Every week between May and October, the Maricopa County Department of Public Health in Arizona releases a heat morbidity report. The most recent report said that 180 people have succumbed to heat-associated illness in the county this year so far. But everyone agrees that number is off.

If previous years are any indication, the true number of heat-related deaths in Maricopa County, which includes Phoenix, is much higher: At the end of last summer, the county revised its initial reports upwards by a factor of five, ultimately reporting a sobering 425 heat-related deaths in total.

This lag plagues not just heat-related mortality reporting, but climate-related death data in general. It’s hard to get a full picture of the true number of mortalities connected to a given disaster in real-time. The full death toll often isn’t revealed until weeks, months, even years after the event occurs. And an unknown fraction of deaths often slide by undetected, never making it onto local and federal mortality spreadsheets at all. For example, a recent retrospective study found the number of people who died from exposure to hurricanes and tropical cyclones in the U.S. in the years between 1988 to 2019 was 13 times higher than the federal government’s official estimates.

That study and others like it indicate that the U.S. is gravely underestimating the health impacts of climate change. “The system of death surveillance wasn’t designed for a climate-changed world,” said Robbie Parks, who coauthored the study on hurricane-related mortalities and works as a researcher at Columbia University’s Mailman School of Public Health.

As temperatures rise and weather extremes worsen, finding better ways to monitor and report climate-related illnesses and deaths will become increasingly urgent. A full understanding of the climate-driven death toll in the U.S. isn’t just good practice, public health officials and researchers told Grist — it’s also essential for preventing future deaths.

But major obstacles stand in the way. The biggest is that properly diagnosing a death as climate-related requires time, training, and resources that many of the nation’s roughly 3,500 health departments don’t have. While Maricopa County carefully combs through every suspected heat-related death that occurs in the county during Arizona’s long summer, it’s an outlier in that respect.

Read the full article about climate-driven deaths by Zoya Teirstein at Grist.