The Union of Concerned Scientists recently released Killer Heat, a report analyzing how the frequency of days with a dangerously hot heat index will change in response to the carbon emissions choices we make in the coming decades. The heat index is the combination of temperature and humidity that the National Weather Service calls the "feels like" temperature.
By late in this century today’s elementary school kids may be retiring into a much hotter world. If the rise in the world’s carbon emissions is allowed to outpace our emissions-cutting efforts, for a week or longer each year about 120 million people across the country will be exposed to conditions so hot that the "feels like" temperature will surpass the limits of the Weather Service's heat index charts. The upper limit of the heat index scale is about 127°F, depending on temperature and humidity. In other words, when those people step outside they will be hit with a wall of heat that feels upwards of 130°F. How hot, we don't know.
In nearly all states, these "off-the-charts" conditions will occur in at least one county at least once a year. When this happens, the National Weather Service will not be able to reliably calculate the heat index. Historically, such "off-the-charts" conditions have only occurred in the Sonoran Desert, along the California-Arizona border, and only for a few days each year.
By late this century the number of days per year that feel like 105°F or hotter will be eight times higher than it has been historically. Every major city — more than 290 in the US — will swelter for the equivalent of a month under such conditions in an average year. Every major city in Michigan, Wisconsin and Minnesota will experience 30 or more days per year with a heat index above 100°F. City residents in Florida and Texas will endure 150 days or more per year with a heat index above 100°F. Notably cooler places like Seattle and the two Portlands will average 30 days or more with a heat index above 90°F.
In an average year more than 80 counties across Texas and Florida will experience a heat index above 90°F for half the year (180 days). That means that spring and fall will be very hot, and summer downright deadly.
This is an unfathomably hot future, one we're on track to hand off to our children and grandchildren. Where can we encourage them to settle in such a world? Will it be safe for their children to play outside in the summer? What do we tell Greta Thunberg’s Generation Z today when they strike and march and plead with us to change course?
In an alternative future, rapid, aggressive action to reduce global emissions would limit global warming to 2°C above pre-industrial temperatures. These ambitious actions would still lead to a future in which the number of days with a dangerously high heat index above 105°F would be nearly four times what it is today, and 85 of the bigger U.S. cities would still endure a month or more of such conditions. Clearly, we need to aim for even greater, even faster cuts to our emissions. Rapidly and aggressively reducing emissions would limit the number of cities with 30 days or more per year with a heat index above 105°F.
Ensuring even this marginally safer future will require us to act on two major fronts: (1) reduce emissions swiftly so that we can achieve net-zero carbon emissions by midcentury and limit future global warming; and (2) build our resilience to extreme heat through such common sense measures as enacting a national heat safety standard for outdoor workers and ensuring that communities have heat warning systems in place that draw on the best available public health information.
This future may be the next-best thing to the one we would ideally hand off to our children, but it is a lot better than the one we are recklessly barreling toward. Can we do even better? Definitely. But only if we act boldly to replace the current economic system with one that serves everyone’s human needs without breaching the ecological limits of this finite planet. The current system and our leaders are failing us on both counts.
The best time to boldly begin pursuing this safer future occurred 30 to 40 years ago. The next-best time is now.