Extremely Warm Days to Continue Under Climate Change

You’ve probably had this experience. It’s an excruciatingly warm summer day. You check your phone. Your phone says it’s 95 degrees Fahrenheit, but it feels like it’s over 100 °F. The difference between the temperature displayed on your phone and the temperature you perceive in your body isn’t just your imagination.

When it’s especially hot and especially humid, the human body has trouble cooling itself by sweating. We experience this as a feels like temperature that surpasses the air temperature, a kind of warning, if you will, that we might have trouble keeping cool.

Yet for as important as the combined effects of humidity and heat are, most studies use temperature alone—minus humidity—to calculate the likely ill effects of climate change’s rising temperatures on human health. Bucking this trend is a recent study published in the journal Environmental Research Communications.

The study—undertaken by the Union of Concerned Scientists and including CIRC’s John Abatzoglou—calculates just how many days each year the United States is likely to experience under future climate change that will be too hot and humid to just sweat it out. To figure this out, the study employs the heat index, a measure combining temperature and relative humidity to create that feels like temperature you’re probably familiar with.

So, how many too-hot-and-humid days are we likely to see under future climate change? The short answer: the numbers—like the temperatures in question—are high. The long answer is the subject of this post.

For their study, the authors primarily looked at two heat index ranges: days when the heat index exceeds 100 ° F, let’s call these +100 °F days; and days when the heat index exceeds 105 °F, let’s call these +105 °F days. These ranges roughly correspond to the National Weather Service’s Heat Advisory and Excessive Heat Warning system. Expect more of both in the future.

By the middle of this century, the contiguous US (i.e. from sea to shining sea, minus Alaska and Hawaii) is expected to see a doubling in the number of +100 °F days and a tripling in the number of +105 °F days.

To get their projected heat index numbers, the researchers used 18 global climate models—numerical models of the climate run inside a computer—in combination with two different emissions scenarios: the high emissions scenario, RCP 8.5, which simulates a world in which we humans do nothing (or close to nothing) cut greenhouse gas emissions and hence the warming principally caused by the release of those emissions continues on its upward trajectory throughout the century (and beyond); and the lower emissions scenario, RCP 4.5, which simulates a climate were humans have agreed to collectively lower our greenhouse gas emissions and, as a consequence, warning continues but at a more subdued pace for much of the 21 century, starting to level off around 2100.

The two emissions scenarios are frequently used back to back. Taken together, the two scenarios act as kind of big thought experiment that helps frame what are arguably the biggest questions facing humanity: What does our future look like if we address climate change?; and What does our future look like if we don’t address climate change?. Up until the middle of this century, the answer to both of these questions is not much really. Then, things change.

So, for instance, the growth in the number of heat index days described above—double for +100 ° F days and triple for +105 °F days—are the combined results of both the lower scenario (RCP 4.5) and the high scenario (RCP 8.5). However, once our climate change calendar moves out of the middle decades of this century, then the effects of either cutting or not cutting emissions comes into play, and the two scenarios branch off into two very different possible futures: hot and very hot.

Under the high emissions scenarios (RCP 8.5) by the end of the century (the 2070s, 2080s, and 2090s), the study’s authors estimate, the contiguous US is likely to experience 54 +100 °F days a year. By comparison, the lower emissions scenario (RCP 4.5) for the same decades results in a mere 34.2 +100 °F days a year. To put it another way, the high emissions sees a quadrupling in the number of +100 °F days by the end of the century, while the lower emissions scenario see a doubling.

But that’s just the US as a whole. Rising temperatures—unlike cake at a socialist’s birthday party—will not be distributed evenly. Instead, projected future temperatures will hit certain US regions harder than others. Consider just two regions: the Northwest and Southeast.

The US Northwest—our Northwest-regional readers will happy to note—does okay compared other US regions. By the end of the century under the high emissions scenario, the Northwest as a whole is expected to see 10.6 +100 °F days and 4.8 +105 °F days a year. Contrast this with the Southeast US, which the study’s authors estimate will see 95.8 +100 ° F days and 72.9 +105 ° F days by the end of the century under the high emissions scenario.

Things are notably better for both regions by the end of the century under the lower emissions scenarios. The Northwest sees just 4 +100 ° F days (1.9 +105 ° F days). While the Southeast sees just 59.5 +100 ° F (34.3 +105 ° F days). A similar story can be teased out for every other US region reviewed in the study. Every region sees fewer really hot days under the lower emissions scenario. But don’t take our word for it…

Readers should check out page seven of the study. There you will find a table displaying US regions and their projected heat index numbers under the different emissions scenarios. In addition to this, the Union of Concerned Scientists has put together a really cool…um, I mean hot…online tool that puts this study into a searchable form that allows users to track projected heat index numbers at the level of individual US counties. And CIRC researchers John Abatzoglou and Katherine Hegewisch have built a similar capacity into CIRC’s Northwest Climate Toolbox. The feature, now part of the Climate Mapper tool, displays for the contiguous US the number of days when the heat index is + 90 ° F, +100 ° F, and +105 ° F. Both online tools allow users to make the comparison between cutting emissions and not cutting emissions.

If all this weren’t sobering enough, consider the study’s final piece: the number of people expected to be affected by these rising temperatures. To calculate that number, the researchers used the measure persons-days.

A person-day, as used in this study, combines the number of heat index days in a location and the population density of that location using three different population growth scenarios for the contiguous US (basically slow growth, medium growth, and high growth). Things can get a little confusing here because, as the number of heat index days goes up, the number of people exposed not only goes up, it can actually exceed the number of people in a given region. The reason: the same populations are often exposed to multiple days of extreme heat. All this makes for some really big numbers.

Under the high emissions scenario, the study’s authors calculate, exposure to +100 ° F days—which historically reached 107 million person-days per year during the late 20th century—is likely to exceed 2 billion person-days per year by the end of this century.

So, what is the role of projected population growth in increasing exposure to extreme heat events?

To be clear, as the authors point out, the increase in the US population does increase the number of people exposed to those too-hot-to-sweat-it-out days. However, before you assume the wrong conclusion, there is a caveat.

If the US population as it stands now were to remain steady, according to the authors, the US would still see a three- to four-fold increase in the number of people exposed to +100 ° F days and seven- to 10-fold increase in exposure +105 ° F days, depending on the emissions scenario. This means population growth is not—we repeat, is not—the key driver of human exposure to the effects of climate change, climate change is. And if climate change is the key driver of human exposure to the effects of climate change, that should make you sweat.

Publication: Dahl, Kristina, Rachel Licker, John T. Abatzoglou, and Juan Declet-Barreto. “Increased frequency of and populations exposure to extreme heat index days in the United States during the 21st century.” Environmental Research Communications. (2019). https://doi.org/10.1088/2515-7620/ab27cf.

Resources:

Killer Heat in the United States: The Future of Dangerously Hot Days (by the Union of Concerned Scientists)
Climate Projections Mapper Tool (by CIRC, The Northwest Climate Toolbox)

Feature Image: “Thermometer 115” (Photo Credit: Conservation Law Foundation, Some Rights Reserved.)

Nathan Gilles is the managing editor of The Climate Circulator, and oversees CIRC’s social media accounts and website. When he’s not writing for CIRC, Nathan works as a freelance science writer. Other Posts by this Author.