Because you’re a smooth-skinned mammal, no weather feels quite as oppressive as a humid heat wave. The more water vapor in the air, the less efficiently your sweat can evaporate and carry excess heat away from your skin. That’s why 90° Fahrenheit in humid Miami can feel as bad as 110° in arid Phoenix.
Climate change has supercharged this summer’s exceptionally brutal heat all around the world—heat waves are generally getting more frequent, more intense, and longer. But they are also getting more humid in some regions, which helps extend high temperatures through daytime peaks and into the night. Such relentless, sticky heat is not just uncomfortable, but sometimes deadly, especially for folks with health conditions like cardiovascular disease. 
One of the more counterintuitive effects of climate change is that a warmer atmosphere can hold more water vapor than a colder one. A lot of it, in fact: Each 1.8° Fahrenheit bump of warming adds 7 percent more moisture to the air. Overall, atmospheric water vapor is increasing by 1 to 2 percent per decade. That additional wetness is why we’re already seeing supersize downpours, like the flooding that devastated Vermont earlier this month.
Water vapor is actually a greenhouse gas, like carbon dioxide or methane, responsible for about half of the planet-warming effect. (It's supposed to be up there, whereas humans have been pumping in way too much extra carbon.) More warming evaporates more water, which causes more warming—a climatic feedback loop.
In landlocked areas, heat waves evaporate water from plants and soils. But humidity gets especially oppressive near the ocean, where water is more readily available. “Coastal regions in general are seeing more humid conditions as ocean temperatures warm,” says Alexander Gershunov, a research meteorologist at the Scripps Institution of Oceanography, who studies humidity and heat waves. “Air sitting over a water body tends to be close to saturated. It has a lot of moisture in it—close to 100 percent relative humidity.”
Sea surface temperatures have been steadily climbing globally, as the oceans absorb something like 90 percent of the excess heat that humans are adding to the atmosphere. But since March, global sea surface temperatures have been skyrocketing above the norm. The North Atlantic, in particular, remains super hot, loading Europe’s air with extra humidity.
The waters around Florida are also logging truly astonishing sea surface temperatures: On July 24, a buoy recorded a temperature of 101° Fahrenheit. “You have incredibly warm Gulf water that warms the atmosphere, which can then absorb more moisture. So it's kind of a feedback loop,” says Kent State University biometeorologist Scott Sheridan. “In a lot of the areas around the Mediterranean, where there's been really bad heat, and then in Florida and the Gulf Coast, those have been the really big driving factors for why the humidity is so high.”
Accordingly, in Miami the heat index—a measurement that combines temperature and relative humidity—has been above 100 for over 40 days in a row, smashing the previous record of 32 days in 2020.
Meanwhile in California, Gershunov’s research has confirmed that heat waves are getting stickier. “It's not just more frequent, more intense, and longer-lasting heat waves, like is the case all over the world with the warming climate,” says Gershunov. “Here, the heat waves are also changing flavor. They're becoming more expressed disproportionately in nighttime temperatures. It turns out it's because of humidity, and that's related to the warming of the ocean.”
If you’re in a desert and suffering days of 110-plus-degree heat, you can at least look forward to those temperatures coming down at night, as the landscape sheds built-up heat. But when it’s humid, the atmosphere stubbornly holds onto that heat. “With more and more humidity, more people will be impacted during the night. And I don’t think we’re ready at all for that,” says Tarik Benmarhnia, an environmental epidemiologist at the UC San Diego. “There's basically no break, no pause in the stress that heat is going to cause to humans.”
The more humid it gets, the harder it is for water to evaporate off the body and the less effective sweating becomes. “If that’s not effective, the only way is to have more and more exchange between the blood and the skin,” says Benmarhnia. “To do that, our body sends more blood, faster and faster.”
That’s why skin flushes if it’s hot out—the body is trying to expel heat via the water in the blood. That means blood is diverted from vital organs to the skin, a sort of physiological panic that’s especially dangerous for people with cardiovascular disease. “But if it's not effective, we just waste a lot of energy, and our circulation system is going to be overwhelmed and lead to very severe complications,” says Benmarhnia. “This is the main cause of hospital admission and emergency department visits during a heat wave.” High heat is correlated with risk of heart attacks and strokes; indeed, heat kills more Americans each year than any other kind of disaster.
It can also potentially cause issues for babies developing in the womb. “For people who are pregnant, blood flow is also diverted from the placenta when the core body temperature increases,” says Rupa Basu, chief of the air and climate epidemiology section at the California EPA’s Office of Environmental Health Hazard Assessment. “That also could provide less nutrients to the fetus, and sometimes, in more extreme cases, could cause preterm delivery.”
Getting more people access to air conditioning will go a long way in preventing heat-related deaths, since AC both reduces indoor temperatures and humidity. “Cooling centers” are a key tool—facilities where people who don’t have AC, or the unhoused population, can take refuge. But because high humidity extends scorching temperatures through the night, people often need that respite through the evening, when cooling centers are closed.
City planners are increasingly turning to green spaces to lower temperatures in the first place. Vegetation “sweats,” which significantly cools the landscape. (Thanks to their lack of greenery, plus all that concrete and brick, urban areas can get way hotter than rural ones.)
Adding vegetation can be helpful, says Edith de Guzman, an environmental researcher at UCLA—but it depends on how you deploy it. “In an arid environment, that's a very good thing, because you create basically an evaporative cooler,” says de Guzman, who is also the director and co-founder of the Los Angeles Urban Cooling Collaborative, a partnership of researchers who work with communities on cooling strategies. “But in a more humid environment or during a more humid heat wave, it's not necessarily good. You have a bit of a penalty for that.”
Basically, sweating greenery adds more humidity to already humid air. And there are trade-offs based on the kind of plants you pick. Big trees have the additional benefit of providing a lot of shade, which makes people feel much cooler, regardless of the added humidity. Vast expanses of lawn are stupid for a number of reasons—they waste water and are awful for biodiversity—plus they provide extra humidity but not a bit of shade.
As the world continues to rapidly warm, humidity will grow worse. But with the right infrastructure and social policies, people won’t have to suffer for it. “Any heat-related death is preventable,” says Benmarhnia. “There is no exception.”
This story originally appeared on wired.com.