CNN
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Another record-breaking summer in Arizona has left hundreds of people likely dead from heatstroke in Maricopa County.

Heatstroke has killed 27 people in the county, which includes Phoenix, and is believed to be the cause of 396 deaths so far this year. According to figures released on Tuesday.

A total of 645 people died last year during the county's deadliest heatwave since the department began tracking deaths in 2006. Nearly half of last year's deaths occurred during the hottest period, from July 10 to July 25. That period saw fewer deaths than the 396 currently under investigation.

Maricopa County Chief Medical Examiner Jeff Johnston told CNN that heatstroke death investigations typically take two to three months, and that 75 percent of the deaths investigated last year were confirmed as heatstroke.

Suspected heatstroke deaths have surged in recent weeks as temperatures have risen. Nearly 100 of the suspected heatstroke deaths this year occurred between July 7 and 13, when temperatures in Phoenix reached 118 degrees Fahrenheit — the exact conditions scientists predict will make heat waves more frequent, intense and last longer as a result of global warming caused by fossil fuel pollution.

“We know that higher temperatures do lead to more heat strokes and more heat stroke deaths in people who are most at risk,” Nick Staab, deputy medical director for the Maricopa County Department of Public Health, told CNN. “The correlation between temperature and deaths has been well-documented for a long time.”

Phoenix is ​​currently experiencing the start of its hottest summer on record, with June this year being hotter than June last year and July this year being nearly as warm as July last year. Hottest month in U.S. city history.

Maricopa County said it is working to reduce heatstroke deaths by increasing the number of cooling centers in the county and extending their hours to serve unsheltered people, the county's most vulnerable population.

Fever The deadliest weather On average, they kill more than twice as many people in the United States each year as hurricanes and tornadoes combined.

But heatstroke deaths are still often underestimated, and in many places they aren't reported accurately or regularly. 2020 Survey They found that heat deaths were undercounted in 297 of the nation's most populous counties, and the researchers said mortality records tend to downplay other causes of death that may be heat-related, such as heart attacks.

Maricopa County last year changed how it investigates heatstroke deaths, allowing investigators to select multiple potential contributing factors for a death, including factors related to heatstroke. Before the change, the number of possible heatstroke cases under investigation was being undercounted, Johnston said.

Arizona's summer and heat aren't over yet. Although the heat peaks in Phoenix in July, the average high temperature doesn't drop below 100 degrees until mid-September.

With the number of deaths from heatstroke already so high, the coming months are expected to become even more deadly.

Correction: An earlier version of this story misstated how Maricopa County investigates heatstroke deaths. It has been updated.