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

The medics loaded Sgt. Sylvester Cline into an ambulance with the air conditioning running at full blast. It was 4:20 p.m., 20 minutes after he’d been helped off a live-fire training range at Fort Chaffee in Arkansas, where it was 93 degrees and humid.

Cline, an Iraq combat veteran, and two other soldiers were being evacuated to a nearby barracks to rehydrate and cool off after nine hours of drills on parched training grounds on that sweltering day in June 2016.

Despite a forecast for extreme heat, base safety officers who prepared the daily risk assessment had decided soldiers faced only moderate danger. Later that morning, the temperature had reached 90 Fahrenheit, triggering “black flag” conditions, the military’s signal for a high risk of heat casualties. Commanders were supposed to allow at least 40 minutes rest for each hour of training, but they did not heed the requirement, an Army investigation found.

As the temperature climbed in the afternoon, reports went out of multiple soldiers falling ill in the heat, while requests to assess whether to halt the training were dismissed. Commanders refused to allow soldiers to shed heavy gear, and it took almost three hours for supply personnel to respond to field leaders’ urgent requests for ice, water and Gatorade, the military’s investigation reports show.

Photos of Sgt. Sylvester Cline on a table at his mother's home. Andrea Morales / for NBC News

In the ambulance, Cline — sweating profusely — began to falter.

Realizing he needed urgent care, the medics rushed him to the base clinic instead of the barracks with the other soldiers. As the doors to the ambulance swung open, Cline, 32, a father of five who was known as "Mr. Mom" for his devotion to his children, tried to step out but couldn’t stand. He was helped onto another stretcher. The medics started an IV and wrapped him in ice sheets. That’s when his condition turned grave.

He began to vomit and soon was unresponsive. Medical personnel started CPR and called a helicopter to take him to a nearby hospital.

At 6:17 p.m., shortly after the helicopter landed, doctors pronounced him dead.

The heat had killed him.

A growing threat

Cline was one of at least 17 troops to die of heat exposure during training exercises at U.S. military bases since 2008, according to the Pentagon. They included an 18-year-old cadet in his first week at West Point, a 21-year-old on his first day of training as an Army Ranger, and a fit 22-year-old Marine who died after a 6-mile hike.

Over the same period, amid scorching conditions, a rising number of military members have fallen ill because of the heat.

In 2008, 1,766 cases of heatstroke or heat exhaustion were diagnosed among active-duty service members, according to military data. By 2018, that figure had climbed to 2,792, an increase of almost 60 percent over the decade. All branches of the military saw an increase in heat-related illnesses, but the problem was most pronounced in the Marine Corps, which saw the rate of heatstrokes more than double from 2008 to 2018, according to military data.

Nigel Chiwaya / Military Health System

The service members who died of heat exposure are among the most extreme examples of how rising temperatures from climate change pose a threat to military personnel, both at home and abroad.

The heat exacerbates challenges the military is facing in some of the world’s most destabilized regions and endangers individual troops — and, by extension, U.S. security and preparedness, the Pentagon concluded in recent studies on climate change risks. Health impacts from heat have already cost the military as much as nearly $1 billion from 2008 to 2018. The warming planet “will affect the Department of Defense's ability to defend the nation and poses immediate risks to U.S. national security,” a recent Department of Defense report said.

InsideClimate News and NBC News spent the last nine months investigating heat deaths and heat-related illnesses in the military and the Pentagon’s uneven efforts to safeguard service members. Reporters interviewed current and former military personnel, visited the two largest installations in the Southeast heat belt, reviewed investigative reports on troops’ deaths obtained through the Freedom of Information Act and analyzed a decade of data related to heat illnesses across all military branches.

The investigation found that despite acknowledging the risks of climate change, the military continues to wrestle with finding a sustainable, comprehensive strategy for how to train in sweltering conditions. The military’s investigative reports, often heavily redacted, show evidence of disregard for heat safety rules that led to the deaths of troops. The reports document a poor level of awareness of the dangers of heat illness and the decisions of commanders who pushed troops beyond prudent limits in extremely hot conditions.

The tendency to train through high heat results at least in part from a warrior ethos based on the belief that troops must be hardened to withstand the rigors of combat, the reporters found. Heat risks are defended as necessary to realistically replicate combat conditions.

The nation’s operational theaters, most notably those in Africa, South Asia, the Middle East and the Persian Gulf, are indeed getting hotter, while most military training in the United States takes place at scorched bases across the Southeast, where the lion’s share of heat-related illnesses are recorded.

Carolyn McLemore with mementos of her son, Cayln McLemore, who died during an Army Reserve training exercise in June 2018 Brock Stoneham / NBC News

Current and former defense officials and officers, in numerous interviews, said they are working to reduce heat illnesses and deaths by revising guidelines for assessing heat risks, updating prevention measures, refining treatment protocols and developing new gear and technology to keep service members cooler.

Heat injuries “are among the most vexing environmental problems the military faces,” said retired Lt. Gen. Eric Schoomaker, who raised the heat issue as the Army surgeon general almost a decade ago. “Almost all are preventable with the appropriate command emphasis and support for troops,” he added. And “none need to be fatal.”

Yet the deaths have continued. Most recently, Cayln McLemore, 25, an Army reservist, died last summer of heat exposure after getting lost during a training exercise at Camp Blanding in Florida. Heat also is being investigated as a factor in the spring 2019 deaths of two Air Force service members who collapsed during physical training at Shaw Air Force Base in South Carolina during an extreme heat wave.

One challenge in getting commanders to treat the heat threat as an urgent priority is that global warming is an increasingly taboo topic in the military under President Donald Trump, who has called climate change a hoax. In testimony before Congress, generals and admirals continue to flag climate change broadly as a threat to national security. But Trump’s stance makes it difficult for leaders at some levels to frame the heat problem as an urgent climate change threat, according to interviews with retired officers, defense academics and current military personnel.

“No one is going to talk about climate change because of the political aspect and who is in the White House,” a military official, who asked to remain anonymous, said. “It’s a career killer to talk about something in opposition to that of the administration.”

There is a ripple effect to this silence, said Alice Hill, a National Security Council official under President Barack Obama who focused on climate and security.

If the risk is not being communicated from the top, then leaders down the chain of command may not see heat and climate change as a priority in planning and training, she said.

“That’s when you can expect consequences to operational readiness,” she said.

Maj. Meghan Galer was the exception to this silence on climate change among the roughly dozen active military personnel interviewed for this story. The Army doctor at Fort Benning in Georgia sees heat as an existential threat to military personnel and said that climate change is the driving factor.

Maj. Meghan Galer began pushing for a "heat center" after seeing the high number of heat-related illnesses coming into the ER at Fort Benning's on-base hospital. Brock Stoneham / NBC News

“I’m going to choose my words carefully,” she said in an interview accompanied by a military public affairs officer. “Temperatures are increasing; heat waves are more frequent and putting people at increased risk. We believe there is an overwhelming amount of scientific evidence that points to global warming. That is an obvious statement of fact. The thing about science and fact is – it doesn’t matter if you believe them or not, it remains fact.”

Galer wrote a 2018 white paper on heat-related illnesses as a blueprint for how the military can face this climate threat. She cited a “tragedy loop,” in which heat awareness is redoubled after a training death but then fades with time — until another service member dies.

“As prudent physicians,” Galer said, “we need to prepare for a future where temperatures are increasing.

A day of unheeded warnings

Almost 12 hours before Sgt. Cline stopped breathing, an orange morning sun had signaled the start of the day for him and 1,300 soldiers from Arkansas' 39th Infantry Brigade Combat Team mustered for training at Fort Chaffee. The forecast, just after 6 a.m., called for temperatures to push well beyond 90 with high humidity.

The account that follows is drawn from investigative reports by the Army and the Arkansas State Police; a less-redacted version of the Army’s report provided by a National Guard source; and an Army document provided by Cline’s mother.

The base safety officers who indicated only moderate danger in the daily risk assessment did so even though the prior two days had been so stifling that 10 soldiers fell ill from the heat, according to the Army’s investigation.

After breakfast, Cline, wearing 40 pounds of combat gear on the hardscrabble fields of Range 100, fired round after round from a .50-caliber machine gun. Uniforms were soaked with sweat and some soldiers had drained their hydration packs to critically low levels in an hour, according to the state police report.

Sgt. Sylvester Cline with his mother, Shirley Cline. Andrea Morales / for NBC News

At 10:53 a.m., the heat and the humidity triggered the “black flag” warning, which should have slowed the exercises based on Army regulations.

Yet there was no relenting of the war games. Radios crackled with the news of soldiers falling to the heat, according to the state police report. Up to six required hospitalizations that day, according to accounts given to state police and Army investigators.

Sometime after noon, Cline left his position and briefly sought shelter in a shaded area. The temperature had hit 92 Fahrenheit and the heat index registered 101.

But the training went on, and Cline pushed through the afternoon heat.

Shortly after 2 p.m., after eight hours in the sun, Cline again took a breather in the shade.

He returned to his position. Less than two hours later, about the time medics were warning field commanders that soldiers were not being given enough rest, the heat finally took its toll. Cline was exhausted. His head throbbed, his back was seized by spasms and his extremities went numb, with his legs cramping so badly he needed assistance walking, witnesses told state police investigators.

This time, Cline needed more than shade. He needed medical help. On wobbly legs and leaning on a fellow soldier, Cline made his way toward medics and the nearby ambulance.

A medic who treated Cline told state police investigators he had never seen a heat victim decline so rapidly. The medic likened it to “falling off a cliff.”

Military leaders accused of negligence

After investigating Cline’s death, the Army condemned the field commander, Lt. Col. Gib Richardson, and the highest ranking noncommissioned officer in charge that day, command Sgt. Maj. Charles Franks.

The two leaders “failed to adequately address known hazards associated with the predicted hot weather environment prior to training commencing,” the 23-page findings and recommendations report issued by the Army a month after Cline’s death said. Richardson and Franks also “failed to execute adequate heat illness prevention procedures while conducting training,” the report found.

That poor judgment “rose to the level of negligence in their duty to protect soldiers from the adverse effects of heat,” the report said.

Richardson lost his command but remains in the National Guard as a warrant officer at the Arkansas National Guard Armory. Franks said he was given the choice between retiring from the National Guard or facing disciplinary action, and he chose to retire.

The state police investigation was turned over to a local prosecutor, who determined no criminal charges were warranted.

Franks said in an interview that he was a “fall guy” for the military scrambling to account for Cline’s death. He said there was nothing out of the ordinary that day. “It’s Arkansas. It was hot and muggy,” he said. “What can you say?”

In a rebuttal to the Army’s finding of negligence, Franks wrote that he strictly adhered to the Army’s guidelines for resting soldiers. He said he was never alerted to any life-threatening conditions.

Richardson wrote a rebuttal as well, saying that he took proactive measures to safeguard soldiers, including assigning six medics and two ambulances to the range when only one medic and one ambulance were required.

“My actions and decisions were thought-out and deliberate,” Richardson wrote. “At no time did I disregard safety concerns or the well-being of my Soldiers.”

Richardson wrote that many statements made to investigators were “patently untrue.”

He said he was absolved of the negligence allegation because no mention of it was made in the brief memorandum relieving him of command.

“It was a tragic accident,” Richardson said in an interview. “We do our best to mitigate all risks but still do what we have to do to prepare our soldiers for the real world they face if called on to deploy. We train for combat.”

He added that he remains saddened by Cline’s death.

A flag recognizing Sgt. Cline's service. Andrea Morales / for NBC News

“That weighs heavy on me. Sgt. Cline will be in my mind forever,” he said.

Spec. Kristopher Fields served as a medic the day Cline died. Afterward, commanders began paying more attention to heat safety, he said.

Fields said he had been ordered by his National Guard superiors and base public affairs officials not to talk about Cline’s death. He said in a brief conversation that conditions that day were not unusual.

“It was hot, but we were used to it,” he said.

Army Ranger trainees rest after the 12-mile march during which they wore sensors to measure their core temperatures and heart rates. Brock Stoneham / NBC News

The military’s heat problem

Slightly more than 40 percent of the heat-related illnesses and deaths over the last five years occurred at five military installations: Fort Benning in Georgia, Fort Bragg in North Carolina, Fort Campbell in Kentucky, Fort Polk in Louisiana and Camp Lejeune, a Marine Corps installation in North Carolina, according to the Defense Health Agency.

When the heat and humidity collide in those regions, the air becomes almost molten.

About 60 percent of the Southeast’s major cities are already experiencing worsening heat waves — a higher percentage than in any other region in the country — according to the National Climate Assessment. During the most recent 10 years, average summer temperatures were the hottest on record.

If greenhouse gas emissions continue on the current path, global average temperatures could rise 8.5 degrees Fahrenheit by the end of the century, the assessment found. The resulting extreme heat could lead to tens of thousands of premature deaths every year across the United States.

The impact of climate change on heat is even more extreme in Africa, South Asia, the Middle East and the Persian Gulf, where thousands of U.S. forces remain deployed. On the day in June 2016 when Cline died in Arkansas, it was 107 in Baghdad.

Many U.S. military leaders fought in the intense heat of Iraq and Afghanistan and want their troops to be able to do the same, dozens of current and former military personnel said.

Augusto Giacoman, a former Army captain, said the Army’s "black flag" heat warnings aren’t always reasonable when training combat forces.

“If you want to be prepared for a fight in the heat, you have to train in the heat under the same conditions you’ll encounter,” he said.

Yet training commanders also remain vigilant – demanding waters breaks and checking for signs of heat exhaustion, Giacoman said. “It’s a fine line,” he said.

Joy Craig, a retired Marine Corps warrant officer and drill instructor, said service members often don’t want to acknowledge their vulnerability to heat.

“It doesn’t matter that you’re about ready to collapse, you don’t let on,” she said. “You push through it.”

Efforts to adapt

Service members are particularly vulnerable to heat illnesses early in their training, when they may be less physically fit or acclimated to the heat. Even in moderate temperatures, high exertion coupled with multiple layers of clothing can result in heatstroke, which can damage organs and result in seizures, coma or even death. Those who survive may face permanent brain damage.

At Fort Benning, the emergency room at Martin Army Community Hospital sees soldiers suffering from a heat illness nearly every day, said Galer, the Army doctor there who has been outspoken about the need for changes.

She was moved to action after Michael Parros, 21, a second lieutenant and a graduate of West Point, died in the first week of training in the sweltering heat and humidity at Fort Benning in July 2016.

Galer and her colleagues at Fort Benning saw 1,504 heat-related illnesses from 2014 to 2018, ranking first among 250 U.S. military installations.

Researcher Julio Gonzalez prepares a sweating mannequin for a heat test at the U.S. Army Research Institute of Environmental Medicine. Brock Stoneham / NBC News

Galer was instrumental in establishing a “heat center” at Fort Benning last year to train medics to treat heat illness on the ground, teach field leaders to prevent heat illness and encourage research that explains why some soldiers are more susceptible than others. Researchers hope to develop an alarm badge — similar to a radiation badge — that would warn when a soldier is nearing heat stress.

“We think we have a good model that we can push out to the rest of the Army's hot-climate training posts,” Galer said, “in the hopes of saving some lives.”

The military’s research and development arm is also developing new gear for hotter environments, including lightweight uniforms that wick moisture and a cooling vest that circulates refrigerated liquid. It’s not clear when these innovations could reach troops in the field.

‘He was proud to do his duty’

When Cline’s mother, Shirley Cline, talks about what happened to her son that sweltering June day two years ago, she pauses to gather her thoughts – and to temper her anger.

“It was a death that should not have happened and the way it happened,” she said. “They needed to pay attention to the heat and how it was hurting people.”

It fell to her to break the news of their dad’s death to three of his five children, who are now between the ages of 6 and 16.