Brutally hot days when the temperature rises above 95°F are still a relative rarity in most of the United States. On average, Americans only experience about 13 such days a year (and most people get far fewer than that, especially in the Northeast).
But if greenhouse gases keep accumulating in the atmosphere and global temperatures keep rising, that's expected to change. A new report from the Risky Business Project shows where 95°F days are expected to become much more common under unchecked global warming:
Average days over 95°F: Projections over a lifetime
By mid-century, the report notes, Americans could see two to three times as many days hotter than 95°F — about 27 to 50 per year, on average. By the end of the century, that could rise to 45 to 96 days each year. And there's huge regional variation, with the South seeing a much bigger rise in extremely hot days.
"To put this in context," the report notes, "by the end of the century, Oregon, Washington, and Idaho could well have more days above 95°F each year than there are currently in Texas."
Note that this is based off a modeled forecast for the most extreme climate scenario — in which greenhouse gases keep rising unabated through the century. The actual number of days hotter than 95°F could be fewer if, say, greenhouse gases don't keep rising (or if global warming is lower than the "likely" range of projections).
But climate models do tend to agree that any significant amount of global warming will mean more extremely hot days. So it's worth taking a look at what that would mean:
What a rise in 95°F days would mean
First, there are huge regional variations here. The Southwest, Southeast, and Midwest get hit especially hard. Under high-end scenarios, the report notes, they could spend the equivalent of several months each year with temperatures higher than 95°F.
That could have major implications for industries that require workers to stay outside — like construction, landscaping, or agriculture. Some research suggests that productivity in these sectors could drop as much as 3 percent in the Southeast. Those industries will have to figure out some way to adapt.
If the number of unbearably hot days rises, the use of air conditioning is also expected to skyrocket. Grid operators will have to figure out how to deal with the sharp rise in electricity demand during the summer.
Then there's health to consider. In cities like Atlanta or New Orleans, hospitalizations and deaths already tend to rise when the temperatures climb past 95°F, especially among the elderly. That will be something to watch if these days become more common. The "Risky Business" report projects an additional 11,000 to 36,000 deaths per year in the Southeast from heat. (Though that's without adaptation — more on that crucial point below.)
Could it ever become too hot to go outside?
Perhaps most jarringly, the Risky Business report raises the prospect of occasional days toward century's end in which it'll be too dangerous for anyone to go outside for more than an hour or so.
Normally, the human body cools down during hot days by sweating, so as to keep skin temperature below 95°F and the body's core temperature from rising above 98.6°F. But high humidity can prevent that sweat from evaporating and cooling the skin, which can cause core temperature to rise until a person collapses of heat stroke.
The United States has never seen a day in which the combination of heat and humidity — dubbed the "Humid Heat Stroke Index" — was this severe. In fact, the only country that has ever seen heat like this is Saudi Arabia in 2003 (when the outside temperature reached 108°F and the dew point was 95°F).
But the report's modeling suggests that, with unchecked global warming, the eastern US could see an average of one such day a year by century's end, and more thereafter. "Less than an hour of activity outdoors in the shade [on these days] could cause a moderately fit individual to suffer heat stroke," said Rutgers climate scientist Robert Kopp, a lead scientific author of the report.
How can we adapt to a rise in 95°F days?
Unfortunately, the Risky Business report doesn't really dwell on possible adaptation strategies to a rise in extremely hot days. And that's a crucial angle here, since adaptation to severe heat will likely prove necessary even if we only get moderate global warming.
For a better discussion of this, it's worth turning to another recent report from the Intergovernmental Panel on Climate Change. The scientists who compiled that report looked at past deadly heat waves in places like Europe and tried to figure out what helped and what didn't.
For instance, many of the deaths during heat waves occur among the elderly with weak social networks or poor health care. (A 2003 heat wave in Europe was particularly deadly because it followed on the heels of a nasty flu outbreak.) Better public-health infrastructure and monitoring could help.
The IPCC also noted that the urban heat island effect tends to exacerbate heat waves. Because of all the buildings and cars and black pavement, cities tend to be even hotter than their surroundings. But there are ways to mitigate that. One study found, for instance, that introducing more green spaces into a city could reduce the need for medical assistance during scorching heat waves by 50 percent.
Adaptation is getting more and more attention lately as scientists realize that we've already locked in some global warming into the system — and it's unclear how much the world will reduce greenhouse gases in the coming century. No matter how much the number of 95°F days rises, most of these strategies will prove necessary.
- Here's the full Risky Business report, which looks at the economic risks of climate change in regions around the United States. The group was chaired by Michael Bloomberg, Tom Steyer, and Hank Paulson, although the modeling was done by the Rhodium Group along with climate scientists Robert Kopp of Rutgers and Solomon Hsiang of Berkeley.
- Here's the full IPCC report on adapting to the effects of climate change. Here's the relevant chapter on extreme heat.