The melting of the Greenland ice sheet is causing more extreme heat in Europe, contributing to the major heatwaves experienced there so far this year.
The surge of cold meltwater into the North Atlantic is altering ocean temperatures and shifting the jet stream to favour heatwaves in western Europe and the Mediterranean, a study undergoing peer review has found. As Greenland ice loss accelerates, Europe may suffer more frequent and intense heatwaves than climate models currently predict.
This newly discovered “atmospheric bridge” played a major role in the unprecedented heatwaves this summer, according to at the UK’s National Oceanography Centre, who is one of the study’s authors.
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“Europe heats up faster. This is because of the jet stream shift,” she says. “In future years… it will be even more extreme. In the future we will think this [year’s heatwave] was not a heatwave; this was nice.”
The steady rise in global temperature has increased heatwaves. But it’s still debated whether changes in prevailing weather patterns are also contributing.
In the past 30 years, high-pressure ridges – large areas of elevated air pressure that look like a ridge or upside-down U on weather maps – have been forming more frequently over Europe, the study finds, leading to sunny skies and extreme heat. These ridges are caused by waviness in the jet stream, the band of strong winds that circles the northern hemisphere. When the more or less straight-flowing jet stream meanders northward around western Europe, it allows warm air from the central Atlantic and Sahara desert to pour into the ridge that’s been created.
Years with extensive high-pressure ridges over Europe have resulted in heatwaves like the one that 70,000 people in 2003.
The researchers say the formation of the ridges can be traced back to Greenland through a cascade of climate events. In the summer before years of extensive ridging, an average of 81 billion tonnes more meltwater rushes off the ice sheet, they found. The cold meltwater lowers sea surface temperatures as it spreads through the North Atlantic in the autumn and winter. Its freshness inhibits mixing with saltier, warmer deep layers, further cooling the surface.
These pulses of meltwater cooling come on top of the “cold blob,” a long-term decline in ocean temperatures in the North Atlantic that some scientists blame on the weakening of the Atlantic Meridional Overturning Circulation (AMOC) system of currents. Together, they create a sharp temperature front between the cold waters and the warmer surface waters further south in the Atlantic.
As warm air masses rise to the south of the front and cold masses descend to the north of it, they begin to swirl and generate cyclones. This makes the jet stream wavier. It tends to meander southward of these cyclones, then back northward over western Europe, creating the persistent, high-pressure ridges and heatwaves there.
Most worryingly, climate models don’t fully capture this process, according to Oltmanns.
“This is a big problem,” she says. “It’s going to be even more intense in terms of heatwaves and droughts, in particular in the Mediterranean region, so the weather extremes are likely underestimated.”
A in July found that the jet stream has shifted southward in the Atlantic in recent decades, similar to the pattern shown in the study. But it concluded that this was due to natural fluctuations in the climate and would eventually reverse.
Meltwater from Greenland could be forcing the jet stream to meander, according to at the University of Reading, UK, who was one of the authors of the July study. But the weakening of the AMOC or changes in atmospheric circulation over the North Atlantic could also be causing this shift.
“That’s the weak link as far as I’m concerned, the link between the cold blob and whatever’s melting in Greenland is not established, and there might be other things going on,” he says.
If North Atlantic cooling continues to affect the jet stream, it could cause not just heatwaves but also other extreme weather, according to Oltmanns. The research linked it to an unprecedented derecho, a stream of thunderstorms and strong winds, in the Mediterranean in 2022. Winds reached more than 60 metres per second, and fist-sized hail .
“We [will] have more droughts at some locations, flooding events at others, and the variability on top of this increases,” she says.
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