closed, increasing the need for home air conditioning even as cooling costs rise. While many cities are shifting response plans and subsidizing AC units or utility bills, it will take efforts at all levels to keep people safe. Simply checking on neighbors could help save lives.

Read Climate Central’s partnership story on humid heat with WJCT News in Florida: Warming Brings Muggier Weather to Jacksonville, Threatening Most Vulnerable


Hourly wet bulb temperature data was collected by researchers from Raymond et al. (2020), using 474 contiguous U.S. weather stations for this analysis. For humid heat extremes in each location, the change in frequency was found by calculating the value that happened (on average) on 5% of days in the 1980s and 1990s, and then seeing how often that happened in the 2000s and 2010s.

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