The last five years (2014 through 2018) have been the hottest since NOAA began keeping records nearly 140 years ago. Across the world, this is increasingly putting people at risk for heat-related illness and death. As climate change warms the planet, temperature is not the only factor that is increasing heat-related health risks. In addition to temperature, relative humidity–that is, how much water vapor is in the air relative to the temperature of the air–is a critical component of heat-related health risks because humidity influences how we experience high temperatures and how our bodies cool off. That is why the National Weather Service (NWS) uses a metric called the heat index to communicate to the public the “feels-like” temperature.
The heat index is critical to communicate heat risks to the population. Many projections of extreme heat typically look at temperature, providing only a partial view of the story that involves people and the places we live in, and the experience of heat. I lived in Phoenix, Arizona for a long time, and this makes me recall that a temperature of 90°F with relative humidity at a low 12 percent would feel like 86°F. Compare that to a similar temperature in Silver Spring, Maryland, where I live now—but with a much higher relative humidity of 60 percent—and this combination feels like 100°F!
In part because of this gap in heat index-based studies, neither the public nor policymakers have the practical information they need in order to make decisions about how to best protect themselves or the communities they serve. Climate change is increasing temperatures and altering relative humidity patterns, which will have implications for how people experience high temperatures. So, at UCS, we took a look at projections of future climate conditions in the contiguous United States, and used projections of temperature and relative humidity to construct heat indices for the midcentury and late century under no action, slow action and rapid action scenarios of warming emissions reduction.
The results are profoundly concerning. We found that if we don’t take action now to reduce heat-trapping carbon emissions, we will experience many more days of killer heat in many parts of the country, putting millions of people at risk of heat-related illness or death. We estimate that without action, by midcentury there could be more than four times the amount of days with a heat index over 105°F across the country.
What does a “no action” future look like?
We looked at what the choices of no action, slow action, or rapid action to reduce emissions would mean for large urban areas (defined as population centers with 50,000 or more persons). With no action, 292 of these urban areas would see an average of 30 or more days per year with heat index values above 105°F by late century. The good news is that we can limit such conditions to 85 cities if we take rapid action. In cities with large heat-vulnerable populations, the implications for these communities unequivocally drive home the need for rapid action to reduce carbon emissions.
Historically, large metropolitan areas have housed many communities of color, which are also often low-income communities (though it’s also true that recent gentrification patterns have pushed many of these communities to suburban areas; however here I focus on urban populations). People in communities with low socio-economic status often do not have the means to seek preventative health care for pre-existing conditions that can be aggravated by killer heat, may not live in energy-efficient housing, or often can’t afford the purchase or operation of an air conditioner unit. In addition, many residents of low-income communities often rent, which prevents them from making modifications to increase energy efficiency or plant shading vegetation around their dwellings to protect from killer heat. Many of the most vulnerable are also the very young or the very old, homeless individuals, persons with mental or physical disabilities, or individuals who are socially isolated. These factors increase urban dwellers’ sensitivity to adverse heat-health outcomes. In addition, the urban environment in which many low-income people can afford to live can further increase their exposure to extreme heat because many poor neighborhoods are deprived of “park cool islands” with shading vegetation that can help mitigate the urban heat island, which typically makes poorer parts of cities hotter. In other instances, abutting hazardous land uses that also contribute to poorer air quality or environmental quality in general such as highways, can increase urban heat island intensity and hence extreme heat.
The legacy of long-standing racial segregation, racially restrictive covenants, disinvestment, and residential redlining—historically directed against communities of color—can be plainly seen in environmental inequities across large American cities, and disparities in high temperatures and adverse heat-health outcomes show that. In study after study, the most vulnerable populations live in the hottest neighborhoods and disproportionately succumb to heat-related illness or death. This has been documented in urban areas with large vulnerable populations such as Phoenix, Chicago, New York City, Houston, Milwaukee, and Atlanta. Additionally, many rural populations are also at high heat-related health risks. Disparities in access and proximity to health care facilities and cooling shelters are in part responsible for some of the highest heat-related mortality and morbidity rates in rural areas of the United States, as is living in a mobile home. Farmworkers and others who work outdoors in particular are exposed to outdoor heat. In addition, a large share of farmworkers in the United States are undocumented migrants with little access to health care or worker protections, and many of them are children. Many workers in these situations may not be granted the heat stress breaks required by the Occupational Safety and Health Administration (OSHA) and this disincentivizes workers to speak up and request cooling measures when they may be in a serious heat stroke situation.
In our study, we found that many large metropolitan areas where climate-vulnerable populations live will face many more days of killer heat if we don’t take action. Miami and Houston, for example, could see almost four months worth (nearly 120 days!) of heat index values above 105°F. Humid, East Coast cities like New York City, Washington, DC, and Baltimore could get the equivalent of nearly 1 to 1.5 months’ worth of days above 105°F. You can see in the graph below that historically, the number of days above 105°F in these cities has been small, and that if we take rapid action to reduce carbon emissions, we can avoid large increases in the number of days with such killer heat conditions. While feels-like temperatures above 90°F and 100°F elevate health risks mostly for outdoor workers and the very young or very old, respectively, exposure at or beyond 105°F threatens everyone who endures such temperatures.
But our future does not have to be this dire. We have solutions at our disposal to avoid the worst impacts of climate change if we act decisively now. In the global arena, the United States needs to remain in the Paris Agreement, signal to the world that it is taking its climate change obligations seriously and reduce the country’s significant carbon emissions. The Environmental Protection Agency should implement the Clean Power Plan—not dismantle it, as it is on course to do—in order to make a dent in the power sector, which accounts for the largest share of the U.S.’s carbon footprint.
Climate change is amplifying heat-health risks to urban and rural dwellers—especially among the most vulnerable. Making drastic cuts in emissions that trap heat by transitioning to a low-carbon economy and energy system, increasing energy efficiency, putting a price on carbon emissions, and making investments in land use and forest management practices to increase natural carbon storage are some of the ways in which we can increase our chances of keeping global average warming below 3.6°F (2°C). Staying in the Paris Agreement is critical if the United States is to uphold its obligation to cut down emissions and signal to the global community that it is taking the climate crisis seriously.
Check out our report, Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days and learn how your area could fare under various emissions choices that we can make.