Why Are Heat Waves Becoming So Common in Europe?
Heat waves are becoming increasingly intense, frequent and long lasting around the world because of climate change. But the pattern of heat waves unfolding in Europe is a global outlier.
"We have had an outstanding increase in the number [and] intensity of heat waves," climate scientist Robert Vautard of the Climate and Environment Sciences Laboratory in France told VOA. "The last one is just the continuation of the series."
The European heat wave of 2003 was blamed for more than 70,000 deaths. Subsequent heat waves in 2006, 2010, 2015, 2018, 2019 and 2020 killed thousands more.
Europe warming disproportionally fast
Now 1.94 to 1.99 degrees Celsius hotter on average than the preindustrial average, Europe has warmed by nearly twice the global average of 1.1 degrees. Recent European heat waves reached temperatures three to five degrees higher than was recorded prior to the current period of climate change, Vautard said.
But while climate models capture a bit of this extra heat, their predictions fall short of the real warming in Europe.
"We do not understand why we have such an increase [in temperature] that the models do not predict," said Vautard. "Models predict easily an increase of 1.5 to 2 degrees in the extreme heat waves since about 100 years, but they do not predict 4 degrees. So, it's really outstanding."
Dim Coumou, a climate scientist at Vrije Universiteit Amsterdam, told VOA that Europe is warming three to four times faster than the rest of the midlatitudes.
"But it's not well understood why heat waves in Europe [have] been increasing faster than in other regions," he said.
Scientists continue to look for answers. Coumou is researching how changes in the jet stream encourage heat waves in Europe. Other potential factors include parched soils and sluggish ocean circulation.
Disturbed jet stream
Europe's climate is moderated by the jet stream, a current of fast-moving air that loops around the northern hemisphere from west to east. Sometimes the jet stream splits in two — what scientists call a double jet. Double jets are normal, but climate change seems to be making them happen more often and last longer.
Earlier this month, Coumou and his colleagues published results in Nature Communications, linking frequent and persistent double jets to European heat waves.
"We showed that especially for Western Europe, the increased frequency in this particular jet state can explain ... the increase in heat waves here," he said.
Coumou's results show that for Western Europe, almost all the "extra" heat not predicted by climate models can be explained by double jets. For Europe as a whole, double jets explain about 30% of the excess heat.
Under a double jet, airflow over most of Europe is more sluggish than usual, which can set the stage for heat waves. The current heat wave, for instance, involved a pocket of low-pressure air that stalled off the coast of Portugal because it was cut off from the swift winds of the jet stream.
Dry soils, hot days
This year's heat waves saw "strong waves, strong patterns in the jet," said Coumou. But while it's still too early to say if a double jet was involved, Coumou said the ongoing droughts in Europe were "very likely" a factor in the July heat wave.
Wet soils act as a buffer against extreme heat, climate scientist Sonia Seneviratne of ETH Zurich explained in a voice memo to VOA.
"When the soil gets drier in regions where normally those soils are humid, you get less evapotranspiration, so less water that is evaporated through plants or directly from the soils," Seneviratne said. "Evapotranspiration normally takes up a lot of energy, which means that if it doesn't take place, because the soils are too dry, this energy is used instead to warm the air."
Seneviratne agreed with Coumou that aridity likely contributed to this year's heat waves.
"There is a clear indication that risers have contributed to the intensity of the heat waves that are currently being seen in Europe," she said.
Sluggish ocean currents
A slowdown in a major ocean current — the Atlantic Meridional Overturning Circulation (AMOC) — could also be contributing to European heat waves.
The AMOC shuttles warm and salty water north, and brings deep, cold water southward. Climate models have long predicted that the AMOC will slow as the climate warms, and some researchers think this is already happening.
"It's still not 100% sure … but there's actually a lot of evidence pointing at an AMOC slowdown over the last decades," climate physicist Levke Caesar of Maynooth University in Ireland said in a voicemail to VOA.
An AMOC slowdown would cool the North Atlantic, promoting changes to the jet stream that would channel more warm air from the south into central Europe. That could cause heat waves in Europe, said Caesar.
Still, Caesar cautioned that despite the long-term AMOC slowdown, there are still years when it runs at full speed. She also said there are some indications that the AMOC wasn't involved in the most recent European heat waves.
"The North Atlantic is not particularly cold at the moment, which might indicate [that] in this case, the AMOC did not play a big role," she said.
Cause and effect
All of the potential drivers of European heat waves can interact, making it hard to determine a single cause for any given heat wave — or for the unusually strong pattern of extreme heat waves in Europe as a whole.
"It is difficult to assign cause-effect relationships because it's all dynamically happening together," Coumou said.
More research is needed to understand exactly why Europe is heating up fast, most of the researchers who spoke to VOA agreed.
What is clear is that summer in Europe will only get hotter. Vautard said European policymakers should prepare for future heat waves as intense as 50 degrees now and not wait for climate models to catch up with reality.
"Forty degrees [Celsius] reached in London — I think if one would have told me that 20 years back, I wouldn't have believed it. But now, it's here," Vautard said.