Cities across the United States are preparing for what could be a sweltering summer, enacting rules to protect people during heat waves and experimenting with new ways to communicate the risks of extreme temperatures.

The measures can’t come soon enough, even with the official start of summer still weeks away.

More than a week ago, temperature records were set in Mississippi and Texas, after conditions reached nearly 100 degrees Fahrenheit in some areas on May 21. That same weekend, a heat dome baked much of the Southeast up through New England.

It was the kind of early season heat wave that serves as an important reminder of the risks of extreme temperatures, said Kristina Dahl, a principal climate scientist with the Union of Concerned Scientists. And with heat waves becoming both more frequent and more severe because of climate change, Dahl said it should be a wake-up call for the entire country.

“Heat is dangerous no matter where you are or how used to it you think you might be,” she said.

Even in parts of the country known for milder conditions, climate change is forcing states to confront extreme heat events.

Earlier this month, Oregon’s workplace safety agency adopted some of the nation’s strongest rules to protect workers, particularly those in agriculture and other outdoor professions, from heat-related illnesses and death. The rules, which kick in when temperatures exceed 80 degrees Fahrenheit, require employers to provide more breaks as well as access to ample shade and drinking water.

Last year, a devastating heat wave hit the Pacific Northwest, causing more than 100 deaths across Washington and Oregon. The dayslong event, in a region generally unaccustomed to temperatures in the triple digits, offered a shocking glimpse of what could happen in places that are not adequately prepared.

Efforts to implement strong heat protections in the region show just how far-reaching the effects of climate change are, said Jamie Pang, an environmental health program director for the nonprofit Oregon Environmental Council.

“It’s powerful that a state in the Pacific Northwest took the lead in formulating these protections for workers,” she said. “It’s really a sign that climate change will impact everyone.”

Last week, the National Oceanic and Atmospheric Administration released its summer outlook, forecasting above-normal temperatures for nearly every part of the contiguous United States from June through August.

Warmer-than-usual conditions fueled by climate change essentially load the dice for extreme heat events, Dahl said.

“As you increase the average temperature in any given region, it makes it that much more likely that you’re going to cross the threshold to conditions that we could consider extreme,” she added.

In addition to making heat waves more frequent and intense, climate change is also lengthening the “heat season,” with extreme heat events occurring earlier than usual — in April and May, in some cases — and often stretching into fall.

“We have to decouple summer and heat,” said Kathy Baughman McLeod, director of the Adrienne Arsht — Rockefeller Foundation Resilience Center at the Atlantic Council, a Washington, D.C.-based think tank. “There’s this cultural concept of summer, where we think about vacations and beaches, but a lot of people don’t understand how dangerous heat can be.”

Baughman McLeod calls heat a “silent killer” because symptoms of heat-related illness can go unnoticed until it’s too late. And though forecasts can give people advance warning of coming heat waves, it’s often difficult to convey risks that can’t be seen, she said.

“It’s not like the drama of hurricanes, where you have roofs ripping off of houses or cars floating down streets,” Baughman McLeod said. “If you can’t see it, and you can’t hear it, it makes it really hard to convey how deadly it is.”

Her research at the Atlantic Council includes how to better communicate the risks of extreme heat to the public. One initiative involves naming and categorizing heat waves to make it clear when people should exercise caution and what steps to take.

A pilot program to gauge the effectiveness of these communication strategies will be underway this summer in Seville, Spain, and Athens, Greece, as well as in Los Angeles and Miami in the U.S.

California Assemblymember Luz Rivas, D-San Fernando Valley, introduced legislation earlier this month that would similarly create an early warning system that ranks extreme heat events based on their severity and potential health impacts. The bill passed this week in the Assembly and will next be voted on in the state Senate.

Rivas said her first eye-opening experience with extreme heat came in 1995 in Chicago, during her first job out of college. That July, an intense heat wave sent temperatures soaring into the triple digits for three straight days, leading to more than 700 heat-related deaths in the city.

More recently, Rivas said she has seen firsthand how extreme heat has impacted people in her community.

“My district is majority low-income essential workers and a lot of people who work in construction and other fields where they have to be outside,” she said.

She said her work took on a new sense of urgency after seeing what unfolded in Oregon and Washington last year.

“What happened in the Pacific Northwest made it real,” Rivas said, “It could happen in California. And it already is.”

Pang said it’s encouraging to see states introduce legislation to address extreme heat, but more still needs to be done. Initiatives that expand access to air conditioning, for instance, or provide assistance to people who are struggling with energy costs could prevent deaths during heat waves, particularly in low-income communities, she said.

“We need national standards and state standards before tragedy hits, and not as a reaction to it,” she added.

Pang said she hopes awareness that nobody is immune to global warming and its consequences, including extreme heat, is growing.

“The idea that the Pacific Northwest or any other region is somehow a refuge is clearly not true,” she said. “Climate change is an existential threat to everyone, everywhere.”