Even for the Southwestern U.S., a record-breaking heat wave that gripped the region was particularly relentless—temperatures in Phoenix topped 115 degrees Fahrenheit for five days in a row. The extreme heat was not just uncomfortable or inconvenient, it quickly transformed the city into a deadly landscape: Hospitals reported treating people for third-degree burns and severe dehydration as temperatures hovered in the triple digits well into the night. A total of 12 people died from heat just in Phoenix.

The most worrying thing about the unusually high temperatures that sizzled the Southwest last week was that they’re not all that unusual anymore. A new study by the Natural Resources Defense Council (NRDC) shows the direct correlation between climate change, extreme heat, and summertime deaths. Heat-proofing cities and curbing emissions by adhering to the Paris agreement, it argues, could help the U.S. save thousands of lives in the near future.

“This report carries a dire warning: Reneging on our climate commitments could cause tens of thousands of Americans to die,” says Juanita Constible, special projects director in NRDC’s Climate & Clean Air program. “If carbon pollution isn’t reined in, climate change will continue superheating summer with terrible consequences for public health in some of our biggest cities.”

The study looks at the number of deaths on “dangerous summer days” in 45 U.S. cities with a population of one million or more. If climate change goes unchecked, about 150 Americans will die daily each summer due to extreme heat by 2040, with about 29,850 summertime deaths each year by the end of the century. That’s double the number of homicides the U.S. experiences annually today.

Due to urban heat island effect—where the hard surfaces of buildings and pavement can intensify excessive heat—residents of cities are at greater risk than their suburban or rural counterparts when temperatures rise. When a heat wave hits a large, densely populated city, it puts the most marginalized residents at risk, including elderly and lower-income Americans, and the tens of thousands of homeless residents living unsheltered on U.S. streets.

But extreme heat is particularly dangerous from a public health perspective because of the way it can exacerbate other conditions like poor air quality, which leads to long-term cardiovascular disease, and trigger bigger disasters like wildfires and drought, which can cause food insecurity. Additionally, cities must shoulder the costs of increased emergency room visits, which spike during heat waves—Phoenix ERs and burn centers were the busiest they’d been in years last week.

Phoenix is a city that routinely experiences 100-degree days. The situation may be even worse in cities that aren’t prepared for their suddenly intolerable summers.

Phoenix is, of course, a city that routinely experiences 100-degree days and has been working to address extreme heat for some time. The situation may be even worse in cities that aren’t prepared for their suddenly intolerable summers. Even cities with more temperate climates like Los Angeles must explore new ways to keep cool because a majority of residents don’t have air conditioning—they’ve never really needed it until the last few years. In a 2003 European heat wave that was estimated to be the continent’s hottest summer since the 16th century, at least 35,000 people died as a result of record-breaking temperatures. More recent analysis revised the death toll to 70,000.

Cities can combat extreme heat by rolling out simple, effective solutions, according to the NRDC report: Planting more trees, painting roofs white, using alternatives to asphalt, building structures that don’t act like giant magnifying glasses.

But since the fastest-growing contributor to climate change is transportation, this is where the decrease in emissions really needs to occur. And of course, more hot days means more people running their air conditioners, putting more stress on an already overtaxed grid (which is likely running on fossil fuels that make climate change worse), so any switch to renewable energy generation helps.

If all 400 U.S. mayors who have pledged to uphold the Paris accord follow through with their climate commitments, it will not only make their neighborhoods more livable but also avoid needless deaths. Cooling cities will mean saving lives.