As temperatures spiked across a large part of the Northern Hemisphere last summer, I got an alarming call from my mother, who was living in Linz, Austria. She was dizzy and disoriented, and she hadn't been sleeping.

The region had been suffering through several weeks of above average day and nighttime temperatures, and when I arrived, her apartment building felt like a concrete oven. Her symptoms sounded like heat exhaustion.

We helped her pack a bag, checked on an elderly neighbor with similar symptoms, and then left the sweltering city for a mountain lake like we were first-world climate refugees.

A study presented this week at a scientific conference in Vienna now shows that last summer's extreme heat was an "unprecedented" hemispheric event that would not have happened without heat-trapping greenhouse gas pollution, the researchers said, and that it lasted longer and was more widespread across the Northern Hemisphere than previously realized.

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All summers will be like last year if the world warms 2 degrees Celsius above pre-industrial levels, said the study's lead author Martha Vogel, an extreme-temperature researcher with ETH Zürich Institute for Atmospheric and Climate Science. Even with 1.5 degrees Celsius warming, 25 percent of the Northern Hemisphere will experience a summer as hot as the summer of 2018 two out of every three years, she said.

Research has shown that increasing global temperatures are causing more frequent, prolonged heat waves. But this study maps the spreading geographical extent of extreme heat events and shows last summer's heat as a hemispheric event.

In places like my mother's apartment building in Austria, that prolonged heat and high nighttime temperatures meant that the unairconditioned apartments—and the people inside—didn't have a chance to cool down. Similar conditions led to deaths elsewhere during the summer, including in Canada and Japan.

Such Widespread Heat Is New, and Worsening

The new study, expected to be published soon, analyzed climate data from 1958 to 2018, and measured the geographic extent of the 2018 heat waves, which affected 17 countries, from the Scandinavian Arctic across the wheat belts of France and Germany, to densely populated cities in North America and Asia.

"We found that it could not have occurred without human-induced climate change," Vogel said at the annual European Geosciences Union meeting in Vienna this week. None of the climate models could simulate such a widespread series of linked heat waves without including heat-trapping pollution in the calculations, she said.

From May to July, the heat waves affected 22 percent of the agricultural land and populated areas in the mid-latitudes of the Northern Hemisphere, from Canada and the United States to Russia, Japan and South Korea, killing hundreds of people, devastating crops and curtailing power production. On an average day during those heat waves, 5.2 million square kilometers (about 2 million square miles) were affected by extreme heat, Vogel said. At its peak extent in July, the affected area was twice as big.

The hemisphere experienced similar large-scale, deadly heat waves in 2012 and 2010, but the scientists did not find evidence of such large areas being affected simultaneously by heat before 2010. They calculated that, for every degree Celsius of global warming, the area of the Northern Hemisphere affected by extreme heat will grow 16 percent.

A separate attribution study of the 2018 heat wave in Europe concluded that global warming made the event twice as likely as it would have been in a climate unaltered by humans. The 2017 European heat wave, dubbed Lucifer, was made 10 times more likely by global warming, another attribution study determined. Last year, the top European science panel documented a big surge in extreme weather events of all kinds in Europe.

The new study shows that even small increases in global average temperatures can lead to significant increases in risks from extreme events.

It's a "stark reminder that global warming of only 2 degrees Celsius actually represents a climate very different from the one humans are accustomed to, and will bring a greatly increased risk of extreme events that will threaten human well-being," said UCLA climate researcher Daniel Swain, who was not involved in the new study.

And This Still Might Underestimate the Risk

The growing likelihood of widespread heat waves raises concerns about food security, as well as human health, and the impacts can ripple well beyond the affected countries, said co-author Sonia Seneviratne, a climate researcher with ETH Zürich.

The 2018 heat wave caused total losses of some crops in Germany and Austria, and spurred large-scale outbreaks of tree-killing bugs.

In the Scandinavian Arctic, temperatures soared to above 30 degrees Celsius (86 degrees Fahrenheit) for the first time on record, leading to unusual fires in the boreal forests of Sweden. Across Europe, river flows reached record lows in late summer and autumn, hampering commercial shipping and power generators that rely on rivers for cooling.

Expect more of the same in the future, said Colorado State University climate scientist Scott Denning.

"There's no question that as average temperature warms, the likelihood of what used to be extremely hot weather increases dramatically. In fact, the more 'above average' we're talking about, the bigger the increase in the likelihood of encountering weather hotter than that threshold," Denning said.

If anything, the new study may underestimate the impact of human-caused warming on extreme heat events, said Pennsylvania State University climate scientist Michael Mann. The computer models used for the study don't fully capture recently discovered changes to the jet stream that also affect the persistence and intensity of extreme events like heat waves, he said.

Watching Climate Change's Consequences

Austria's 2018 heat wave persisted. It ended up as the hottest year in the 251-year record, with millions of people facing dangerously warm conditions. We brought my mom to our home near Vienna, where she slept for most of two days.

I was flustered by the effects of climate change that were playing out in real time and affecting people close to me. And I was frustrated that the warnings about climate change and heat waves weren't getting through to some of her neighbors.

It can be hard to convince someone that their safe home could be a threat, but I explained the urban heat island effect as best I could. As a first step, we'll install an effective awning for the main south-facing window and balcony on her apartment, and we'll research air conditioning options.

Just a few months earlier, my partner and I had moved out of the urban downtown core of Vienna to a cooler clime, based in large part on concerns about the long-term livability of our apartment if we didn't add air conditioning. With the thick walls of our new apartment in a 500-year-old building, the temperature is cooler, offering at least a temporary refuge.