If the number of extremely hot, humid days already seems like too many, a national study finds it’s likely to get worse.

The average number of days in Houston with a heat index above 105 degrees will jump from 10 to 74 by 2065 if there’s no action on climate change — making Space City among the hottest in Texas, according to a new report by the Union of Concerned Scientists, a nonprofit science advocacy organization.

The group released early Tuesday its report, “Killer Heat in the United States: The Future of Dangerously Hot Days.”

“Extreme heat is already extremely dangerous in the U.S. and it can even be deadly,” said Kristy Dahl, senior climate scientist with the group. “We also know that extreme heat is getting worse and it’s on track to get a whole lot worse in our lifetimes, so we wanted people to see this coming and to have a chance to take action to steer our future in a different direction.”

About 600 people already die of heat-related causes in the U.S. each year, according to the Centers for Disease Control and Prevention, Dahl said.

Nationwide, the average number of days per year with a heat index above 105 degrees would more than quadruple from an historical baseline of five up to 24 if no action is taken to reduce heat-trapping emissions by mid-century, which the report describes as 2036 to 2065.

“If we take no action and global heat-trapping emissions continue to rise unabated, as they have in recent decades, our findings indicate that across broad swaths of the U.S., extreme heat conditions once measured in days per year would need to be measured in weeks or months by mid-century and by late-century,” the authors wrote. “Few refuges from extreme heat will remain.”

The report, with an interactive map and associated quick reference guide, lays out data projections for hundreds of cities and every county and state in the continental U.S.

Texas among the hardest hit

The scientists presented their results nationally, by region, state and cities with more than 50,000 people. It used a heat index, a combination of temperature and relative humidity that can be more relevant than the actual temperature in places such as Houston, which has a humid climate.

They used historic data to calculate what levels could be reached by mid- and late-century, and what those numbers would look like if heat-trapping emissions are reduced.

Dahl said states on average are projected to experience 16 more days of 105-plus temperatures by mid-century compared to now. Texas would experience an additional 40 days with a heat index above 105 — more than five additional weeks, according to the calculations.

“Texas is one of the states that has the most frequent and most extreme heat,” Dahl said. “But we also found that when compared to other states, Texas sees one of the largest increases in number of extremely hot days.”

Houston would go from having a week-and-a-half with a heat index above 105 degrees to about two-and-a-half months with a heat index at that level. Brownsville, Victoria and Conroe are also among the Texas cities that would see bigger changes for days with a triple-digit, heat index, according to the study.

But a heat index at that level doesn’t even merit a heat advisory for Houston from the National Weather Service, said John Nielsen-Gammon, the state climatologist and Regents Professor in the Department of Atmospheric Sciences at Texas A&M University.

In Houston and across southeast Texas, he said, a heat advisory is generally issued when temperatures reach 108 degrees and a heat warning is issued when they exceed 113 degrees.

“Part of the issue of why we have higher thresholds for heat advisories and heat warnings is that it takes a hotter temperature to have health impacts here compared to farther north where it’s normally cooler,” he said.

Regardless, he said, as the world continues to warm up, places such as Houston will cross heat thresholds more often.

“It makes things more unpleasant, leads to more days where outdoor activities are limited and fewer full days of outdoor construction,” he said, impacting not only people’s health, but also the economy.

And the heat will be worse in urban areas, he said.

“I’ve done preliminary analysis on that which suggests the number of days over 100 degrees are increasing twice as rapidly in urban areas versus the rest of state,” he said. “If anything, I think the problem for the majority of Texas is going to be worse than the paper itself indicates because we’ve got not just global climate change going on but local climate change due to urbanization.”

A deadly problem

Extreme heat is among the deadliest weather hazards society faces, according to the new report.

During extreme hot days, heat-related deaths spike and hospital admissions for heat-related illnesses rise, especially among people experiencing poverty, elderly adults and other vulnerable groups.

During a 2011 heat wave, the number of emergency department visits by older adults increased by about 9 percent, said Meredith Jennings, a post-doctoral research scientist specializing in community and climate resilience at the Houston Advanced Research Center.

Also, a 2017 survey indicated, 87 percent of Houstonians reported having air conditioning, though only 44 precent knew that the city had opened cooling center, she said. Only 7 percent had used one of the centers, the survey showed.

Having more days of extreme heat means cities such as Houston need to think about where they are planting trees in relation to the areas that are the hottest, rethink cooling centers and make sure there are enough programs to assist families with utilities, Jennings said.

Planning for the future

The good news, the Union of Concerned Scientists concluded, is that with changes and limits to greenhouse emissions, the nation can change course.

“Unlike a lot of effects of climate change, extreme heat responds relatively quickly to emissions reductions,” Dahl said. “Less carbon dioxide in the atmosphere means less-frequent, less-severe extreme heat.”

The group has several recommendations for all levels of government.

For local and state governments, the group suggests investing in developing localized heat-adaptation plans and heat emergency response plans. It calls on the federal government to invest in scientific research, data, tools and public communications related to extreme heat risks and protective actions.

But the most consequential thing the world’s wealthiest nations can do, the report concludes, is make deep cuts in heat-trapping emissions and continue to strengthen the Paris Climate Agreement.

The United States’ withdrawal from the Paris Climate Agreement “sends signals to the world we aren’t serious of reducing our emissions in this country,” Dahl said.

States and cities have stepped up to fill some of the gaps, she said, but the effort needs to be at the federal level to seriously contribute to the declines in global emissions that will be needed in coming decades.

While climate change can’t be completely avoided, “anything we do to reduce its magnitude will be beneficial,” said Nielsen-Gammon. “Knowing the climate is changing is helpful for planning for the future.”

Staff writer Josh Baugh contributed to this report.