Planetary waves amplifying wild weather have tripled, scientists say
Climate change has tripled the frequency of atmospheric wave events linked to extreme summer weather in the last 75 years, a new study has found. It may explain why long-range computer forecasts keep underestimating the surge in killer heatwaves, droughts and floods.
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 the study in Monday's Proceedings of the National Academy of Sciences (PNAS) journal.
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 visualise the planetary waves in the northern hemisphere, the easiest way to visualise 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. 'Heatwaves will last longer, grow larger and get hotter. Worsening droughts will destroy more agriculture.'
Spain is preparing for an unusually warm summer, hot on the heels of a record-breaking May.
In an update on Friday, national weather agency AEMET said there is a 60 per cent chance the country will be hotter than average from June through to August.
It follows searing temperatures at the end of last month, when the mercury climbed to 40.7C at the airports of Córdoba and Seville, and 37.5C in Zaragoza. Temperatures on 30 May averaged 24.08C - the highest reading for May since records began in 1950.
There is no doubt, as the Intergovernmental Panel on Climate Change (IPCC) has warned before, that 'human-caused greenhouse gas emissions have led to an increased frequency and intensity of temperature extremes'.
The last three summers have been Spain's hottest on record - and summer 2025 looks set to continue that trend.
The likelihood of a warmer than usual summer rises to 70 per cent along Spain's Mediterranean coast and the Balearic and Canary Islands.
The eastern coast - including Barcelona, Valencia, and the Balearics - is expected to experience frequent 'tropical nights', with temperatures failing to drop below 20C, and many consecutive days above 35C.
AEMET has issued several orange‑level warnings for heat today (17 June), including in Córdoba and Seville countryside - where temperatures could hit 41C - and Vegas del Guadiana in Extremadura.
The average June high for Seville and Córdoba is around 33C, Lars Lowinski, a meteorologist at WetterOnline and Weather and Radar, points out.
'While these parts of Spain are used to summer heatwaves, it is part of an increasing trend of hot spells developing earlier during the season while also becoming more intense/longer-lasting, something that can clearly be attributed to climate change,' he tells Euronews Green.
To help people prepare for its increasingly hot summers, last year Spain's Ministry of Health released a new map with more detailed heat alerts. It breaks the country down from 52 provincial areas into 182 meteosalud (or metro health) zones, providing a colour-based warning system for each.
The hyper-local heat alerts range from Green or no risk through to Red or high risk to health and life. Alongside a colour, these alerts come with information about sun exposure, hydration and symptoms of heat-related illness. There is even an English language version of the official heat alert website, to help alert tourists, students and newly arrived residents to the risk.
This year, Spain's annual heat plan supplements the new meteosalud areas with a guide advising different administrations, health professionals and citizens when intense heat strikes. It follows a study from the Carlos III Health Institute, which determined the threshold for heatwaves in these different areas, taking into account variables like heat-related deaths.
Every heatwave in the world is now made stronger and more likely to happen because of human-caused climate change, World Weather Attribution (WWA) states.
This is borne out by numerous previous analyses - including one that found extreme heat felt in Spain and Portugal in April 2023 would have been almost impossible without human-caused climate change.
2024 was the warmest year on record and the first calendar year where the global temperature exceeded 1.5°C above pre-industrial levels. Carbon dioxide is the biggest contributor to global warming; its concentration in the atmosphere exceeded 430 parts per million (ppm) last month.
As average temperatures rise, the amount of weather at the 'extremely hot' end of the spectrum increases, making extreme heat events more frequent, longer, and more intense.
The heatwave impacting Spain this week is also likely to build into France, the southern UK, parts of Germany and Switzerland, particularly from Saturday, Lowinski says.
Paris could see up to 37C early next week, while 33 or 34C are possible in London.
'Extreme heat is a silent killer, affecting people's health, social, environmental, and economic well-being, particularly women and vulnerable communities,' says Kathy Baughman Mcleod, CEO at Climate Resilience for All (CRA).
The women-led climate adaptation NGO shares five points to help people in Spain, Europe and beyond prepare for upcoming brutal heatwaves.
Recognise early symptoms like dizziness, headache, nausea, rapid heartbeat, and confusion. Heatstroke is a medical emergency. Watch closely for these in vulnerable groups:
Download and follow AEMET for real-time, science-based heat alerts via app, SMS, or website. AEMET issues heatwave warnings by region, severity, and duration - use it to plan your day and check in on others.
With rising temperatures, even healthy adults are suffering from heat exhaustion and heatstroke, especially during physical activity or prolonged exposure.
Don't assume you're immune. Hydrate constantly, rest in the shade or air conditioning and schedule demanding tasks for early morning.
When nighttime temperatures stay above 25C, the body can't recover from the day's stress. Poor sleep increases heart risk, reduces productivity, and worsens mental health.
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