This post is a part of a series on Ask a Scientist
I’m sure you’ve heard that old adage, “It’s not the heat, it’s the humidity.” Living in Washington, DC, for the last three decades, I certainly know what it means. That said, it would be more accurate to say, “It’s not only the heat, it’s also the humidity.”
Indeed, humidity is critical when estimating the impact higher temperatures triggered by global warming will have. Scientific projections of future heat stress that rely on temperature increases alone underestimate the problem. After all, a 100°F summer day in the nation’s capital with 70 percent humidity feels a lot different than a 100°F summer day with 25 percent humidity in Phoenix. When there is a lot of water vapor in the air, it feels hotter.
With that phenomenon in mind, the Union of Concerned Scientists (UCS) published the report Killer Heat in the United States, along with a companion peer-reviewed study in the journal Environmental Research Communications, in July. Both the report and study use heat index—the measure of how hot it feels when air temperatures are combined with the amount of moisture in the air—to project the impact of rapid increases in extreme heat nationwide if industrialized nations don’t act quickly to reduce heat-trapping emissions.
It turned out that the report and study’s release date—July 16—was smack dab in the middle of an extended heat wave that settled over the central and eastern portions of the United States. The entire month of July in Washington was brutal. We suffered through 22 days with a high temperature of at least 90°F and humidity averaging 65 percent.
As if on cue, we recently received a question about water vapor and global warming from Henry J., a UCS member in Lexington, Kentucky, who must be familiar with high humidity. Lexington experienced temperatures in the high 80s with humidity hovering between 50 and 65 percent the first week of August.
It was a made-to-order question for the lead author of the Killer Heat report, Kristina Dahl, a senior climate scientist in our Climate and Energy Program. Kristy, who has a Ph.D. in paleoclimate from a joint Massachusetts Institute of Technology-Woods Hole Oceanographic Institution program, was the associate director of a Rutgers University campus-wide climate change initiative and a long time consultant to UCS before joining UCS full time in 2018.
Q: As global warming increases, is there an upper limit as to how much additional water vapor the atmosphere can hold? – Henry J., Lexington, Kentucky
K.D.: The short answer is no, there is no upper limit that we know of. But the long answer is that it’s a bit more complicated.
Experiments have shown that air can continue to accumulate water vapor at temperatures well above what we expect with global warming. As the atmosphere warms, a few things happen: Evaporation from bodies of water increases, which adds water vapor to the atmosphere, and water molecules in the air become energized and start moving faster. Energized water molecules are less likely to condense, so it is often said that warm air can “hold” more water vapor.
Condensation tends to happen around particles—such as dust and salt—in the atmosphere. There are enough particles that humidity typically doesn’t exceed 100 percent, though it theoretically could in completely pure air, before water molecules start to condense. That said, the amount of water vapor it takes to reach 100 percent humidity depends on the air temperature.
E.N.: What difference does humidity make?
K.D.: When your body’s cooling system is trying to cope with a hot day, humidity makes all the difference. People sweat to release heat, because when sweat evaporates, it has a cooling effect. A breeze or a fan can help us to cool down by quickening the pace of that evaporation. But humidity in the air around us limits sweat evaporation and reduces the associated cooling effect. So high temperature and humidity cause our bodies to accumulate heat. For this reason, weather forecasters often talk about the heat index—or the “feels like” temperature—which is a combined measure of temperature and a measure of humidity or “relative humidity.”
In Arkansas, Georgia and other humid states, the heat index may be much higher than the air temperature, while in Arizona, New Mexico and other arid states, the temperature and heat index may be the same.
As the heat index rises, more people become susceptible to heat-related illnesses, including heat stress and heat stroke. Recognizing the importance of the heat index to our bodies’ experience with heat, the National Weather Service uses heat index-based thresholds as the basis for issuing heat advisories and excessive heat warnings. For example, with a heat index of 105°F, the weather service typically issues an excessive heat warning that includes such language as “heat illness is likely” or “the very young, the elderly, those without air conditioning, and those participating in strenuous outdoor activities will be the most susceptible.”
E.N.: How will the heat index change with global warming?
The importance of humidity prompted my colleagues and me to focus on projecting future extreme heat using the heat index rather than temperature alone in our recent Killer Heat in the United States report. Without that humidity component, most previous studies were only capturing part of the story about how rising temperatures will threaten our health.
One thing that’s really interesting about the heat index, though, is that it was intended to encompass the range of conditions we’ve experienced historically. As global warming progresses, we’ll increasingly find ourselves in the black “off-the-charts” zone in the chart above. That zone represents conditions that are so hot that they exceed the National Weather Service’s heat index range and formulas. Such conditions have, thankfully, been exceedingly rare in the United States. They’ve been limited to just a couple of days per year in Arizona and the Sonoran Desert in Southern California.
Our Killer Heat analysis found that if industrialized nations worldwide fail to reduce global heat-trapping emissions, the United States would see a staggering—and frankly terrifying—increase in the frequency and severity of days with extreme heat in the coming decades. For example, we found that by midcentury, the average number of days per year with a heat index above 105°F would quadruple across the county. And while just three U.S. cities that have historically experienced an average of 30 or more days per year with a heat index above 105°F, by midcentury more than 150 cities across the country would experience that frequency and level of extreme heat.
The number of people in the country exposed to off-the-charts conditions also would jump dramatically if global emissions do not fall precipitously. Between now and midcentury, the number of people exposed to a week or more of off-the-charts conditions in an average year would rise from fewer than 2,000 to roughly 6 million. And by late century, more than a third of the U.S. population—120 million people—would be frequently exposed to these dangerous conditions.
The good news is that this unrecognizably hot future is far from a done deal. Aggressive emissions reductions that limit future global warming to at most 3.6°F—or 2°C—would contain the expansion and spare millions in the United States from suffering relentless summer heat.
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