Sweltering heat broke records across Europe last week, the latest in a series of heat waves that have baked the continent since June. Temperatures crept near or above 40 degrees Celsius in much of western Europe, and the heat is now moving east, where it is expected to linger into August.
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 was blamed for more than 70,000 deaths. Subsequent heat waves in 2006, 2010, 2015, 2018, 2019 and 2020 killed thousands more.
Now than the preindustrial average, Europe has warmed by . 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.
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 , 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.
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 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.
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.