Heat waves are dominating summer, killing thousands, and fueling wildfires. The world needs to prepare for more.

• Deadly heat waves are repeatedly pummeling most of the Northern Hemisphere this summer.
• This could be the new normal, since climate change has made heat waves more common, severe, and persistent.

Extreme heat has besieged nearly the entire Northern Hemisphere this year.

Currently, Europe is simmering in its third heat wave of the summer, fueling devastating wildfires and threatening millions of people. On Sunday alone, Portugal and Spain reported more than 1,000 heat-related deaths. In France, thousands of people fled wildfires. One United Kingdom airport suspended flights after its runway melted, according to Sky News, and another after its runway buckled in the heat. Wales recorded its highest temperature ever. UK meteorologists expect still higher temperatures on Tuesday.

Europe isn't the only place sweltering. Almost all of the Northern Hemisphere has experienced record heat this month, from China to North Africa to the United States, where forecasts indicate extreme heat could continue for two more weeks. These are the latest in a series of simultaneous heat waves across the planet this year.

"I wish I could say that it was abnormal," Deepti Singh, a climate scientist at Washington State University, told Insider.

"The likelihood of getting a heat wave is increasing simply because it's warming. And that's happening pretty much anywhere across the globe," said Singh, who lived through an unprecedented heat wave that caused more than 1,400 deaths in the Pacific Northwest last year. At the time, Europe was suffering extreme heat, too.

Concurrent heat waves are becoming more common as global temperatures rise. This summer may stand out as especially hot in recent memory, but scientists warn this could become normal. With the looming reality of vast, long-lasting, historic heat waves dominating summers, researchers say the world must cut greenhouse-gas emissions that are causing temperatures to rise.

At the same time, cities should get ready to take the heat. That means increasing greenery, to provide residents more shade and covering surfaces like asphalt, which heats up significantly in direct sunlight. Governments can adapt infrastructure to new extreme temperatures, to avoid railways buckling, cables melting, and power failing. It also means adding social infrastructure, like cooling centers, policies to protect people who work outdoors, and robust warning systems.

"There needs to be kind of a shift in the perception of what a heat wave actually means, that the heat wave is not some fun day at the beach, but that it's potentially dangerous to health," Kai Kornhuber, a climate physicist at Columbia University, told Insider.

The climate crisis is making heat waves worse

In March, record heat struck both poles at once, despite the Arctic and Antarctic being in opposite seasons. At the same time, a two-month heat wave was blanketing India and Pakistan. In June, heat-trapping weather systems scorched the US and Europe, setting all-time temperature records for the month, as more heat and wildfires devastated crops in Tunisia.

"These temperatures are occurring with only 2 degrees Fahrenheit of global warming and we are on track for 4 degrees Fahrenheit more warming over this century," Andrew Dessler, a climate scientist at Texas A&M University, told The Associated Press in June, as temperatures in his town breached 100 degrees Fahrenheit. "I literally cannot imagine how bad that will be."

Scientists don't always directly attribute particular heat waves to climate change, but further analysis can allow them to do so. At the tail end of the India-Pakistan heat wave, in May, scientists at World Weather Attribution conducted a historical data analysis to determine that climate change made that event 30 times more likely.

On the whole, rising global temperatures are making heat waves more common, severe, and lengthy. The 2018 National Climate Assessment found that the frequency of US heat waves has tripled since the 1960s, and that the average heat-wave season has increased by 45 days. The United Nations' Intergovernmental Panel on Climate Change expects a similar trend across the planet.

Simultaneous large heat waves are becoming the norm

As they happen more frequently and last longer, simultaneous heat waves in different locations are bound to occur more often.

"The way we define the concurrent heat wave is there's any two regions in the mid latitudes that are simultaneously experiencing large heat waves. And that is pretty much almost every day in the summer season," Singh said.

That's a recent development. In the 1980s, concurrent heat waves only occurred for 20-30 days each summer, Singh said. Global warming has driven a sixfold increase in the frequency of simultaneous heat waves over the last 40 years, according to a study she and Kornhuber co-authored, which was published in the Journal of the American Meteorological Society in June. The study also found that concurrent heat waves covered about 46% more space and reached maximum intensities that were 17% higher than 40 years ago.

This summer is only unusual in the context of a stable climate, according to Kornhuber.

"We're in a climate that is constantly shifting towards more extremes. From that perspective, it's exactly what we would expect, and what scientists have projected to happen for the last decade," Kornhuber said.

"We don't have to go down this path," he added, calling for a rapid reduction in greenhouse-gas emissions. "But if things keep developing the way they are, it's pretty clear that we will see more record-breaking extremes, and more concurrent extremes just as this year, and even more extreme."