Extreme heat becomes common in Denver as city seeks climate solutions

One of the most life-threatening impacts of climate change in Denver is silent, invisible and becoming increasingly common.

As temperatures reaching triple digits become less unusual, the same oppressive heat that wilts gardens and sends hordes to local pools can pose a serious risk to people’s health. The frequent headlines noting record-breaking heat are unlikely to stop as climate change shifts weather patterns and makes Denver’s summers hotter.

“We’ve had hot days in Denver, but these long stretches of extreme heat aren’t something that everyone is used to,” said Lis Cohen, climate adaptation and resiliency manager at the city’s Office of Climate Action, Sustainability and Resiliency.

Before a much-anticipated cooldown Thursday, Denver’s temperatures broke a series of records. The high of 102 recorded on Aug. 4 was the highest for that date, greatly exceeding the previous record of 98 degrees. Similar records were broken on July 12, 14 and 29, according to the National Weather Service.

As global temperatures continue to rise, Denver is not the only city working to adapt to rising temperatures while contending with infrastructure that wasn’t built for extended periods of heat. NASA on July 21 announced that that day was the hottest in Earth’s recorded history.

The record was promptly broken the next day — and July 22 remains the hottest day ever recorded on Earth, according to NASA.

Average summer temperatures in Denver have risen 3 degrees since 1970 and are expected to continue rising as the climate changes, according to Climate Central, a nonprofit news organization that reports on climate science.

Even when Denver’s heat isn’t record breaking, it’s persistent. Twenty days in July exceeded 90 degrees. Data from the National Weather Service show that 100-degree days are becoming more frequent, as are streaks of consecutive 100-degree days.

So far this summer, Denver has recorded six days with temperatures above 100 — the third-highest number of 100-degree days on record, behind 2005 and 2012, according to the weather service.

Those sweltering days can be dangerous.

“It’s important for us to understand that extreme heat is a risk for everyone and everybody,” said Lisa Romero, a senior community health specialist for Kaiser Permanente Colorado.

“Becoming a more significant risk”

Heat illnesses range from dehydration to heat stroke, which can be deadly and cause permanent disability. Heat illnesses occur when the body becomes so hot it cannot properly cool itself through sweating.

Heat can also exacerbate other health conditions, such as asthma and chronic obstructive pulmonary disease, because it increases the amount of ground-level ozone, according to the U.S. Department of Health and Human Services.

Those most at risk of heat illness include older people, children, people with certain preexisting medical conditions, outdoor workers and people without adequate cooling in their homes, Romero said.

“In places where heat is emerging and becoming a more significant risk, there is that work to do,” she said.

On a broader scale, the rate of medical visits for heat-related illnesses has reached the highest level since the Colorado Department of Public Health and Environment started collecting data in 2019. The data is collected between May and September in 13 counties, mostly on the Front Range. The others are Mesa and La Plata counties.

In 2019, the state recorded 258 visits for heat illnesses in those counties. That is a rate of 5.6 visits per 10,000 residents. So far in 2024, the state has documented 653 visits for heat illnesses — a rate of 10.5 visits per 10,000 and the highest number recorded in any year.

“We know heat is already a risk — it’s already the deadliest weather phenomenon — and we know that heat is going to increase with climate change,” Romero said. “So thinking about how we can mitigate that risk and those vulnerabilities is really important. We need to have an overlapping comprehensive approach to addressing that risk.”

Same city, different heat

Cohen, the city’s climate adaptation manager, sometimes watches the temperature reading on her car dashboard as she drives around the city. It can swing up to 12 degrees, depending on where she is, from the hot concrete of downtown to the greener spaces along City Park.

“It’s getting hotter,” she said. “The heat waves are happening more often. And people are not experiencing the same heat in different parts of the city.”

It’s part of her job to help city residents find solutions for heat risks. Those solutions vary widely, but Cohen and her team are focusing first on the neighborhoods most affected by rising temperatures.

To know where to go, Denver’s climate action office uses a map that shows how vulnerable each neighborhood is to heat. The analysis crunched a wide swath of data on neighborhoods: tree coverage, access to vehicles, surfaces that increase heat, race and ethnicity, education levels, household incomes, age distribution, age of the housing stock, health indicators.

Many of the poorest and most racially diverse neighborhoods are also the most at-risk to heat, the map shows.

People with lower incomes are more impacted by heat in myriad ways, Cohen said. Those who don’t own a car must walk or bike outside and wait in the sun for buses. They are less likely to be able to afford a newer home with air conditioning or portable cooling units, not to mention the higher energy bills associated with hot days.

Areas with many buildings, lots of pavement and few trees can be several degrees hotter than greener spaces — a phenomenon known as an urban heat island. An analysis released last month by Climate Central showed that Denver’s downtown core can be more than 12 degrees hotter than other areas of the city.

Those areas also stay warmer longer into the night, as asphalt, concrete and tall buildings retain heat, Cohen said.

Part of the city’s long-term plan is to plant more trees in neighborhoods without much tree coverage. Not only do trees provide shade, they also improve air quality and mental health, Cohen said. So far, the city has worked with local nonprofits to plant more than 1,300 trees on private property.

That work will continue in coming years, as part of the climate office’s $5 million allocation from the federal Inflation Reduction Act.