By the middle of this century, summer heat in the Bay Area will feel similar to conditions in Los Angeles now. And Sacramento will feel more like Texas.
Those are among the findings of a new scientific report out Tuesday that concludes that at the current rate of greenhouse emissions worldwide, cities across the United States are in for significantly more days of extreme heat in the coming decades, raising the risk of everything from wildfires to heat stroke, with infants, the elderly and people who work outside particularly vulnerable.
"Extreme heat is dangerous and can even be deadly," said Kristina Dahl, lead author of the study.
"It's getting more frequent, and it's on track to get a whole lot more extreme in our lifetimes," said Dahl, a senior climate scientist at the Union of Concerned Scientists, a nonprofit group founded 50 years ago by scientists and students at the Massachusetts Institute of Technology. "We want people to see what's coming and have a chance to take action."
The study, published in the journal Environmental Research, looked at the "heat index," which the National Weather Service defines as a measure of what the temperature feels like to the human body when humidity is combined with the air temperature. It's similar to wind chill for cold weather, only for heat. A temperature of 90 degrees, for example, feels like 100 degrees when the humidity is 60 percent.
Using 18 computer models, the researchers found that the number of days nationwide on average per year when the heat index exceeds 100 degrees Fahrenheit will more than double from 14 between 1971 and 2000 to an average of 36 by mid-century (2036-2065) at the current rate of emissions.
Further, those 100-degree days are on pace to increase four-fold to 54 days a year by late this century (2070-2099). But that number can be limited to 31 days a year, on average, by century's end, if the world meets the targets of the 2015 Paris Climate Agreement. That agreement, signed by 197 countries, including the United States, calls for limiting warming to 2 degrees Celsius, or 3.6 degrees Fahrenheit, by the end of the century.
The world currently is on pace to increase temperatures by about 8 degrees Fahrenheit by the end of this century. President Trump has promised to pull the United States out of the Paris Agreement and has worked to increase the burning of coal, a leading cause of greenhouse gas emissions.
"This study is completely consistent with other studies I've seen," said Chris Field, a professor of biology at Stanford University who was not involved in the research. "I hope it connects with people."
Field, a climate expert who also is director of the Stanford Woods Institute for the Environment, echoed the report's findings that there is still time to reduce emissions through ramping up renewable energy, electric vehicles and other technologies, and for other states to copy many of California's climate and renewable energy policies, a trend already underway.
"The main theme from the study is that we should avoid warming on the high end and do everything possible to meet the targets of the Paris Climate Agreement," Field said.
From 1999 to 2010, extreme heat was blamed for 7,415 deaths in the United States -- roughly 600 a year, according to the U.S. Centers for Disease Control.
High temperatures can cause heat exhaustion, with dizziness, fainting and dehydration, and also heat stroke, a potentially fatal condition that can lead to seizures, heart attacks and coma, particularly in people older than 75, infants and pregnant women.
In addition to national projections, Tuesday's study also looked at 481 urban areas.
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The Southern states and the Midwest, where humidity is highest, are facing the worst risk. But California and other parts of the West also are in for more uncomfortable, even at times dangerous, summer weather.
For California, the study found:
— San Jose will go from having 7 days a year on average above a heat index of 90 degrees between 1971 and 2000 to 24 days a year by mid-century and 53 days by late century, at the current rate of emissions. That number could be cut to 20 days by the end of the century if the Paris Agreement climate targets are met.
— The San Francisco-Oakland area will go from 2 days a year on average above a heat index of 90 to 10 days by mid-century and 30 days by late century. That number would be cut to 8 days by the end of the century if the Paris climate targets are met.
— Los Angeles will go from 20 days on average with a heat index of 90 or higher to 56 days by mid-century and 96 days by the end of the century. But that could be reduced to 49 days by meeting the Paris targets.
vFresno will go from 77 days with a heat index of 90 or higher to 119 days by mid-century and 143 by the end of the century. Those could be limited to 113 by meeting the Paris targets.
— Sacramento will go from 47 days a year on average with a heat index of 90 or higher to 94 days by mid-century and 125 days by the end of the century. Those could be limited to 86 days a year on average if the Paris climate targets are met.
Regardless, the warming will affect the entire United States, the study found.
There currently are 29 cities that experience a summer heat index of 100 degrees or more for at least 30 days a year. But at the current rate of warming, that will jump to 251 cities by mid-century and will include Sacramento, Cincinnati, Omaha, Chicago, St. Louis, Washington, D.C., and even Fargo, North Dakota.
"We are looking at the types of conditions that we only see in places like Yuma, Arizona, becoming very widespread in places like Tampa, New Orleans, Houston, and southern Indiana," Dahl said.
The warmer weather also will mean more cooling centers are needed for people without air conditioning during heat waves, Field said. New building codes encouraging cooler roofs, reflective paint and other features will need to be passed. And in many cases, people who work outside, like farm workers and construction workers, will require better safety standards and the ability to work at night during the summer to avoid the hottest parts of the day, Field said.
What about people who say "I don't care? I'll be dead."
"The amount of warming we have seen to date has already led to a dramatic increase in heat waves, Western wildfires and coastal flooding," Field said. "It's really clear. The impacts are on us now. And the benefits of acting soon are on us now."