U.S. Soldiers Falling Ill, Dying in the Heat as Climate Warms

At least 17 service members have died from heat illnesses in the past decade, and the rise in heat stress injuries suggests the military isn't prepared for worse.

This story was published as part of a joint investigation with NBC News.

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 the heat index had reached 103.

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 degrees, 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 training were dismissed. Commanders refused to allow soldiers to shed heavy gear, and it took nearly three hours for supply personnel to respond to field leaders' urgent requests for ice, water and Gatorade, the military's investigation reports show. 

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

Shirley Cline with a photo of her son, Sgt. Sylvester Cline, an Iraq combat veteran who died in June 2016 during routine training exercises. Credit: Andrea Morales for NBC News

Shirley Cline sits with a photo of her son, Sgt. Sylvester Cline, an Iraq combat veteran who died of heat-related injuries in June 2016 after falling ill during routine training exercises. Credit: Andrea Morales for NBC News

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.

Investigations Show Military Struggling with Rising Heat Risks

Cline was one of at least 17 service members to die of heat exposure during training exercises at U.S. military bases since 2008. 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.

Dangers Without Borders: An ICN Series on Military Readiness in a Warming World

In 2008, 1,766 cases of heat stroke 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 a rise in heat-related illnesses, but the problem was most pronounced in the Marine Corps, which saw the rate of heat strokes more than double from 2008 to 2018, according to military data.

The troops who died of heat exposure are among the most extreme examples of how a warming world poses a threat to military personnel, both at home and abroad.

The rising heat exacerbates challenges the military is facing in some of the world's most destabilized regions and endangers individual service members — 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 in lost work, retraining and medical care. The warming of the 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.

Chart: The Rising Danger from Heat

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 war fighters. Reporters interviewed current and former military personnel, visited the two largest installations in the Southeast heat belt, reviewed investigative reports on troop 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 service members. 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.

Chart: Confirmed Military Heat Deaths

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.

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.

Map: Military Bases with the Most Heat-Related Injuries

Heat injuries "are among the most vexing environmental problems the military faces," said retired Lt. Gen. Eric Schoomaker, who raised the heat issue as Army surgeon general nearly 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. Last summer, Cayln McLemore, 25, an Army reservist, died 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.

"Although numerous effective countermeasures are available, heat-related illness remains a significant threat to the health and operational effectiveness of military members and their units and accounts for considerable morbidity, particularly during recruit training in the U.S. military," the Defense Health Agency said in an April report.

When a soldier falls to a heat illness, especially heat stroke, it's akin to a battlefield casualty. "It takes troops out of training," said Col. James Mancuso, chief of Epidemiology and Analysis at the health agency. "It takes service members who are deployed away from their mission."

[If you have a story to share about heat illnesses or heat-related deaths in the military, contact us at insideclimatenews.org/contact.]

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.  

Profile: Reservist Cayln McLemore

"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 climate silence 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 at Fort Benning's hospital. Credit: Brock Stoneham/NBC News

Maj. Meghan Galer began pushing for a "heat center" after seeing the high number of heat-related illnesses coming into the emergency room at Fort Benning's hospital. Credit: 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 soldier dies.

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

Chart: Heat's U.S. Death Toll

Heat has been responsible for more deaths in the United States over the last 30 years than any other natural hazard—more than floods, hurricanes or cold. Climate change is making the heat worse. Globally, the average annual temperature is now about 1.8 degrees Fahrenheit above the temperature of the late 19th Century, and the last three years have been the warmest years on record for the globe, according to the National Climate Assessment, a multiagency federal report.

"The consequences become disproportionate to the rise in temperature," said Retired Army Lt. Col. Frank Galgano, an associate professor of geography and the environment at Villanova University. "The effects of heat must be planned for in the greater scope of operations, soldier training and preparation and weapons employment. The ripple effects are immense."

A Day of Unheeded Warnings Ended with a Soldier Dead

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.

Photos of Sgt. Sylvester Cline on a table in his mother's home. Credit: NBC

Photos of Sgt. Sylvester Cline on a table in his mother's home. Credit: Andrea Morales for NBC News

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 as part of the day's training. Uniforms were soaked with sweat and some soldiers had drained their personal hydration packs to critically low levels in an hour, according to the state police report.

By 10 a.m., the temperature reached 86 degrees and the humidity made it feel like 93, hot enough for some field leaders to worry it was getting dangerous, documents show. Yet soldiers were kept in full "battle rattle," a military idiom for wearing full gear.

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

Profile: Sgt. Sylvester Cline

Yet there was no relenting of the sun or the war games; it was a hell-bent day of training for Cline and the National Guard members of Alpha Company. A request to bring up a large water tank to resupply soldiers' personal hydration packs was initially denied. Field leaders asked for more water, Gatorade and ice; it took hours to arrive. They sought permission from senior commanders to allow soldiers to shed their heavy gear; they were refused. There was talk in the ranks that training should be halted. Radios crackled with the news of soldiers falling to the heat, according to the state police report. Up to six required hospitalization 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 nearby shaded area. The temperature had hit 92 degrees and the heat index registered 101.

But the training went on, and Cline pushed through the afternoon heat. Breaks were only occasional.

Infographic: Heat Exhaustion or Heat Stroke — a Guide

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, multiple witness 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."

Leadership Failures or 'A Tragic Accident'? 

The documents obtained by InsideClimate News describe leadership failures from dawn, with the inadequate daily risk assessment, to the end of the day, when commanders had failed to allow soldiers enough rest to comply with the Army's "black flag" regulations. 

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. 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 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.

Infographic: What Heat Stroke Does to the Human Body

Staff Sgt. Samuel Kell was a platoon leader for the training exercise. He pulled Cline out of action several times to cool off and to hydrate, and he expressed his concerns about the heat to his higher-ups.

"Kell said the lieutenant colonel [Richardson] told them that the soldiers needed to know the difference between being tired and having a heat injury," according to the state police report. "Kell said the lieutenant colonel kept telling them they needed to push harder."

When contacted for comment, Kell said he had been ordered not to talk about Cline's death. Richardson denied making that statement.

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

Franks said 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, including the blistering heat that is common for June at Fort Chaffee. "It's Arkansas. It was hot and muggy," he told InsideClimate News. "What can you say?"

Marines with an 926th Engineer Brigade are loaded with gear to practice bridge construction at Fort Chafee, Arkansas, in late July 2018. Credit: Lance Cpl. Quentarius Johnson/U.S. Marine Corps

Marines loaded with gear practice bridge construction at Fort Chaffee, Arkansas, on a July day in 2018 when temperatures neared 100 degrees. Credit: Lance Cpl. Quentarius Johnson/U.S. Marine Corps

In his defense, Franks wrote a rebuttal to the Army's finding of negligence. He said he strictly adhered to the Army's guidelines for resting soldiers and reassessed conditions for safety throughout the day. He said he was never alerted to any life-threatening concerns.

"As command, all we can do is rely on our medics and our first line leadership," his rebuttal said. "If we are not aware of the problems, we cannot correct them."

Richardson wrote a rebuttal as well, saying he took proactive measures to safeguard soldiers, including assigning six medics and two ambulances to the range when only one medic and ambulance were required. He also directed safety officers be stationed at each site where soldiers were positioned with machine guns and appointed the battalion's safety officer as second in command to ensure soldiers were safe.

"My actions and decisions were thought-out and deliberate," Richardson wrote to the adjutant general of the Arkansas National Guard. "At no time did I disregard safety concerns or the well-being of my Soldiers."

The military has projects underway to better monitor troops' health in the heat. A soldier at Fort Benning participates in a 12-mile march while wearing sensors to measure his core temperature and heart rate. Credit: Brock Stoneham/NBC News

The military has projects underway to better monitor troops' health in the heat. A soldier at Fort Benning participates in a 12-mile march while wearing sensors to measure his core temperature and heart rate. Credit: Brock Stoneham/NBC News

Richardson wrote that many statements made to investigators were "patently untrue" and were refuted by other witnesses.

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.

"There are kids without a father, and a mother and father without a son. That weighs heavy on me. Sgt. Cline will be in my mind forever," he said.

The disregard for safety on the day Cline died prompted one insider to send an anonymous letter to Cline's mother several weeks later. The author, who appears to have military knowledge and had been close to the training exercise, suggested commanders were more concerned with a successful training exercise than with the welfare of those under their command.

Shirley Cline places glow-in-the-dark crosses at her son's grave at their family plot. Credit: Andrea Morales for NBC News

Shirley Cline places glow-in-the-dark crosses at her son’s grave at their family plot. Credit: Andrea Morales for NBC News

"There are many more issues to address but heat related deaths are unacceptable as they are too easily avoided when soldiers are put first and not the mission," the typed letter said.

After Cline's death, commanders began paying more attention to promoting safety and adhering to the prescribed work/rest cycles mandated by the military, said Spec. Kristopher Fields, who served as a medic on the day Cline died.

Fields said he had been ordered by his National Guard superiors and base public affairs officials not to talk to InsideClimate News 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.

The Military's Climate-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. 

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

Profile: Sgt. Aaron Scales

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.

Profile: Cpl. Alexis Alcaraz

Army medical researchers found heat illnesses were already responsible for more than 20,500 lost or limited-duty days in 2017, the first time the data was collected. 

Heat illnesses cost the military as much as $917.9 million between 2008 and the first half of 2018, according to data provided by the combat readiness centers for the Army, Air Force, Navy and Marine Corps. The costs include medical care, the lost investment in training if ill service members cannot return to their duties, the cost to retrain them and any ongoing expenses for rehabilitation and disability. A Defense Health Agency study found that while 90 percent of all recruits finish basic training, only 66 percent of those who suffer a heat illness do so.

The impact of climate change on heat will be 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.

Profile: Sgt. Demekia Cola

Training in hot environments poses two competing demands: the necessity of conducting realistic exercises and the need to protect personnel against heat-related illness. 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. To them, a few degrees seem insignificant when compared with the rigors of combat. This temperament can raise the risk of heat illness at home and abroad, according to interviews with dozens of current and former military personnel.

Augusto Giacoman, a former Army captain who trained at Fort Bragg's airborne school and later managed the training of platoons of up to 45 soldiers, 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. "It was always about my soldiers being ready. To be prepared, you have to go beyond the charts; you have to be aware of that, but training and being prepared has to come first."

Profile: Cadet Jacob Bower

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. And when they do, she said, it's taken as a sign of weakness.

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

There are a number of safety badges personnel wear during training. One indicates a poor swimmer; another indicates allergies to bee stings. A red badge signifies susceptibility to heat illness.

Craig wore one. "It's not a badge of honor," she said.

Efforts to Adapt: Monitoring and Innovation

Troops 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 heat stroke, 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 climate change and the need for changes to safeguard soldiers.

Profile: 2nd Lt. Michael Parros

She was moved to action after Michael Parros, 21, a second lieutenant and graduate of West Point, died in the first week of training in the sweltering heat and humidity at Fort Benning in July 2016. The cause of death was listed as "exercise-associated hyponatremic encephalopathy," a condition triggered by drinking too much water that results in sodium in the blood becoming so diluted it causes brain swelling.

Following a two-month Army investigation, authorities recommended administrative or disciplinary action be taken against some members of Parros' command team and instructors based on the delayed medical care, the investigative report stated. There is no indication in the report of such action being taken.

Galer and her colleagues saw 1,504 heat-related illnesses from 2014 to 2018, the highest number among 250 U.S. military installations. In 2017 alone, the base had 300 heat stroke cases. "When we looked at the numbers," Galer said, "I was like, 'God, this is a ton of freaking heat strokes.'"

Galer was instrumental in establishing a "heat center" at Fort Benning last year to train medics to treat heat illnesses on the ground, teach field leaders to prevent heat illnesses and encourage research that explains why some soldiers are more susceptible to heat illnesses than others.

A volunteer runs with a chest sensor that measures his core temperature in an environmental chamber that military medical experts use to simulate extreme hot and cold temperatures. Credit: Brock Stoneham/NBC News

A volunteer runs with a chest sensor that measures his core temperature in a room that can simulate extreme hot and cold temperatures at the U.S. Army Research Institute of Environmental Medicine. Credit: Brock Stoneham/NBC News

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

The Army's Research Institute of Environmental Medicine has joined the Heat Center to study soldiers during intense physical training, in the hopes of developing an alarm badge — similar to a radiation badge — that would warn when a soldier is nearing heat stress. The data also would enable leaders to observe the condition of their soldiers, and better tailor training to avoid debilitating heat illnesses. That is critical to maintain readiness.

"If you are not having individuals fall out of training because you can predict onset of a heat illness, then that allows you to focus on training rather than managing people through recovery periods," said Dr. Beth Beidleman, an institute research physiologist working on the study.

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 soldiers in the field.

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

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

Other bases are paying attention, too. At Fort Bragg in North Carolina, which ranked second after Fort Benning in the number of heat-related illnesses from 2014 to 2018, precautions have changed "significantly" over the past several years, said Rich Eppler, director of installation safety.

The turning point there came in summer 2011, when a young paratrooper died of heat stroke at the base, the home to the Army Special Forces and the 82nd Airborne Division. Sgt. Joshua Mann, an airborne infantry team leader, was on a 6.5 mile run on an 86-degree day with fellow paratroopers when he collapsed in the heat. His internal body temperature surpassed 105 degrees. The 22-year-old Iraq war veteran died the next day of kidney and liver damage brought on by heat stroke.

The heat also forced 500 soldiers to drop out of a 10K run that summer. Ten of those soldiers had to be hospitalized, which prompted base commanders to order a "stand down." All training stopped while safety precautions were evaluated, overhauled and reiterated to every soldier and officer.

Dr. Mark Buller, a researcher investigating heat illness in the military, observes a training march at Fort Benning during which soldiers wear sensors to measure their core temperature and heart rates. Credit: Brock Stoneham/NBC News

Dr. Mark Buller, a researcher investigating heat illness in the military, observes a training march at Fort Benning during which soldiers wear sensors to measure their core temperature and heart rates. Credit: Brock Stoneham/NBC News

The base's casualty prevention plan now takes into account nutrition and fluid intake, the types of gear and clothing worn during exercises, how hard to press training and how to recognize the onset of heat stress. Sophisticated instruments register temperature and humidity and calculate the heat index to determine the amount of rest soldiers need. The base schedules strenuous physical exertion for the cooler morning hours, and medics and other safety personnel constantly monitor training.

The full impact of these changes has not been calculated. The number of "black flag" days at the base, meanwhile, has increased from 13 in 2015 to 40 last year.

"We try to make sure we get ahead of it and look at it from various perspectives," said Eppler, a retired Army chief warrant officer and attack helicopter test pilot who served multiple tours in Iraq and Afghanistan.

Still More Deaths

Even with the military's deepening awareness, Cayln McLemore, an Army National Guard soldier, was found dead in the woods in June 2018 two days after he disappeared during a land navigation training class at Camp Blanding in Florida. The heat index that day reached 107 degrees. He had thrown off some of his heavy gear in the hot and humid conditions and failed to return as scheduled.

The official cause of death of the 25-year-old specialist was environmental heat exposure, according to an Army Findings and Recommendations report released to InsideClimate News in response to a Freedom of Information Act request.

Carolyn McLemore looks at the program from her son's funeral at his home in Memphis. Cayln McLemore died during an Army Reserve training exercise in June 2018. Credit: Brock Stoneham/NBC News

Carolyn McLemore looks at the program from her son’s funeral at his home in Memphis. Army Reservist Cayln McLemore died during training exercises in June 2018. Credit: Brock Stoneham/NBC News

His squad leaders described him as a "real go-getter," and he was on track to be named to the commandant's list for his high scores. In high school, he had taken summer school classes so he could graduate early and join the military, and the National Guard was preparing him for leadership through its Basic Leader Course.

"He saw that as a way to make a better future for himself," his mother, Carolyn, said. "He wanted to be a success, and he knew he'd have to work for it."

McLemore's death was declared an accident. Carolyn McLemore said she doesn't understand what happened. Her son was in top shape, she said. He'd been in the Army Reserves for seven years, participating in the same kind of training exercises.

The military expressed its condolences, but his family was not satisfied. His mother had to file a Freedom of Information Act request to get the Army's report on her son's death. It took five months for the Army to send her the redacted document, which shed little light on what had happened.

Soon after InsideClimate News began asking questions about her son's death, McLemore said the Army asked to meet with her. Six officers in uniform came to her Memphis home with yet another report of her son's death.

"They said they were going to tell me what happened to Cayln," she said. "They didn't."

"The big question isn't answered: How could this happen?" she said. "They said my son's death was an accident. Well, these kinds of accidents shouldn't happen."

'A Death That Should Not Have Happened'

When Cline's mother, Shirley Cline, talks about what happened to her son that 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."

That day remains heavy on her mind. She lost a son, and her grandchildren lost their father. 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.

Nephews of Sgt. Sylvester Cline look at photos of their uncle at their grandmother's home. Credit: Andrea Morales for NBC News

Nephews of Sgt. Sylvester Cline look at photos of their uncle at their grandmother's home. Credit: Andrea Morales for NBC News

"First I told them, 'Your dad loves you,'" she recalled. "They were like, 'yes.' I said, 'The Lord has called your dad, and he's not coming back to Earth.'

"That's something you never forget."

There are still nights Shirley Cline cannot sleep. "I get up every morning missing my son," she said from her home in Pine Bluff, Arkansas. "I go through the day missing my son. If I didn't trust in God, I would have lost my mind."

Cline said her son saw it as a privilege to serve in the military, particularly in Iraq. "He was proud to do his duty," she said.

She asks the same questions about her son's death as the military did in its investigations, though hers are suffused with grief.

"They know it's going to get hot; they know it," she said. "So when they know it, what are they doing about it? They have to accept that this heat is going to be there and that things will have to change so other boys don't die."


NBC News data reporter Nigel Chiwaya and NBC News data editor Joe Murphy contributed to this report.

Top image: U.S. troops carry a fellow soldier who suffered heat stroke during a patrol in Iraq. Photo illustration by Paul Horn based on photo by David Furst/AFP/Getty Images

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