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Nov. 5, 2021 — Increasingly extreme and more frequent heat waves are clear signals of the threat climate change poses to human health, but heat isn’t the only important factor. High humidity increases the dangers of extreme heat, and high-humidity days are on the rise, too.

New findings, published in Geophysical Research Letters, show that the planet has seen increases in both dry heat and humid heat extremes. The increases are similar across many regions, including Europe, northern South America, Africa, and most of North America. More densely populated areas are seeing the most growth in hot and humid days.

On average, each person worldwide has had 5 extra days of extreme humid heat per decade since 1979. If the calculation is made based on land area instead of per capita, the increase is less, at 3.5 days since 1979. Extreme dry heat, on the other hand, has occurred about 4 extra days per decade across the globe, regardless of population density.

The people hit hardest during those extra hot and humid days are often already sweltering more than the rest of the world. Extremes in dry heat increased mostly in subtropical and desert areas, such as the Middle East and Australia. Extreme humid heat occurred where temperatures and humidity were already at dangerous levels, including northern India, parts of Southeast Asia, and portions of Bolivia and Brazil that border the Amazon rainforest.

In these areas, many people rely on agriculture and other outdoor labor, such as construction, and on human-powered transportation, such as rickshaws. The increasing tempo of extreme heat and humidity events can ruin crops, cause spikes in heat-related illnesses, and prevent outdoor work, threatening productivity in regions where the economy is struggling.

Rainfall patterns have a likely role in these trends, but a human factor may be irrigation for farming. Although this research offers no solutions, it reveals the importance of identifying causes of these extremes and how they affect people living in hardest-hit areas.

The researchers write that those most under threat in these regions include outdoor laborers, unhoused people, older adults, and those living without air conditioning or warning systems for extreme heat.

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