This article has been jointly published by NBCNews.com and InsideClimate News, a nonprofit, independent news outlet that covers climate, energy and the environment.

U.S. troops already sweating through dangerous summer heat at bases across the country could face greater danger as global temperatures rise, a new report warns. The report, from the Union of Concerned Scientists, a nonprofit scientific research and advocacy organization, found that by midcentury, service members on U.S. bases could see an average of an extra month of life-threatening heat every year.

The military is already struggling to develop a sustained, comprehensive strategy for dealing with today’s heat, from training in sweltering summer conditions at home to deploying in war zones.

The report, released Monday, shows how quickly that risk could rise if countries don’t rein in the greenhouse gas emissions that are driving global warming.

“Thousands of service people suffer from heat-related illnesses every year, and the problem is set to grow much worse,” said Kristina Dahl, a senior climate scientist at the Union of Concerned Scientists and the lead author of the report. “The growing number of dangerously hot days could pose a challenge to the military’s efforts to protect service members’ health while also ensuring mission readiness.”

Dahl and her colleagues began looking into the impact of heat on the military after an investigation by InsideClimate News and NBC News this year revealed the rising dangers military personnel face from extreme temperatures and the cascading consequences when the military fails to prepare.

The InsideClimate News/NBC News report found at least 17 heat deaths during military training in the past decade and a 60 percent surge in heat-related injuries — primarily heat exhaustion and heat stroke — over the same period. The Air Force is investigating two additional deaths of service members who collapsed in the heat during training at Shaw Air Force Base in South Carolina this year.

The military has taken steps to reduce heat illnesses, including updating prevention measures, refining treatment protocols and developing specialized medical units and new equipment and technology, among other measures.

But the InsideClimate News/NBC News investigation found that while generals and admirals continue to flag climate change as a threat to national security, denial in the Trump administration has made it difficult for leaders at some levels to frame the heat problem as an urgent climate change threat.

Steep increase in dangerous heat days

The Union of Concerned Scientists’ military threat report indicates that by 2050, with no action to reduce global heat-trapping emissions, U.S. military installations will experience an average of nearly five times as many days with a heat index above 100 degrees Fahrenheit compared with the 1971-2000 average. The findings are based on data from an earlier national report, published in the journal Environmental Research Communications, showing that nearly every part of the United States will face a significant increase in extremely hot days by midcentury.

The results mean that living, working and training at U.S. military bases could become increasingly risky for service members and their families, Dahl said.

“We were pretty astounded in the increase in the number of extreme heat days in such a relatively short period of time,” she said.

“It’s already bad,” she added. “But when we put in context what is coming down the pike, the implication is really sobering.”

Heat has become an increasingly formidable enemy of military personnel, accounting for tens of thousands of lost duty days because of heat-related illnesses each year and costing nearly $1 billion over the last decade. Much of the military training in the U.S. occurs in sweltering regions with drenching humidity, where service members are sometimes pushed to their limits based on the belief that troops must be hardened to withstand the rigors of combat.

Retired Army Capt. Jon Gensler, a West Point graduate who served as a tank commander in the first Gulf War and later as a training officer, told InsideClimate News that temperatures as severe as predicted in the Union of Concerned Scientists report would have calamitous consequences for training and preparedness.

“It will severely limit the amount of outdoor training and induce uncertainty as to what you can accomplish,” Gensler, who was not involved in the report, said. “It reduces the amount of time available to make sure troops are ready to deploy and trained to the skill levels required to complete missions.”

Heat has already disrupted training schedules, forcing the military to find alternative ways to make sure personnel are prepared without exposing them to deadly conditions. Yet there is no complete substitute for training under realistic conditions, he said.

“The fact of the matter is that missions won’t stop,” Gensler said. “We could be sending troops into harm’s way who are not as well trained as they should be, and that will likely have tragic consequences.”

Which bases will see the fastest rise?

The Union of Concerned Scientists report calculates that, with no decrease in global emissions, 169 major military installations in the contiguous U.S., each home to more than 1,000 personnel, would experience an average of an additional 33 days a year with a heat index above 100 degrees by midcentury.

For some bases, the increase could be much larger. Fort Sill in Oklahoma, for instance, is projected to experience an additional 53 days a year of dangerous heat by midcentury.

Other bases expected to see leaps in extreme heat days — days with a heat index topping 100 degrees — over the next three decades, according to the report, include:

Homestead Air Reserve Base, Florida, with an additional 102 extreme-heat days per year;

Naval Air Station Joint Reserve Base New Orleans, increasing by 74 extreme-heat days per year;

Lackland Air Force Base, Texas, with 69 more extreme-heat days per year;

Marine Corps Recruit Depot Parris Island, South Carolina, with 61 more extreme-heat days per year; and

Fort Benning, Georgia, increasing by 57 extreme-heat days per year.

Historically, only nine major military installations in the U.S. have experienced 30 or more days per year with a heat index above 100 degrees. By midcentury, with no reduction in emissions, 100 installations could experience such conditions, according to the report.

Those conditions trigger “black flag” warnings under the military’s guidelines for protecting personnel from heat. Under the most extreme black flag conditions, triggered when temperatures top 90 degrees, the military requires 50 minutes of rest for every 10 minutes of strenuous work.

Heat has become such an urgent issue that the Defense Health Agency issues an annual report that tracks the number of heat illnesses in the military and identifies the bases where heat takes the greatest toll.

A 2019 report by the Defense Health Agency shows heat-related illnesses have been steadily increasing since 2014 across all branches of the service. That year, 1,751 heat illnesses — heat stroke and heat exhaustion — were reported. In 2018, 2,792 heat illness were reported.

With the new report, the Union of Concerned Scientists is urging all branches of the military to review and update heat-related health guidelines to reflect projections of worsening heat.

“Military personnel, especially those in command, should be trained to be more fully aware of the dangers of heat-related illnesses,” Dahl said, “and safeguards around work/rest cycles should be diligently enforced to prevent overexertion on dangerously hot days.”