Is a broken jet stream causing extreme weather that lasts longer?

At the end of October 2024, a grey gloom descended over the British Isles. Nothing unusual there. But this murky shroud was particularly persistent, even for the UK. Some regions barely saw the sun for a fortnight. Residents of the village of Odiham in Hampshire, for example, enjoyed just 12 minutes of sunshine in the first 11 days of November. And according to the Met Office, the UK’s weather service, the country as a whole saw just 8.3 hours of sunshine across that period, well below average for that time of year. Meanwhile, in Spain, a slow-moving storm over the Valencia region unleashed torrential downpours, causing flash floods that killed 231 people.

For both events, you can blame the jet stream – the fast-moving air currents that flow west to east around the globe. In October 2024, the polar jet had buckled, trapping a high-pressure anticyclone system over the UK and a low-pressure system over Spain. That isn’t unprecedented: the jet stream buckles from time to time. But even the casual observer might have noticed that weather events seem to be lingering longer across the northern hemisphere, from Europe to North America. Now, climate scientists are scrambling to figure out whether global warming is making the jet stream more erratic, as some have predicted.

We urgently need answers. If we don’t get a clear picture of how the jet stream is changing, and what that means for our weather, we could be dramatically underestimating the extreme events coming our way. “We really need to keep pushing for an understanding of these extremes,” says Tiffany Shaw at the University of Chicago.

While we tend to talk of the jet stream, singular, there are in fact four main jets circling the globe – two in the northern hemisphere and two in the southern hemisphere. Those nearest the poles are known as the polar jets, steered by the boundaries between temperate air and cold polar air. A second pair, called the subtropical jets, runs each side of the equator, sandwiched between the temperate air of the mid-latitudes and the warmth of the tropics.

The jets push weather systems along west to east, typically ensuring that we experience a change in outlook every few days. But they often meander, dipping and rising in altitude and latitude. Sometimes, especially if the winds weaken, the jets wander more extravagantly, looping back on themselves with the result that weather systems move more slowly than usual, or even get trapped in one location for an extended period.

Meteorologists call this an atmospheric block. “Blocking happens when those meanders get really large, the whole jet overturns and turns back on itself,” says Tim Woollings at the University of Oxford, author of Jet Stream: A journey through our changing climate. Atmospheric blocks come in different forms. One of the most common is called an omega block, where the jet stream forms a large loop similar in shape to the Greek letter omega, with a high-pressure system squeezed between two low-pressure systems. There is also a dipole block, where the jet stream splits, trapping a high-pressure system towards the pole and a low-pressure system towards the equator.

Atmospheric blocks

Sometimes stretching over 3000 kilometres, blocks can consign those living under them to days, even weeks, of persistent weather – the nature of which depends on the time of year. “The impact will be really different in different seasons… if you are under the block in winter, that makes you colder, and in summer it makes you hotter,” says Woollings.

The anticyclonic gloom that lingered over the UK in October and November, and the days of heavy rain in Spain, were the result of one such block. As were several other recent extreme weather events, including the Siberian heatwave of 2020 and the intense heat that engulfed parts of western North America in June 2021. “We’re watching the real world unfold and deliver these unprecedented extreme events… all associated with very unusual jet stream patterns,” says Jennifer Francis at Woodwell Climate Research Center in Falmouth, Massachusetts.

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The problem for climate scientists, when it comes to figuring out how the jet stream is shifting in response to climate change, is twofold. Firstly, there is and always has been natural variation in the jet stream, from year to year and decade to decade – and indeed across regions. That makes it difficult to identify long-term trends. More importantly, however, we still don’t fully understand how and why atmospheric blocks develop. “This is somewhat embarrassing,” says Lei Wang at Purdue University in West Lafayette, Indiana. “No one really knows what the fundamental mechanism for blocking is.”

There is no shortage of ideas. Some suggest that changes in north-to-south undulations in the atmosphere, known as Rossby waves, sometimes pile into each other, causing a block to develop in the jet streams. Others see jet streams as “highways” for weather systems. They suggest that changes to the speed of the winds, prompted by topography or atmospheric shifts, can cause “traffic jams” on this highway. “The more crowded the weather systems, the slower the jet stream,” says Noboru Nakamura at the University of Chicago, who pioneered the hypothesis. “That creates a logjam of traffic… this is just like traffic congestion on a highway.”

If researchers don’t fully understand what causes blocking, climate models can’t account for it. Most of these project a general decrease in blocking events as the world warms, and therefore a decrease in persistent weather extremes. “If you look into the climate change predictions in the next 100 years, winter blocks especially are projected to be reduced,” says Wang. In this scenario, jet streams will shift polewards in response to rapidly warming air in the tropics. As they do, the idea goes, the winds will strengthen, resulting in fewer slow-moving or blocked weather systems.

But not everyone buys that. Some researchers suggest, for example, that the northern hemisphere polar jet will weaken and become wavier as the Arctic warms. That is because a warmer Arctic reduces the temperature gradient between polar air and temperate air, slowing the winds and making the jet more likely to buckle, pushing cold air masses much further south than normal.

The trouble is that evidence to support either idea is thin on the ground because satellite observations only began in 1980. This is compounded by the fact that winter in the northern hemisphere presents a noisier picture anyway, largely due to the stratospheric polar vortex over the North Pole, which influences how the jet stream behaves and ensures there is much more natural variability.

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For the northern hemisphere summer, however, a clearer picture is starting to emerge. The polar and subtropical jets are naturally weaker at this time of year because the temperature gradient between low and high latitudes is smaller. But there is now evidence to suggest the summer jets are weakening further because the high latitudes are warming faster than elsewhere, reducing the temperature gradient even more.

“In summer there is growing evidence, with observation and climate models agreeing, that climate change is leading to a weakening of the summertime circulation,” says Kai Kornhuber at the International Institute for Applied Systems Analysis in Laxenburg, Austria. A weaker jet means weirder weather, he warns: “A weakening of the jet, or a slower jet, would lead to slower moving weather patterns and more persistent weather patterns.”

Heatwave hotspots

“Heatwave hotspots” are already emerging across Europe, where heatwave temperatures are outpacing the extremes predicted by climate models, according to work from Samuel Bartusek at Columbia University in New York. These new extremes could be down to changes in blocking, he says. “One of the biggest influences on allowing these extreme heatwaves to happen is having a high-pressure system stuck in the same place for many days.”

As well as identifying the extremes, researchers are also starting to explicitly connect these shifting weather patterns with jet stream changes. Working with Dim Coumou at the University of Amsterdam in the Netherlands and others, Kornhuber published a paper in 2022 that showed a rise in the frequency and intensity of heatwaves in Europe over the past 40 years and also connected it to an increase in the frequency and persistence of dipole blocks. These split jet formations, also known as double jets, push storms blowing through from the Atlantic northwards, leaving western Europe under persistent heat. In 2003, for instance, a double jet configuration lingered for 29 days over Europe during July and August, causing a prolonged, record-breaking heatwave that killed an estimated 70,000 people. According to Kornhuber and Coumou, these double jets have increased in frequency at a rate of around three days per decade and increased in persistence at a rate of around two days per decade since 1980.

That study didn’t link these changes in the jet stream explicitly to climate change, but a paper published last year by Coumou and Rei Chemke at the Weizmann Institute of Science in Rehovot, Israel, made the crucial connection. That study demonstrated that human-induced climate change was the main driver behind a significant weakening in jet wind speeds between 1979 and 2020 during the northern hemisphere summer. It is one of the first pieces of research to demonstrate human-caused climate change is responsible for an increasingly erratic jet stream.

Climate change link

The upshot is that for the northern hemisphere summer at least, it looks like warming temperatures are causing the jet stream to weaken, leading to more persistent weather. The question then is, what can we expect to happen in the future as the world continues to warm?

As those recent studies suggest, prolonged heatwaves look almost certain – particularly in Europe, which is known as a hotspot for atmospheric blocks. Days on end of sunshine might sound wonderful, especially to those who often labour under weeks of grey gloom. But prolonged heatwaves are dangerous, even deadly. The 2021 heatwave that hit parts of Canada and the US is testament to that. Hundreds died and infrastructure buckled: highways were closed, train power lines melted and crops withered in fields. On 30 June of that year, the village of Lytton in British Columbia was destroyed by a wildfire, killing two people.

Prolonged heat is also profoundly dangerous for the human body, especially when there is no opportunity for people to cool down during the night. Longer-lasting heatwaves significantly increase the risk of death from cardiac arrest and stroke. The risk is highest in regions where persistent hot weather is unusual, as homes will rarely have air conditioning.

Long spells of extreme heat are damaging to wildlife too, particularly when high temperatures strike temperate climates. In the UK, 40°C (104°F) temperatures in the summer of 2022 led to birds falling out of the sky. And while most plants and animals can cope with a day or two of high temperatures, prolonged exposure is often fatal to crucial species such as bumblebees, ecologists warn.

But lengthy heatwaves aren’t the only danger posed by an increasingly wavy jet. Wet, stormy weather can be just as deadly, as the recent flooding in Spain demonstrates. Mid-latitude nations such as the UK are no strangers to wet weather, even in the summer. But such weather fronts usually blow through quickly. “If you are living in a city such as London, [intense rain] comes from the west, sweeps across the city and in a matter of 30 minutes it goes away,” says Abdullah Kahraman at Newcastle University, UK.

As the polar jet weakens and waves during summer months in response to climate change, however, slow-moving rainstorms could become more frequent. Moving at around 3 metres per second, versus the 20 to 30 metres per second of a standard rainstorm, their sluggishness means huge amounts of rain can fall on just one location, overwhelming communities. In July 2021, Germany and Belgium were hit by a series of these slow-moving thunderstorms, resulting in flash floods that destroyed towns and killed at least 200 people.

Kahraman’s analysis suggests slow-moving storms could become 14 times more frequent across land by the end of the century, under a high-emissions scenario. Summer and autumn are the most dangerous periods, with warm sea temperatures providing fuel for intense rainstorms. But the risk posed by this kind of extreme weather is a “mostly unexplored” issue in climate science, he warns.

Wildfires and flash flooding

The danger of slow-moving storms isn’t just limited to Europe. Across the northern hemisphere, the probability of extreme rain falling for at least seven consecutive days will increase by 26 per cent for the mid-latitudes, according to a 2019 study, if global temperatures rise by 2°C above pre-industrial levels. There is also evidence indicating tropical cyclones are slowing across the western North Pacific, the North Atlantic and Australia as jet stream winds weaken.

The issue isn’t just that slow-moving or stuck weather patterns may become more frequent. As the world warms, they will also become more noticeable. Climate change means heatwaves caused by blocked high-pressure systems are already more intense, increasing the risk of wildfires, droughts and ecological breakdown. Meanwhile, warmer air holds more moisture, meaning slow-moving storms can hold more rain, further increasing the threat of flash flooding. “We care about how hot the heatwaves are in the summer, we care about how much rain you get around the edge of a block,” says Woollings. “Those impacts are going to get worse.”

How much worse is the big, urgent question. Currently, climate models may not be accurately reproducing the changes to atmospheric processes wrought by climate change, says Kornhuber. “That then bears the risk of an underestimation of future extremes.”

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As the past few years have shown, we are far from ready for prolonged heatwaves or days and days of intense rain. Without a more accurate understanding of how jet streams are warping in response to climate change, hundreds of thousands of people will be left vulnerable. “We need to help people get ready for these unprecedented events,” says Francis. “I think that’s the most important thing.”