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No scientific consensus yet on whether warming Arctic may lead to more extreme weather

Wednesday July 21, 2021 - 22:13 AEST
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Predicting the impact of global warming on the jet stream is at the frontier of climate science. - ABC

In the past month alone, Germany, Belgium and the Netherlands have suffered horrific flooding, Siberia caught fire, and the Arctic Sea suffered near-record melting. 


Meanwhile, in North America, after record-high temperatures, formerly rare fire thunderstorms have become near-daily events.


There is one big theory connecting climate change to the weather patterns behind events as disparate as fire and floods, heatwaves and melting ice, across three different continents. 


It is elegant, reasonably easy to understand and has profound implications ? but because it is at the frontier of climate science, not all researchers are yet convinced.


Warmer world, hotter heatwaves


In one respect, the influence of climate change on heatwaves is relatively straightforward, according to Andrew King from the ARC Centre of Excellence for Climate Extremes at The University of Melbourne.


"We've warmed the planet by about 1.2 degrees Celsius and the land has warmed faster than the ocean," Dr King said.


"So, this allows heatwaves to be that bit hotter than they would be otherwise."


Dr King said a warmer atmosphere could also hold about 7 per cent more moisture for every degree Celsius of warming.


"That means storms that occur on very short timescales can rain out more than they would be able to in a world without global warming," he said.


Climate change and weather patterns


Instances of extreme weather can be driven by distinct weather patterns that allow rain or heat to build up over time because weather systems stall in one place instead of moving on.


If climate change is causing weather systems to stall for longer, they may build to more intense levels.


Late last month in Canada, a horseshoe-shaped high-pressure system called an Omega Block saw heat build up for days, leading to off-the-chart temperatures.


In Europe last week, a cut-off low pressure system stalled over Germany, dumping a month's worth of rain in a day.


Limit of science


The impact of climate change on how such individual weather patterns move is at the very limit of science.


"It's kind of like having a jigsaw but most of the pieces are missing," Dr King said.


"We have really incomplete observations in many parts of the world and they don't go back long enough in time to really track the climate for long enough."


That missing data ? often from remote places like the Arctic ? is needed to build computer models of unprecedented detail that can better predict weather patterns. 


"We really need high-resolution simulations," Dr King said. 


"We have a few studies with regional, high-resolution simulations that point to a climate change-caused intensification of short-duration extreme rainfall, including in Europe.


"That's quite a powerful line of evidence to suggest that climate change is [enhancing] ? or has likely enhanced ? the recent extreme rainfall we saw in Germany, the Netherlands and elsewhere.


"But we just don't have enough data to really make conclusive statements."


The missing data is crucial to answering one of the biggest questions in climate science: Are weather systems sticking around for longer in the Northern Hemisphere because of climate change?


The answer ? which would connect heat, floods and fire ? has everything to do with the jet stream.


Wavy jet stream theory


Dr King says there is a belt of high-altitude winds that encircle the Northern Hemisphere, called the jet stream, and weather systems often follow that track.


Those winds are related to temperature differences between the cold polar regions and the warm tropics. 


"In a warming world, we're seeing a bit more warming over higher latitudes in the polar regions than we see over the equator," Dr King said.


"And that reduces the temperature difference between the equator and the poles.


"The idea is that, if you reduce that temperature difference between the equator and the poles, both near the surface and higher up in the atmosphere, you might reduce the strength of the jet stream. And you might make it wavier or slower," he said.


A slower, wavier jet stream may allow storms to stick around longer, leading to more extreme weather. 


But there is no conclusive evidence that the jet stream is slowing due to climate change.


"There are a variety of studies looking into this, some of which find evidence to suggest this is happening, particularly from the model-based studies, [and[ others, which suggest this isn't really happening,"  Dr King said. 


One recent study used detailed computer modelling to show a warmer world would lead not only to more intense rain in Europe but also to slower storm movement. 


However, its lead author ? Abdullah Kahraman from Newcastle University in the United Kingdom ? was at pains to qualify the limits of the study, saying it related to one very detailed computer simulation.


"This study does not really tell you that this will definitely be happening like that, because this is one scenario," Dr Kahraman said.


"However, when it comes to the jet stream issue, this is not the only simulation that is projecting some kind of decreasing of wind speed in the higher atmosphere."


Other scientists have recently shown there may only be a modest decrease in high altitude winds due to a warming Arctic. 


"It's basically an area of very active research, there are quite a few people around the world looking into this. And there is a diversity of views among scientists," Dr King said.


"At the very least, I think we can say that we don't have a great deal of confidence that this is a clear effect of climate change.


"But there is some indication that there might be more persistence of weather systems, as the jet stream may be allowing them to remain in place for longer.


"This could be contributing to some extreme weather events."







- ABC

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