A New Measure for Weather: “Outdoor Days”

It’s just a little more than a month into spring, and I’m already wondering if the summer of 2024 will set a new heat record just as the summer of 2023 did. But, this time, more than setting records and reading dire statistics, I’m thinking about how those temperatures will truly feel to us. And I’m not alone, for climate change is now becoming personal.

To reflect that immediacy and intimacy, a new measure termed “outdoor days” is being used to describe climate change impacts by noting the number of days per year that outdoor temperatures are comfortable enough for you to engage in your normal outdoor activities.

We’re not the only ones feeling this difference. Climate change is now personal for wildlife, too. On the unprecedented hot days that will most likely soon be coming, the cool of a forest will be a welcome escape. This is especially true for mammals in North America’s hottest regions. That means that as the climate warms, preserving forest cover will be increasingly important for wildlife.

The rising heat index, or how hot it really feels

Texans have long endured scorching summer temperatures, so a global warming increase of about 3 degrees Fahrenheit might not sound like much to worry about. But a new study, conducted by researchers at the University of California, Berkeley, concludes that the heat index—essentially, how hot it really feels—has increased much faster in Texas than has the measured temperature: about three times faster. That means that on some extreme days, what the temperature feels like is between 8 and 11 degrees Fahrenheit hotter than it would without climate change.

Using Texas data from June, July and August of 2023, the study—published March 15, 2024, in the journal Environmental Research Letters—highlights a problem with communicating the dangers of rising temperatures to the public. The temperature alone does not accurately reflect the heat stress people feel. The heat index, defined in 1979, is based on the physiological stresses induced by heat and humidity, but the calculations of the heat index do not extend to the heat stress humans feel today during the extremes of heat and humidity accompanying climate change. Even the heat index itself, which considers the relative humidity and thus the capacity to cool off by sweating, gives a conservative estimate of heat stress, according to the study’s authors. This leads people to underestimate their chances of suffering hyperthermia and/or dying on the hottest days.

And Texas is not just an outlier. Recently, Arizona’s Maricopa County, which covers most of Phoenix and is the state’s most populous county, reported that heat-associated deaths last year were 50% higher than in 2022, rising from 425 in 2022 to 645 in 2023. Two-thirds of Maricopa County’s heat-related deaths in 2023 were of people 50 years or older, and 71% occurred on days when the National Weather Service had issued an excessive heat warning.

The reason that it feels much hotter than you’d expect from the increase in ambient temperature alone is that global warming is affecting the interplay between humidity and temperature. In the past, relative humidity typically dropped when the temperature increased, allowing the body to sweat more and thus feel more comfortable.

But with climate change, the relative humidity remains about constant as the temperature increases, which reduces the effectiveness of sweating to cool the body. So, the University of California, Berkeley, researchers extended the calculation of the heat index to all combinations of temperature and humidity, enabling its use in even the most extreme heat waves, like those that buffeted Texas in the summer of 2023. Using their revised heat index, the researchers found that the 3-degree-Fahrenheit rise in global temperatures has increased the state’s heat index as much as 11 degrees on the hottest days.

Over the decades, the nation’s major weather forecaster, the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service, has dealt with the lack of calculated values for high heat and humidity by extrapolating from the known values. The researchers found, however, that commonly used extrapolations fall far short when conditions of humidity and temperature are extreme.

After spending a sweltering summer in Texas last year, the lead scientist in the University of California, Berkeley, investigation decided to use the state as a case study to determine how global warming has affected the perceived heat stress represented by the corrected heat index. He found that while temperatures peaked at various places and times around the state last summer, one place, Houston’s Ellington Airport, stood out. On July 23, 2023, he calculated that the heat index was 167 degrees Fahrenheit. Global warming accounted for 12 degrees of that heat index. That insane heat is beyond the physiological capacity of a young, healthy person to maintain a standard core temperature. While hyperthermic, a person may still be able to survive—but not happily.

To deal with the temperature increases we’re already experiencing, say the researchers, we need to take precautions to avoid hyperthermia. For those in extreme heat situations who are unable to take advantage of air-conditioning, they suggest coating yourself in water by running a rag under the faucet, wetting your skin with it and getting in front of a fan. And, they say, drink plenty of water.

The scientists conclude that stopping the burning of fossil fuels is the priority. If we don’t, then it is conceivable that half of Earth’s population will be exposed to unavoidably hyperthermic conditions, even young, healthy adults. People who aren’t young and healthy or who are laboring outside in the sun will suffer even more, including potentially life-threatening levels of heat stress.

Outdoor days, or being comfortable outside

Scientists are hoping that another new measurement will help people understand how fast and high the heat is rising.

For most people, reading about the difference between a global average temperature rise of 1.5 degrees Celsius versus 2 degrees Celsius doesn’t conjure up a clear image of how their daily lives will be affected. So, researchers at the MIT Climate Grand Challenges project have come up with a different way of measuring and describing what global climate change patterns in specific regions around the world will mean for people’s daily activities and quality of life.

The new measure, called “outdoor days,” describes the number of days per year that outdoor temperatures are neither too hot nor too cold for you to go about your normal outdoor endeavors, whether for leisure or work, in reasonable comfort. Describing the impact of rising temperatures in those terms reveals some significant global disparities, say researchers, who revealed their findings in a research paper published in the Journal of Climate in March 2024.

One of the MIT researchers says that he got the idea for this new system during his hour-long, daily walks in the Boston, Massachusetts, area. He found that recently there have been more winter days when he could walk comfortably than in past years. Originally from Sudan, he says that when he returned there for visits, the opposite was the case: in winter, the weather tends to be relatively comfortable, but the number of these mild winter days has been declining. There are fewer days that are suitable for outdoor activity.

Rather than predefine what constitutes an acceptable outdoor day, the MIT scientists created a free website where users can set their own definition of the highest and lowest temperatures that they consider comfortable for their outside activities. They can then click on a country within a world map, or a state within the U.S., and get a forecast of how the number of days meeting those criteria will change between now and the end of this century. Hopefully, this will facilitate a deeper understanding of how climate change will impact individuals directly.

While working on the website, the scientists made several significant discoveries. First, there will be winners and losers; and the losers will tend to be concentrated in the global South, in places such as Bangladesh or Sudan, where there will be significantly fewer outdoor days. In the North—such as in Canada or Russia—people will gain many outdoor days.

While the North-South disparity in climate change exposure and vulnerability has been broadly recognized in the past, this way of quantifying the effects of changes in weather patterns helps to bring home how strong the uneven risks from climate change will be on our quality of life.

The same kind of disparity even shows up in Europe, and it’s apparent in travel patterns: the researchers note a shift where people are spending more time in northern European states. For example, Sweden is gaining visitors, while Mediterranean nations are losing them.

Forest-seeking, or finding refuge from the heat

What will these upcoming hot days mean for the nonhuman animals that share our world? According to a new study conducted at the University of California, Davis, that was published in the journal Proceedings of the National Academy of Sciences of the United States of America, as the climate warms, preserving forest cover will be increasingly important for wildlife conservation.

The study’s findings showed that North American mammals—from bears, pumas and wolves to deer, opossums and rabbits—consistently depend on forests and avoid cities, farms and other human-dominated areas in hotter climes. In fact, on average, mammals are 50% more likely to occupy forests than open habitats in hot regions. But the opposite is true in the coldest regions.

In other words, different populations of the same species respond differently to habitats based on where they are, and climate is mediating that difference. For example, the eastern cottontail, a common rabbit, prefers forests in hotter areas while favoring human-dominated habitats, such as agricultural areas, in colder regions.

This illustrates what’s called intraspecific variation, which the study showed to be pervasive across all North America’s mammals. This runs contrary to a long-standing practice in conservation biology of categorizing species as those that live well alongside people and those that don’t. The authors say that there is a growing recognition of ecological flexibility, and that species are more complicated than those two categories suggest.

That’s why we can’t take a one-size-fits-all approach to habitat conservation. Climate plays a large role in how species respond to habitat loss. For example, if elk are managed under the assumption that they can only live in protected areas, then conservation managers may miss opportunities to conserve them in human-dominated landscapes. On the other hand, if we assume a species will always be able to live alongside us, then we might be wasting our efforts trying to improve the conservation value of human-dominated landscapes in areas where it is simply too hot for that species.

For the study, the authors leveraged Snapshot USA, a collaborative monitoring program with thousands of camera-trap locations across the country. They analyzed 150,000 records of 29 mammal species using community occupancy models, which allowed them to study how wildlife responds to habitat types across their ranges while accounting for the fact that species may be in an area, but their presence is not recorded because they are elusive or rare.

The study provides a pathway for managers to tailor conservation efforts in protected areas, as well as enhance working landscapes, such as developed areas, farms and pastures. For instance, if trying to conserve species in working landscapes, providing more shade with scattered trees and maintaining patches of hedgerows would create local refuges for wildlife and establish a protective buffer against high temperatures, especially in places that are going to get warmer with climate change.

Future forecasting, or thinking of the “others”

In the near future, statistics alone may not be the best way to communicate the dangers of climate change. But emphasizing how we’re going to feel just might be.

The new outdoor days website should help with that. Instead of looking at global averages, by having detailed and localized information at your fingertips, you’ll say—according to your own definition of what a pleasant day is—how climate change is going to impact you and your activities. And that, I think, can only help us make quicker decisions regarding this global challenge.

And, just as we measure our lives by how we feel, we’ll be able to consider how various individuals of the nonhuman “others” who live among us feel, too.

It’s about time.

Here’s to finding your true places and natural habitats,

Candy