More dangerous heat waves are on the way

It was the middle of July and already this summer had become a top contender for the hottest in Texas’s recorded history. In San Antonio, which by July would normally experience about three days of triple-digit heat, there had been three dozen. Houston, Waco and Austin were also seeing temperatures 5 to 8 degrees above normal. The state was roasting and Texans were using a record amount of electricity to stay cool.

New calculations suggest that, by the middle of this century, this record-breaking summer in Texas may look normal.

Across much of the United States, millions of people are expected to experience extreme temperatures more frequently and for longer periods of time – a threat that will grow as climate change worsens. The new data, released Monday by the nonprofit First Street Foundation, calculates the heat risk facing each property in the contiguous United States over the next 30 years, the length of a typical mortgage, providing some of the most detailed nationwide estimates. It uses heat index, a measure of how hot it feels outside by including temperature and humidity.

A Washington Post analysis of the group’s data found that today’s climate conditions have caused an estimated 46% of Americans to endure at least three consecutive days of 100-plus degree heat, on average, each year. Over the next 30 years, that will increase to 63% of the population.

Nowhere is the danger more widespread than in the South, where global warming is expected to deliver an average of 20 extra days of triple-digit heat per year. In some southern states, such as Texas and Florida, residents could see over 70 consecutive days with the heat index topping 100 degrees Fahrenheit.

“We’re talking about taking summer, which is already hot, and expanding it for months,” said Jaime González, director of the Houston Healthy Cities program for the Nature Conservancy in Texas. “That’s going to cause all sorts of disruptions to everyday life.”

This data comes as more Americans are moving to some of the hottest parts of the United States. For more than a decade, census data has shown Sun Belt states like Arizona, Texas and Florida drawing in new residents, while Northeastern and Midwestern states are not.

The larger pattern identified by First Street’s model suggests that people living in the South are likely to face some of the most dramatic changes over the next several decades. A previous analysis found that the southern half of the country also faces the greatest risk of wildfire.

First Street’s analysis of property level heat exposure is based on a combination of high-resolution measurements of surface temperature data, tree cover, impervious surfaces – like pavement and asphalt – and proximity to water. It incorporates global climate models from the United Nations Intergovernmental Panel on Climate Change and it relies on a moderate scenario in which global greenhouse gas emissions peak around 2040 and then slowly decline. If countries act more quickly, or fail to enact climate-friendly policies, the outcome could change.

The analysis found that Miami-Dade County in Florida will likely suffer the most extreme change. Whereas the county sees about 50 days of a heat index above 100 degrees, it is likely to have 91 broiling days by 2053.

“We know we have a heat problem here. This is right in line with what we expect,” said Jane Gilbert, Miami-Dade’s chief heat officer.

Unlike the West and the Midwest, which have been scorched this year by extreme heat waves, South Florida has chronic exposure to high heat for months, Gilbert said. This drives up energy costs and endangers outdoor workers, homeless people and those who can’t afford to air-condition their homes all day. The county has designated May 1 through Oct. 31 as an official heat season and launched an awareness campaign targeting neighborhoods with the highest rates of heat-related hospitalization.

Gilbert said having property-level heat projections could help county officials make the case for more tree-planting and painted rooftops that reduce the need for air conditioning.

“If it’s truly good modeling, it’s extremely valuable to help us develop policies to require cooling,” she said.

Florida tops the states that will see the largest increase in days with a heat index over 100 degrees. But residents all along the Gulf Coast and Southeastern Atlantic are also expected to live through more weeks of dangerous heat because of muggy summertime conditions, low elevation and the Gulf of Mexico’s warm waters.

Even more severe temperatures are expected to hit a swath of the country stretching from northern Texas and Louisiana to Illinois and Indiana. Though the central United States is not typically thought of as bearing the brunt of summertime heat, First Street’s analysis found that tens of millions more people living in this region are likely to see a heat index above 125 degrees by mid-century. The group calls this area an “extreme heat belt.”

Situated between the Appalachian and Rocky Mountains, this part of the country “almost forms a bowl that funnels humidity into the area, which drives up those ‘feels like’ temperatures,” said Jeremy Porter, chief research officer for First Street.

According to the National Weather Service, at a heat index of 125 degrees – which the agency classifies as an “extreme danger” day – heat stroke becomes “highly likely.” And although there is no set temperature threshold at which roads, bridges and trains start to fail, or water pipes break, recent examples show that it doesn’t take a 125-degree day to overtax essential infrastructure.

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Heat is the top weather-related killer in the United States. But like other effects of climate change, it is felt unevenly. The poor, the elderly, very young children and people with certain chronic medical conditions are most at risk.

Treeless city neighborhoods, packed with buildings, parking lots and asphalt roads, absorb and retain more heat than areas with tree-lined streets and parks. Scientists call this the urban heat island effect. Nationwide, this pattern reveals itself in city after city, concentrating heat in majority low-income Black and Latino neighborhoods that were designated as risky investments decades ago.

Today, about 64% of all Black people in the U.S. experience a dangerous heat wave, defined as more than three consecutive days of a heat index above 100 degrees. But that will increase to 79% in 30 years, making a population that is already more vulnerable to heat significantly more exposed.

The Post also found that by the middle of this century, 71% of the poorest neighborhoods in the country will likely endure severe heat.

Extreme heat will also make work more dangerous. Today, there are roughly 3.8 million people who work outdoors and experience at least one severe heat wave. In 30 years, that number will increase nearly 30% to 4.9 million.

In the Houston neighborhood of Gulfton, the shift to more days of dangerous temperatures and humidity would expose the area’s 45,000 residents – many of them recent immigrants from Afghanistan, Syria, and Central America – to unbearable levels of heat. The neighborhood has one park and few trees. Two years ago, when Houston officials worked with scientists and volunteers to map its heat island effect, they discovered parts of Gulfton were 17 degrees hotter in the afternoon than the coolest neighborhood they measured.

González said that until recently, conversations about climate change in Houston were dominated by talk of flooding and sea level rise. The destruction unleashed by Hurricane Harvey in 2017 forced the city to confront the likelihood of intensifying storms. Now, this summer’s record-setting heat is forcing another shift in focus, he said.

“We’re getting a little bit of a preview of what it might look like if we don’t take more action,” González said.

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About this story

The 2023 numbers in the analysis reflect the current climate. They are estimates based on 2014-2020 averages, and adjusted to include temperature and humidity projections from the United Nations Intergovernmental Panel on Climate Change’s global climate model ensemble. Population estimates are based on the 2020 5-year American Community Survey census tract data. Regions and divisions were identified by the Census Bureau. Outdoor worker estimates are based on County Business Patterns from 2020 and include the sectors of agriculture, construction, and landscaping services.