Springtime is underway in the southern Hemisphere, but across much of South America it has felt like the depths of summer for months already. A string of heat waves have settled in over the region, pushing temperatures into record-breaking territory month after month.

Last week, temperatures soared in southern Brazil. In Rio de Janeiro, a city of nearly 12 million people, intense heat and humidity pushed a 23-year-old Brazilian university student into cardiac arrest at a Taylor Swift concert. Fans had stood in line for the Eras Tour at the Nilton Santos Olympic stadium in brutally hot, humid, windless conditions for hours before the Friday night show. It was just as hot and steamy inside the venue, concertgoers reported.

The woman who died, Ana Clara Benevides Machado, got medical attention from paramedics at the concert venue, but died later at a nearby hospital.

Rio's temperatures last week topped 100 F. But the heat index–a measure that takes into account both air temperature and humidity–made it feel like it was nearly 140 degrees Fahrenheit. People can only handle heat like that for a few hours before they start to get sick–or even die.

Brazil's Ministry of Culture noted the extreme, dangerous heat in a statement expressing condolences for Machado's death. This is a clear signal that climate change, the ministry said, has to be considered a major risk for events like big concerts or other cultural events now. Swift postponed a concert planned for Saturday night, another day that was supposed to be dangerously hot.

Recording-breaking heat

The heat wave was the eighth major one of the year in Brazil, says Lincoln Alves, a climate scientist at Brazil's National Institute for Space Research. And it was almost certainly intensified by climate change, says Alves. He and colleagues analyzed a similar September heat wave, which was at least 100 times as likely because of climate change.

The past six months have each sequentially broken regional heat records, says Raul Cordero, a climate scientist at Chile's University of Santiago. "October, it was the warmest October on record. September, it was the warmest September on record. And so on, since last May." He pauses, and repeats it. "Six months we have seen [record-breaking heat], in a row!"

It's extra hot across South America in part because the region is in the throes of El Niño, which pushes temperatures up a few degrees both regionally and globally. But that warming sits on top of long-term climate warming, driven primarily by the burning of fossil fuels.

"This is not a coincidence what is happening, not only in Rio de Janeiro and in São Paulo in southern Brazil but also in Bolivia and Paraguay, and in Gran Chaco. All over. And a little further north in Brazil, not only are there high temperatures but very severe drought," says Cordero. "It's a huge problem that's affecting not only southern Brazil but the whole subcontinent."

Average temperatures in São Paulo have gone up by more than 3 degrees Fahrenheit since the 1960s.

A few extra degrees of warming may not sound like much, says Alves, but the increase in the number of extreme heat days has skyrocketed. In the 1960s, there were about seven days of serious heat in the region–about one major heat wave a year. Now, annually there are more than 50 days of extremely hot weather, or about 9 major heat events. That number is forecast to increase further in the future.

How heat kills

Air temperatures in Rio de Janeiro were sweltering last week as concertgoers waited to get into the Nilton Santos stadium for Swift's Eras Tour Friday night. People waited for hours in the sun to get into the venue, and many didn't have water to drink.

High humidity was the other problem. People cool down by sweating: when water evaporates, it pulls away the heat that has built up in the body. But when the air is intensely humid–in other words, when it's holding nearly all the water vapor it can–that sweat doesn't evaporate. It stays beaded up on the skin, useless.

"When we're thinking about the real big dangers to the human body, humid heat stress is one of the biggest," says Daniel Vecellio, a climate scientist and heat expert at George Mason University. "When it starts to get really humid, we can sweat as much as we want to, but if that sweat can't evaporate...that basically shuts off our main physiological mechanism to be able to cool ourselves down."

The air in Rio last week was still and stagnant making it nearly impossible for sweat to evaporate. The air was heavy with humidity.

The body can also cool down by shunting blood toward the tinier vessels near the skin, where it can–hopefully–come into contact with cooler air. That puts stress on the heart, which needs to pump harder to move blood around. That's why heart problems, like the one that killed Machado, go up during heat waves, says Veliccio.

It's not like people in Brazil are unaccustomed to heat, says Alves. "But these times, in September, October, right now, the temperature puts too much pressure. Even these people who are, I would say, more familiar with these kinds of climates, face stress based on these extreme events."

Making heat less dangerous

Heat as all-encompassing and extreme as last week's in Rio will always be dangerous, says Marisol Yglesias-González, a climate expert at the Centro LatinoAmericano de Excelencia en Cambio Climático y Salud in Costa Rica. But warning people in advance of extreme heat, for example, can help reduce the dangers. Designing emergency heat plans at venues like the Nilton Santos stadium and other public venues is another way to lower risks brought on by hot temperatures.

Some of the work to reduce heat risks can come from governments. Cities, where vast amounts of concrete absorb heat and push temperatures up, can develop green spaces or cooling centers. National meteorological systems can send out early warnings to help people plan for the worst times, though it's crucial to design effective warnings that reach everyone they need to, stresses Yglesias-González. Brazil's meteorological agency sent out heat warnings last week.

Efforts need to happen in the private sector, as well. The Brazilian Ministry of Culture stressed in its statement that the new risks of climate change entail coordinated efforts from event hosts. Emergency heat protocols are critical, says Yglesias-González. For example, the Nilton Santos stadium prohibited concertgoers from bringing water bottles inside. That led to dehydration for many fans. Brazil's justice minister said on X, formerly Twitter, that water bottles would be allowed into venues in the future.

"They were not allowing people to bring their bottles of water into the venue? Like really, this is not the 70s, we don't have the weather of the 70s! We're facing an existential crisis with climate change," says Yglesias-González.

"If we're going to do these types of events, we have to acknowledge that climate change is a risk. And be prepared for that, to protect the people that we are bringing to see this type of show."

That means everyone, from private companies and city governments to nonprofits, needs climate plans, she says.

Because of the ongoing heat, Swift postponed a show planned for Saturday. Billboard, which has begun tracking concerts affected by climate-influenced extreme weather, has counted 30 shows postponed or canceled so far in 2023 due to heat, floods, and other weather problems.

Adapting to the heat problems that exist, and will continue to worsen as climate change marches onward, is one half of the challenge, says Cordero, the Chilean climate scientist. The other half is tackling the root cause of human-driven climate change: drastically reducing planet warming pollution.

Swift, like other members of the richest 1% of Earth's inhabitants, has a disproportionately high impact on climate change. That group alone is responsible for about 20% of global emissions, according to a new report from Oxfam released this week.