(CN) – Models used to project future changes in extreme weather behavior may be underestimating the impact of climate change because they are unable to capture a phenomenon that freezes the jet stream – and the extreme weather it brings – in place.
Amplified Arctic warming – called Arctic amplification – associated with human-caused climate change both slows down the jet stream and increases the frequency of extreme weather events like those seen this past summer.
Quasi-resonant amplification (QRA) events produce extreme summer weather when the jet stream exhibits broad north-south meanders and becomes stationary, with the peaks and troughs locked in place. Extreme paths for the jet stream bring about flooding, drought and wildfires.
Michael Mann, professor of atmospheric science and director of the Earth System Science Center, explains in the study published Wednesday that climate change impacts on extreme weather were no longer subtle in summer 2018, and the phenomenon of QRA played an important role in producing that hemispheric array of unprecedented weather events.
"It played out in real time on our television screens and newspaper headlines in the form of an unprecedented hemisphere-wide pattern of extreme floods, droughts, heat waves and wildfires," Mann said.
Weather this past summer included flooding in Japan, record heat waves in North America, Europe and Asia, and devastating wildfires in Greece and even parts of the Arctic. Seemingly unending heat and drought in California led to the worst wildfire season ever recorded.
"Most stationary jet stream disturbances will dissipate over time. However, under certain circumstances the wave disturbance is effectively constrained by an atmospheric wave guide, something similar to the way a coaxial cable guides a television signal. Disturbances then cannot easily dissipate and very large amplitude swings in the jet stream north and south can remain in place as it rounds the globe,” Mann said.
Continued burning of fossil fuels is likely to fuel even more extreme summers than what was seen in 2018 because of its impact on the jet stream. If carbon dioxide continues to be added to the atmosphere, QRA and associated extreme weather events will increase at the same rate they have over the past decades.
The impact of aerosols produced by pollution could be reduced until mid-century if countries like China phase out these fuels, according to the international team of climate scientists. They use climate models to predict changes in the occurrence of so-called QRA events associated with persistent weather extremes.
"If the same weather persists for weeks on end in one region, then sunny days can turn into a serious heat wave and drought, and lasting rains can lead to flooding," Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany said.
Mann noted previous work by he and colleagues showed a connection between extreme climate events and climate-induced changes in the jet stream. While researchers cannot accurately identify QRA events in climate models, one thing the climate models capture very well is temperature change.
"QRA events have been shown to have a well-defined signature in terms of the latitudinal variation in temperature in the lower atmosphere," Mann explained. "The change in temperature with latitude and how it responds to increasing greenhouse gas concentrations depends on physics that are well understood and well represented by the climate models."
The researchers found the pattern of amplified Arctic warming that slows down the jet stream also increases the frequency of QRA episodes.
Potsdam Institute study co-authors Dim Coumou and Kai Kornhuber say the models are still too unrefined to trust them to predict these types of extreme weather episodes.
"However, the models do faithfully produce large-scale patterns of temperature change," Kornhuber said.
The scientists say greenhouse gases are not the only consideration when looking at the future of the Earth's climate. Although the U.S. and Europe have switched to "cleaner" coal-burning methods which remove aerosol-generating pollutants from emissions, many other areas of the world have not.
If these countries switch to cleaner coal-burning technology by 2050, the mid-latitude areas of the world will warm and Arctic amplification will diminish. This will occur because aerosols, especially in the mid-latitudes where there is abundant sun, cool the Earth by reflecting heat away from the planet and the incidence of QRAs will begin to diminish.
While curtailing the burning of fossil fuels can prevent an increase in persistent summer weather extremes, summers like 2018 will likely continue in the immediate future, according to researchers.
"The future is still very much in our hands when it comes to dangerous and damaging summer weather extremes," Mann said. "It's simply a matter of our willpower to transition quickly from fossil fuels to renewable energy."
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