From Miami to Melbourne, a quiet revolution is underway to fend off a silent and invisible killer

A quiet revolution is underway to address a widely underestimated climate challenge: extreme heat.

Local authorities have appointed several chief heat officers (CHOs) in cities worldwide in recent years to prepare residents for increasingly frequent and severe bouts of excessive heat.

"They call it the silent killer," said Eleni Myrivili, who serves as the global CHO for the U.N.'s human settlement program and previously worked in a similar role for the Greek capital of Athens.

Myrivili said she believes that extreme heat is often overlooked because it lacks the visible drama of roofs being ripped from homes or streets being turned into rivers.

"Heat, I believe it to the bottom of my heart, is going to be the number one public health challenge that we will be dealing with in the next decade. And we need to prepare for it now," Myrivili told CNBC via videoconference. "We can — but we really need to make it a priority."

Heat is the leading weather-related killer in the U.S. Data from the Centers for Disease Control and Prevention showed that more than 1,700 deaths were the result of heat-related causes in 2022, roughly double the toll of five years prior. Researchers have said these are likely conservative estimates.

The CDC defines extreme heat as summertime temperatures that are significantly hotter and/or more humid than average.

Older adults, young children and people with chronic diseases are recognized as among the most at risk of heat-related illnesses, such as heat exhaustion or heat stroke. The CDC warns that even young and healthy people can be affected.

The first person in the world to be assigned as a CHO was Jane Gilbert, who was appointed in 2021 to oversee Florida's most populous county, Miami-Dade.

The CHO role was created through an initiative by the Adrienne Arsht-Rockefeller Foundation Resilience Center (Arsht-Rock), a U.S.-based think tank that aims to support 500 million people with heat-resilience solutions by 2030.

"We have relatively high [air-conditioning] penetration, but with our rising temperatures, electricity bills are just through the roof. We've also had the electricity rates go up. AC can be over 50% of what the electricity bill so people are choosing between AC and putting food on the table for their families," Gilbert told CNBC.

A coastal metropolis in the southern U.S., Miami is internationally known for its vulnerability to sea-level rise and hurricanes. Yet Gilbert said community-led surveys have identified chronic heat as the most pressing climate concern.

For six months of the year, Gilbert said temperatures in Miami exceed 90 degrees Fahrenheit (32.2 degrees Celsius) almost daily, posing a particularly big problem for outdoor workers.

To help reduce the risks to the county's population of 2.7 million, Gilbert said her team's action plan focused on informing and preparing people for extreme heat, helping to cool homes affordably and working to cool community neighborhoods to tackle the so-called "heat island effect" — whereby a city incurs much warmer temperatures than nearby rural areas.

In practice, Gilbert said the measures included broad-scale marketing campaigns targeting the zip codes and demographics known to be most at risk, working with the national weather service and emergency management teams to update advisory and warning levels. They also involved installing 1,700 efficient AC units in public housing and ensuring that new affordable housing requires the most efficient cooling systems, such as cool and solar-ready roofs, to keep utility costs down.

"We want to address the root cause of this problem while we're helping people adapt," Gilbert said.

"All of us here have grown up in a typically hot and humid environment. We are used to the heat so that makes it really hard to distinguish between normal heat and unsafe heat," Bushra Afreen, CHO for Dhaka North in Bangladesh, told CNBC via videoconference.

Afreen, who became Dhaka North's CHO in May last year, said stark income inequality in the country's largest city meant excessive heat was not a universally similar experience.

"When you combine that with fragile urban systems like drainage and power outages and poor health management and poor health systems and poor education systems, you get a very bad stew."

Alongside planting thousands of trees in Dhaka North's informal settlements and reintroducing a culture of water fountains in the city, Afreen said her team would roll out a pilot project in one city settlement to create green nooks and corners for reprieve.

Afreen said it would be important to consider the type of trees to plant, such as citrus or neem trees to ward off mosquitoes amid a dengue outbreak. Sufficient lighting, a bench, CCTV cameras, a water fountain and signs urging priority for women and children would also be necessary, she added.

"Right now, the two reactions that we're seeing most are 'good job, keep it up, we need more awareness,'" Afreen said.

"And the other kind is, 'oh, you're going to decrease the heat? Good luck.'"

Tiffany Crawford, co-CHO of Melbourne, told CNBC that extreme heat kills more people in Australia than bushfires, floods and storms.

"There's a reason for that, and it's the lag in the data," she said.

Crawford, who works alongside Krista Milne as CHOs of Melbourne, said the true scale of heat-related deaths and illnesses often doesn't became clear until health authorities have pored through hospital admissions and ambulance data.

With a population of roughly 5 million, the southeastern Australian city of Melbourne is known for its mild and temperate climate — but Crawford says it is prone to spates of summer heatwaves that last for several days and offer scant reprieve through the night.

"There's an extreme northerly wind blows that is just ferocious. I liken it to going outside and it's like someone left the oven door open or the heater on all night and forgot to turn it off," Crawford said.

Some of the short-term interventions that have been put into place in Melbourne include extending public library and pool hours and rolling out so-called cool kits, which contain water bottles, neck towels and old-fashioned fans.

Looking ahead, Crawford said the city was in conversation with Google to provide constituents with so-called online-mapped "cool routes," which help users navigate the city by taking advantage of existing shade or canopy cover.

"In places like Europe, the dialogue in the media is a bit different, the heat is shocking. Whereas in Australia, the heat is something that was consistently lived with, and we will continue to live with it, but it is those variables, like any climate response, they are becoming more and more pronounced," Crawford said.

"We need to plan around that."