Extreme heat in the US has killed at least 147 people in just five counties — a mere snapshot of the fatal toll this searing summer is taking. And experts say that estimate is likely far lower than the actual number of lives lost to the excruciating heat.

The deaths reported here occurred in three states that have endured the worst of what this summer's vicious heat had to offer: As of early August, there were 64 dead in Pima County, Arizona; 39 in Maricopa County, Arizona; 26 in Clark County, Nevada; and seven in Harris County, Texas.

Eleven people in Webb County, Texas, had died of heat-related causes as of June 27. Webb County officials have not been able to tell CNN if there were more heat-related deaths since then.

In addition to the dozens of deaths in Texas, Arizona and Nevada, several heat-related deaths were reported in California, parts of the South and the Midwest, though the tolls have not been as high as those in the five counties reported here. And Maricopa County, which is home to Phoenix and has officially tallied at least 39 heat-related deaths, has 312 more deaths under investigation for possible heat-related causes of death.

The fatalities came as temperatures soared to record-breaking levels at the end of June, and continued to bake much of the South and Southwest through July. Most notably, Phoenix just logged the hottest month of any US city on record, with 31 consecutive days at or above 110 degrees Fahrenheit from June into July.

The death toll from extreme heat to-date is far higher than tornadoes and flooding combined. Scientists have cautioned that heat waves will continue to get worse with the climate crisis, yet there are still many places that fail to accurately or regularly report heat-related deaths.

David S. Jones, a physician and historian at Harvard University, said the numbers reported here and more widely across the country are likely underestimates. He called the counts "mysterious."

"The low numbers of reported death really puzzle me," Jones told CNN. "Less severe heat waves in the US have killed hundreds of people in the past. I think it is very likely that the current mortality reports from the US in summer of 2023 are a significant undercount, though I have no proof of that."

Jones noted that the death toll could be low in the South because people are more acclimated to the heat and generally have more access to air conditioning.

But he stressed that reporting on a person's cause of death has always been "a complicated process." A medical examiner or coroner must list a single cause of death, and in some places, those officials are political appointees or elected officials who may not have any kind of medical background.

"The (cause of death) assessment itself is a complicated one," Jones added. "If someone is found deceased in an apartment, and you're trying to figure out what was the top cause of death to list, many medical examiners will say, 'well, the person must have died of heart disease in some way because their heart stopped.'"

Epidemiologists have said that extreme heat is a risk-multiplier and can be a major contributing factor in most instances where it's unclear that heat ultimately led to a person's death. A 2020 study found that heat-related deaths were being underestimated in 297 of the country's most populous counties. Researchers said mortality records tend to neglect other potentially heat-related causes of death, like heart attacks.

The Centers for Disease Control and Prevention, estimates heat-related deaths based on death certificates which list heat as a primary or contributing cause of death. Around 700 people die from heat each year, according to the CDC.

But the National Weather Service estimates there were only 148 heat deaths last year, with the 10-year average being 153 and 30-year average at 168 deaths.

Both US heat mortality numbers would suggest that far fewer people are dying from heat than countries in Europe, where last summer's scorching heat wave killed nearly 62,000 people.

Jones said that there are a couple explanations as to why US statistics seem inconsistent: the US could be underreporting its numbers, or heat is more lethal in Europe due to the lack of air conditioning — or it could be a combination of the two.

"The US has long had a difficult time getting fast, comprehensive health data," he said. "Cities report the state, states report to the CDC, and that happens at varying speeds, varying degrees of accuracy. And we just don't have a robust system of comprehensive and rapid health data."

The numbers still show how serious a health risk extreme heat can be. In the US, heat kills more Americans than any other weather-related disaster. The climate crisis has been making these extreme events more deadly — and this summer's exceptional heat is proving just that.