The jet stream is a key link in climate disasters
Devastating floods destroyed cities in Germany and Belgium. A relentless heat wave has roasted the western United States and Canada. Heavy rains paralyzed a Chinese industrial hub housing 10 million people. These recent weather phenomena are intensified by climate change.
But the connection between these far extremes goes beyond warming global temperatures. All of these events are affected by jet currents, strong, narrow bands of westerly winds blowing above the Earth’s surface. Currents are generated when cold air from the poles collides with warm air from the tropics, creating storms and other phenomena such as rain and drought.
“Jet streams are weather – they create it and direct it,” said Jennifer Francis, senior scientist at the Woodwell Climate Research Center. “Sometimes the jet stream takes a very convoluted shape. When we see it taking big swings to the north and big troughs to the south, we know we are going to see unusual weather conditions. “
Meteorologists worry whenever these oscillations and troughs form omega-shaped curves that resemble waves. When this happens, warm air moves further north and cold air enters further south. The result is a succession of unusually hot and cold weather systems along the same latitude. Under these conditions, winds often weaken, and dangerous weather conditions can get stuck in one place for days or weeks at a time – rather than a few hours or a day – resulting in prolonged rains and heat waves.
“It’s like when ocean waves hit a beach, topple over and break,” said Tess Parker, a researcher at the ARC Center of Excellence for Climate Extremes at Monash University in Melbourne, Australia. “It can also happen in the atmosphere, and if it does, you tend to get a high or low pressure system that will become stationary.”
This is what plunged parts of Germany into flooding earlier this month, as a low pressure system blocked over the western region of the country. Heavy rains drenched the terrain for the first two days, followed by a few hours of even more intense rainfall that caused rivers to overflow. Water and mudslides have invaded homes and roads, killing more than 170 people and leaving hundreds missing. Heavy rains also flooded parts of Luxembourg, Belgium and the Netherlands.
“Of course, the events in Germany were linked to the position of the jet stream,” said Johannes Quaas, meteorologist and professor at the University of Leipzig. At the same time, he said, there is evidence that the atmosphere can hold 7% more moisture for every degree the Earth warms. Global average temperatures are already around 1.2 ° C above pre-industrial levels.
“We were extremely shocked,” said Stefan Heydt, spokesman for the German armed forces in the state of North Rhine-Westphalia, which suffered severe damage in recent flooding. His home state, Rhineland-Palatinate, was also hit hard. Heydt said a cousin still lives there and now has to travel 60 kilometers to get electricity.
“Entire lives have been destroyed instantaneously,” Heydt said.
The economic fallout from flooding in Europe continues to be felt, and it is already clear that a wide range of sectors will feel the shock. Germany halted operations at an open-pit lignite mine operated by RWE AG, reducing output at the Weisweiler connected power plant. Most of the company’s hydroelectric dams in the west of the country and a power station in the Netherlands have also ceased to function.
When the movements of the jet stream, which were first documented by American bombers flying to missions in Japan during World War II, coincide with climatic extremes – heat, drought, intense precipitation – the consequences can be catastrophic .
Maybe that’s what happened in China this week. A record-breaking rainstorm brought a year of three-day rainfall to Zhengzhou, the world’s largest iPhone manufacturing base and a major hub in central China for food production. At least 33 people died and as many as 380,000 had to be evacuated.
Scientists from the China Meteorological Administration attributed the storm to strong and sustained blocks of high pressure which, with Typhoon In-fa approaching from the southeast, pushed water vapor out of the sea. hit the mountains surrounding Henan, the province where Zhengzhou is located, converged and then flew upward, where it cooled to form the destructive rains.
The situation in the atmosphere may have been caused by the weakening of the jet stream, “but it will take further analysis to confirm,” said Michael Mann, professor of atmospheric sciences at Pennsylvania State University. . “What is certainly true is that a slower summer jet stream, which is a robust prediction of a warming climate, allows these systems to persist in one location for longer periods, contributing to good many of those record precipitation totals that we see in Europe and Eurasia, ”he said.
The question for scientists is to what extent climate change has affected these disturbances in the jet stream. It takes time to do this kind of analysis. A rapid attribution study of floods in Europe has already started, carried out by the German meteorological service, with results expected by mid-August.
The heatwaves that hit the western United States and Canada in late June were so unprecedented that researchers were able to conclude in early July that climate change had made them at least 150 times more likely. A high pressure system, usually associated with hot, dry weather, was made worse by the fact that the land below was extremely dry.
Usually, soil moisture absorbs much of the sun’s heat and cools the air as it evaporates. But the extreme drought over the northwestern part of North America meant temperatures could only rise, creating a bubble of hot air in the atmosphere. Since then, a second heat wave has hit the region, with forest fires now burning across the western United States and parts of Canada.
The extreme heat has helped propel the forward prices of natural gas and electricity as millions of people turn on air conditioners. Droughts are forcing almond growers to uproot trees in California. In the high plains of North America, locusts thrive and eat away at already shrunken fields of grass and wheat. With food inflation skyrocketing amid disruptions from the COVID-19 outbreak, weather could keep crop and meat prices high in the months to come.
Meanwhile, western and central Russia was cold, even as heat and forest fires hit eastern Siberia. And as Germany and Belgium saw heavy rains, high temperatures prompted Britain’s Met Office to issue its first-ever extreme heat warning. “The jet winds its way around these successive weather systems, connecting weather conditions in different regions,” said Tim Woollings, professor of physical climatology at the University of Oxford.
Most scientists agree that climate change is worsening events caused by the jet stream, but there is debate about the direct impact of global warming on currents. Researchers have already linked the jet stream to several natural disasters over the past two years.
Scientists have determined, for example, that a high pressure system blocked over eastern Siberia was causing heat that would have been nearly impossible without climate change. In Australia, a two-year drought coupled with a high pressure system over the state of New South Wales led to decades-long temperature records in 2020 and fueled the country’s worst fire season.
Francis of the Woodwell Climate Research Center is studying how the warming of the Arctic influences the weather elsewhere on the planet. She thinks one consequence is a weakening of the jet stream. As the Arctic warms faster than regions further south, the temperature difference decreases, weakening the winds, making its path more sinuous and extreme events over an area more persistent.
“We’re somewhere between hypothesis and theory at this point,” Francis said. “There’s a lot of evidence to back it up, because it’s such a complicated atmosphere, it’s really hard to pinpoint very clearly what makes a weather event more extreme.”
Other researchers don’t make such a clear link, but see signs of other climate-related changes. There is good evidence that jet streams are approaching the poles, according to Woollings. This change is moving storms further north in the northern hemisphere and south in the southern hemisphere, and helps explain the multi-year drought that southern Australia and central Chile are currently experiencing.
Today, researchers are focusing their efforts on predicting fluctuations in the jet stream. It’s a complex task, and much of the research has focused on the northern hemisphere. The impacts in the south, especially South America and the southern tip of Africa, are less well understood as there is less research and raw data from this part of the world.
Some extreme weather conditions may have nothing to do with the jet stream. Temperatures in Brazil’s coffee-growing regions fell below zero degrees C for hours on Tuesday, damaging bean crops and orange groves. Frost deals a second blow to producers after a severe drought caused by weather conditions in La Nina left fields parched and depleted water reservoirs for irrigation.
Yet understanding the jet stream is becoming increasingly urgent, as warming temperatures lead to more frequent extreme weather events. “We need to think more about how weather systems will change with climate change, rather than how the climate will change,” Parker said of Monash University. “This division between climate and weather is, to a large extent, artificial. “
Story by Laura Millan Lombrana, Hayley Warren and Brian K Sullivan, Bloomberg