If you live in the southern U.S., you don't need us to tell you it is hot.

ASHLEY HARDY: To me, it feels like it gets hotter every summer.

KELLY: For Ashley Hardy in Dallas, this punishing heat wave means walking her dog early, before it gets truly unbearable, and keeping that walk short.

HARDY: We're probably going to be out here for less than 30 minutes total and then be back in the AC.

KELLY: Even the squirrels are suffering. Inks Lake State Park in Burnet, Texas, posted a Facebook video of a squirrel splayed out flat on a shady patch of concrete captioned, I feel ya.

CARLOS BOTERO: What they're doing is they're putting their body in contact with a surface that's a little bit cooler than them.

KELLY: Carlos Botero is a biologist at UT Austin.

BOTERO: Basically, what that helps is in dumping all the extra heat that they have to, like, lower down their body temperature.

KELLY: It's a strategy called splooting. And Botero says if you see a squirrel doing it, you know the heat has gotten bad.

BOTERO: It may look a little bit cute, but it's actually an indication that these guys are in trouble.

KELLY: And millions of people are, too, as the heat wave spreads east. More than 60 million Americans are expected to face dangerous levels of heat over the next week as the Mississippi Valley and central Gulf Coast begin to experience what Texas has been dealing with for weeks.

VICTOR MURPHY: This would have been a case of pick your poison.

KELLY: National Weather Service meteorologist Victor Murphy is based in Fort Worth.

MURPHY: Parts of the state have seen all-time record-high temperatures. The other side of the coin has been just very, very high dew point temperatures, which is a way of measuring the amount of moisture in the air.

KELLY: Across the South, heat index temperatures, meaning what it feels like outside, are expected to top 110 degrees in many places. Extreme heat kills more people every year than any other natural disaster. Already, Texas has recorded more than a dozen heat-related deaths. The high temperatures are driven by a weather phenomenon known as a heat dome.

MURPHY: Well, heat dome is basically a large area of high pressure aloft - very stable, pretty much precludes any development of any clouds or even showers. Just a lot of hot air trapped.

KELLY: You can think of it like putting a lid on a pot. As for what is making this heat dome so extreme...

MURPHY: It's hard to attribute one specific event to climate change. But I do see a couple of climate change fingerprints, shall we say, at the scene of this crime.

KELLY: CONSIDER THIS - climate change is making extreme heat events, like this one, more likely. We'll hear how a county in Oregon is going to court to argue that oil companies should be responsible for the damage heat waves cause.

KELLY: From NPR, I'm Mary Louise Kelly. It's Wednesday, June 28.

It's CONSIDER THIS FROM NPR. The U.S. is not the only place facing record-breaking heat this year. China and India have faced oppressively hot weather, too. Scientists say climate change is a major driver. And something else is coming into play - the El Nino climate pattern. Lauren Sommer from NPR's climate desk has more.

LAUREN SOMMER, BYLINE: When it comes to understanding how climate change is making weather more extreme, heat waves are pretty much the clearest example out there.

KAI KORNHUBER: Heat waves are likely the one type of extreme weather events that is most directly associated with climate change.

SOMMER: Kai Kornhuber is a climate scientist with Columbia University and Climate Analytics, an extreme weather research group. He says the Texas heat wave, where records have been shattered for days in a row, shouldn't be surprising. When temperatures rise due to burning fossil fuels, it pushes heat waves into a new category.

KORNHUBER: They get hotter. They are occurring in higher frequency. So that also increases the likelihood of sequential heat waves.

SOMMER: And heat waves get longer, which means they take a bigger toll on our health. Extreme heat is the deadliest weather-related killer especially in low-income and communities of color. And this year, there's something added - El Nino. It starts when the ocean in the central and eastern Pacific gets hotter. Daniel Swain, a climate scientist at the University of California, Los Angeles, says that makes the planet warmer.

DANIEL SWAIN: That's its role in the global climate system, is moving some of the energy up from depth and dumping it into the atmosphere.

SOMMER: This year, scientists are predicting El Nino could be quite strong. But it's just getting started, and there's a lag time.

SWAIN: That lag is because, of course, it takes some time for that extra heat near the surface of the ocean to actually make it into the atmosphere and be moved around by wind currents.

SOMMER: And because of El Nino, scientists say this year could be one of the hottest ever recorded. The last eight years were already the hottest since record keeping began.

SWAIN: Of course, the long-term driver is human-caused climate change, where we're sort of stair-stepping up along that inexorable upward trend.

SOMMER: El Nino is kind of like an exclamation point on that trend, he says, which means more heat waves are on the way in the near term. In the long term, avoiding more dangerous extremes will take cutting fossil fuels.