Last year Earth sweated through another one of the hottest years on record, continuing a pretty disturbing upward trend. But it wasn't just a matter of more hot days – the summer of 2018 was marked by extreme weather events like droughts and floods. A new study has pinned the blame on stalled giant waves in the jet stream, and unfortunately they're expected to only get worse with the changing climate.
Jet streams are channels of fast-moving wind that circle the globe at an altitude of about 10 km (6.2 mi), and are responsible for driving large-scale weather systems. But sometimes they can meander off-course, creating patterns known as Rossby waves. Normally these clear up fairly quickly but occasionally they can stick around longer, in turn exacerbating weather systems by drawing them out over weeks. That means a few warm days can drag into a heatwave, while rainy days can turn into rainy weeks, leading to flooding.
Historically these patterns have been blamed for heatwaves in Europe in 2003, 2006 and 2015, which are among the worst on record. And now, an international team of climate researchers has found that stalled waves were also responsible for several extreme weather events in June and July 2018. That includes record-breaking heatwaves and droughts in North America and Western Europe, as well as heavy rainfall and floods in South-East Europe and Japan.
"Our study shows that the specific locations and timing of the 2018 summer extremes weren't random but directly connected to the emergence of a re-occurring pattern in the jet stream that stretches around the entire Northern Hemisphere," says Kai Kornhuber, lead author of the study.
Worryingly, the team says we haven't seen the last of these extreme events. The stalling wave patterns seem to be getting worse and more common in recent years, and that trend is projected to continue as the climate changes.
"[The pattern's] frequency and duration have in fact increased over the last two decades," says Dim Coumou, co-author of the study. "In the two decades before 1999, there were no summers that saw a stalling wave pattern lasting for two weeks or more, but since then we have seen already seven such summers."
One possible explanation is that climate change is creating a greater contrast between land and ocean temperatures. That's because land areas tend to heat up faster than oceans do.
"The stalling wave pattern may be favored by this increased land ocean temperature contrast," says Stefan Rahmstorf, co-author of the study. "Another relevant aspect could be the cooler than normal North Atlantic, likely a result of a slowdown of the Atlantic Meridional Overturning Circulation, commonly known as Gulf Stream System. However, this needs further investigation."
If they're going to become more common, these stalling wave patterns need to be taken into account when trying to forecast the weather, and particularly when predicting future extreme events.
The research was published in the journal Environmental Research Letters.
Source: Oxford University
