WASHINGTON (AP) — Climate change has tripled the frequency of atmospheric wave events linked to extreme summer weather in the last 75 years and that may explain why long-range computer forecasts keep underestimating the surge in killer heat waves, droughts and floods, a new study says.

In the 1950s, Earth averaged about one extreme weather-inducing planetary wave event a summer, but now it is getting about three per summer, according to a study in Monday’s Proceedings of the National Academy of Sciences. Planetary waves are connected to 2021’s deadly and unprecedented Pacific Northwest heat wave, the 2010 Russian heatwave and Pakistan flooding and the 2003 killer European heatwave, the study said.

“If you’re trying to visualize the planetary waves in the northern hemisphere, the easiest way to visualize them is on the weather map to look at the waviness in the jet stream as depicted on the weather map,” said study co-author Michael Mann, a University of Pennsylvania climate scientist.

Planetary waves flow across Earth all the time, but sometimes they get amplified, becoming stronger, and the jet stream gets wavier with bigger hills and valleys, Mann said. It’s called quasi-resonant amplification or QRA.

This essentially means the wave gets stuck for weeks on end, locked in place. As a result, some places get seemingly endless rain while others endure oppressive heat with no relief.

“A classic pattern would be like a high pressure out west (in the United States) and a low pressure back East and in summer 2018, that’s exactly what we had,” Mann said. “We had that configuration locked in place for like a month. So they (in the West) got the heat, the drought and the wildfires. We (in the East) got the excessive rainfall.”

“It’s deep and it’s persistent,” Mann said. “You accumulate the rain for days on end or the ground is getting baked for days on end.”

The study finds this is happening more often because of human-caused climate change, mostly from the burning of fossil fuels, specifically because the Arctic warms three to four times faster than the rest of the world. That means the temperature difference between the tropics and the Arctic is now much smaller than it used to be and that weakens the jet streams and the waves, making them more likely to get locked in place, Mann said.

“This study shines a light on yet another way human activities are disrupting the climate system that will come back to bite us all with more unprecedented and destructive summer weather events,” said Jennifer Francis, a climate scientist at the Woodwell Climate Research Center who wasn’t involved in the research.

“Wave resonance does appear to be one reason for worsening summer extremes. On top of general warming and increased evaporation, it piles on an intermittent fluctuation in the jet stream that keeps weather systems from moving eastward as they normally would, making persistent heat, drought, and heavy rains more likely,” Francis said.

This is different than Francis’ research on the jet stream and the polar vortex that induces winter extremes, said Mann.

There’s also a natural connection. After an El Nino, a natural warming of the central Pacific that alters weather patterns worldwide, the next summer tends to be prone to more of these amplified QRA waves that become locked in place, Mann said. And since the summer of 2024 featured an El Nino, this summer will likely be more prone to this type of stuck jet stream, according to Mann.

While scientists have long predicted that as the world warms there will be more extremes, the increase has been much higher than what was expected, especially by computer model simulations, Mann and Francis said.

That’s because the models “are not capturing this one vital mechanism,” Mann said.

Unless society stops pumping more greenhouse gases in the air, “we can expect multiple factors to worsen summer extremes,” Francis said. “Heat waves will last longer, grow larger and get hotter. Worsening droughts will destroy more agriculture.”