Scientists say a closely watched atmospheric pattern — the jet stream — is behind both the Canadian wildfires and the scorching heat in Texas, raising questions about how it shapes extreme weather events and whether climate change is disrupting its flow.
The jet stream, a ribbon of air that encircles the Northern Hemisphere at high altitudes, drives pressure changes that determine weather across North America. The jet stream’s wavy pattern creates areas of high and low pressure.
In recent months, the jet stream’s patterns trapped and stalled a ridge of high pressure over northern Canada, which caused a heat wave and primed the landscape for the wildfires that later sent smoke pouring into the Midwest and the eastern U.S. Earlier this month, another ridge of high pressure centered over Texas, sending temperatures soaring.
More than 100 million people in the U.S. faced either blistering heat or unhealthy air quality Wednesday.
In recent weeks, the jet stream has appeared unusual and disjointed, scientists say. Some researchers think climate change is disrupting its flow and causing it to bake regions in heat longer. They are concerned that changes in the patterns could cause extremes to increase more rapidly than climate models have projected as the world warms.
Michael Mann, a climate scientist at Pennsylvania State University, likened visualizations of the jet stream’s appearance in recent weeks to the swirling brushstrokes of a post-impressionist painter.
“I’m honestly at a loss to even characterize the current large-scale planetary wave pattern,” Mann tweeted this month. “Frankly, it looks like a Van Gogh.”
Those weather patterns are of particular interest to climate scientists because they add to research that suggests alterations in jet stream patterns could play a significant role in driving extreme conditions.
Jennifer Francis, a senior scientist with the Woodwell Climate Research Center in Massachusetts, said the recent weather patterns fit that theory.
“Climate change is pushing heat waves into more extreme territory every year,” Francis said. In a warming world, she added, “these kinds of wavy, blocky patterns” in the jet stream “are certainly consistent with what we expect to see more often.”
Whether a warmer atmosphere is monkeying with the jet stream is difficult to prove. Researchers are still investigating whether there’s a link between climate change and the jet stream and how those specific weather events fit.
Some scientists, like Francis, think global warming is causing the jet stream to grow increasingly wavy.
But Kai Kornhuber, an associate research scientist at Columbia University and a senior scientist at Climate Analytics, a nonprofit policy institute, said the theory is not yet proven and difficult to evaluate. Trends don’t stick out neatly in the available data, he said.
Kornhuber and Francis agree, though, that there is evidence that warming global temperatures are causing the jet stream to slow down and trap high pressure systems like the ones in Canada and Texas.
“We do see more persistent weather patterns, and persistence translates to higher impacts,” Kornhuber said.
Longer heat waves challenge people’s heat tolerances, and they also dry out soils and heat up the land, which can reinforce the jet stream pattern and create a self-fulfilling cycle.
“Once the heat wave gets established over land and the land heats up, that’s warm air that causes the atmosphere to bulge,” said John Walsh, the chief scientist at the International Arctic Research Center at the University of Alaska Fairbanks. “There is a positive feedback loop that contributes to the persistence and intensity of the heat wave.”
Scientists are still unspooling how global warming could affect the jet stream and what mechanisms could be responsible for the most significant changes. Francis said warming ocean temperatures and the melting of sea ice in polar regions could be contributing.
There is little doubt that heat waves are increasing in severity and frequency.
“Heat extremes do scale with mean temperature,” Kornhuber said.
Changes in the nature of atmospheric circulation and the jet stream could intensify those trends even further and more rapidly than scientists once thought.
“We need to consider if climate models are more conservative when it comes to extreme weather,” Kornhuber said.