GEOFF BENNETT: The World Cup kicked off in Mexico today, as South Africa squared off against Mexico, which is one of this year's host countries.

Matches get under way tomorrow in the two other hosting countries.

That's the U.S.

and Canada.

Notably, several American cities hosting these opening matches will be sweltering this weekend, making stadiums feel more like a sauna than a playing field.

The heat and humidity can take a major toll on player performance and fan safety.

Climate Central's Ben Tracy shows us how extreme heat is changing the game in our warming world.

It's part of our series Tipping Point.

MARISA ABEGG, Former Professional Soccer Player: Follow through where you want the ball to go.

Perfect.

BEN TRACY: Marisa Abegg has always gotten her kicks on the soccer field.

MARISA ABEGG: You're going to take that touch across your body and then pass it back.

Kept me sane as a kid.

Got to run around a ton.

BEN TRACY: For her, the World Cup was must-see TV.

MARISA ABEGG: I didn't even really know what Super Bowl was growing up, right?

The '99 World Cup was absolutely my Super Bowl.

ANNOUNCER: Abegg clears it.

BEN TRACY: She played two seasons of professional soccer, where sometimes the rivalry got too heated.

ANNOUNCER: And a sliding tackle from Abegg.

MARISA ABEGG: You just know how miserable it is to run around in hot weather, because it plays such a huge impact on the way you play.

BEN TRACY: This summer, extreme heat could be a real risk for both players and fans at the World Cup.

DOUGLAS CASA, CEO, Korey Stringer Institute: It has the potential to be some of the most extreme environmental conditions for soccer in the history of America.

BEN TRACY: Doug Casa runs the Korey Stringer Institute named after the NFL player who died from heatstroke in 2001.

DOUGLAS CASA: Pick the population.

If they have to do their job or sport in the heat, we're studying them.

BEN TRACY: Inside their heat labs at the University of Connecticut, researchers study how athletes and workers can stay safe and still perform in extreme conditions.

Before the 2021 Tokyo Olympics, the U.S.

women's national soccer team trained here to prepare for Japan's oppressive heat and humidity.

Which World Cup sites are you most concerned about?

DOUGLAS CASA: Without question, Miami.

It could easily be a 100-degree day and 80 percent humidity.

BEN TRACY: Climate Central analyzed temperature trends at all 16 World Cup sites, including this outdoor stadium in Miami, which now sees 14 more extremely hot June and July days due to climate change.

Heat and humidity push an athlete's core body temperature higher, giving those playing indoors an edge.

DOUGLAS CASA: Because, if your temperature is lower, they just perform better.

They can cover more distance, have more sprints, faster sprints.

All of those things can happen if you're less dehydrated and you're less hyperthermic.

BEN TRACY: So you're blowing heat into the room right now to make this uncomfortable?

REBECCA STEARNS, COO, Korey Stringer Institute: Yes.

BEN TRACY: I can feel it.

REBECCA STEARNS: Here we go.

BEN TRACY: KSI chief operating officer Rebecca Stearns showed me how quickly heat can impact performance... REBECCA STEARNS: In the heat, your heart rate is showing much higher stress.

BEN TRACY: ... by cranking up my pace and the temperature.

REBECCA STEARNS: So, right now, the room is about 100 degrees.

BEN TRACY: So it's like going for a run in Phoenix.

REBECCA STEARNS: Yes.

BEN TRACY: The test measures how fast my body heated up.

REBECCA STEARNS: And your body temperature right now is 102.5.

BEN TRACY: All right, so that's toasty.

REBECCA STEARNS: Yes.

BEN TRACY: We don't want to go above 104.

REBECCA STEARNS: One hundred and four is the max.

BEN TRACY: Above that, the risk of heatstroke rises, potentially leading to organ failure or even death.

DOUGLAS CASA: The outcome of our heatstroke all boils down to the number of minutes that the body is above approximately 105 degrees.

We literally don't know of anyone yet on planet Earth who's ever died from a heatstroke if their temp gets under 104 within 30 minutes.

BEN TRACY: Casa says a World Cup heatstroke is unlikely, but believes FIFA's new three minute hydration breaks don't go far enough.

DOUGLAS CASA: That is just absolutely not long enough to have an impact.

My suggestion would make that five or six minutes.

BEN TRACY: Players have been trying to acclimate to warmer temperatures before the tournament begins.

FIFA says it's limiting outdoor matches during the hottest parts of the day and adding cooling areas for fans.

But Casa is still concerned for stadium workers and spectators, especially those in sunbaked locations like Miami.

DOUGLAS CASA: I believe two of their games are happening at 5:00 p.m., but because we're near the summer solstice, 5:00 p.m.

is basically like the middle afternoon in Miami.

And so a big chunk of the stadium is still going to be in full sun.

So I ask myself, a game that's scheduled for 5:00 p.m., why are we not playing at 8:00 or 9:00 at night?

MARISA ABEGG: Look at you.

Oh, my gosh.

Is your name actually Mia Hamm?

(LAUGHTER) BEN TRACY: Marisa Abegg is now coaching youth soccer.

MARISA ABEGG: Nice.

BEN TRACY: Just before our interview, she canceled practice because of a heat wave.

MARISA ABEGG: I think it's just going to play more and more of a role as we go forward.

BEN TRACY: She still has World Cup fever, but is trying to keep it cool.

MARISA ABEGG: I think we're going to a game in Boston in June, and I'm already considering, like, well, what's the temperature going to be like?

BEN TRACY: Do you feel for these teams that are going to have to play in Miami in the middle of the summer?

MARISA ABEGG: Oh, my gosh, 100 percent.

Like, I don't even want to watch in that weather, right?

BEN TRACY: For the "PBS News Hour," I'm Ben Tracy with Climate Central.