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Sunday, April 9, 2017

Northern Hemisphere jet streams stumble as the world warms

The warming of the atmosphere by greenhouse gases is slowing the jet streams which drive the Northern Hemisphere's weather, scientists say. 
by Tim Radford, Climate News Network, April 9, 2017

 – Researchers have once again linked a sequence of devastating climate events to global warming fuelled by prodigal human use of fossil fuels. And this time, they believe they have identified the agency
 behind the blazing summers that have claimed lives and destroyed livelihoods repeatedly during this century.
They argue in the journal Scientific Reports that human impact on the climate now reaches high into the stratosphere, to influence the behaviour patterns of the giant jet streams that carry heat and moisture around the Northern Hemisphere and keep the weather on the move.
Warming driven by carbon dioxide emissions from car exhausts and power stations, they argue, tends to make these giant oscillating waves stall in their journey around the hemisphere – to create enduring episodes of high and low pressure and lingering hazards of drought and flood.
“The unprecedented 2016 California drought, the 2011 US heatwave and 2010 Pakistan flood as well as the 2003 European hot spell all belong to a most worrying series of extremes,” says Michael Mann from Pennsylvania State University in the US.

“Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity”

“The increased incidence of these events exceeds what we would expect from the direct effects of global warming alone, so there must be an additional climate change effect. In data from computer simulations as well as observations, we identify changes that favour unusually persistent, extreme meanders of the jet stream that support such extreme weather events.
“Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity.”
Professor Mann has repeatedly confirmed the link between human action and climate change. His co-author Dim Coumou of the Potsdam Institute for Climate Impact Research in Germany and the VU University in Amsterdam in the Netherlands has separately linked storm tracks to surface temperature extremesmade a connection between torrential rains and planetary warming, and confirmed too that less stormy weather is not necessarily a good sign, because it could be the harbinger of heat waves.
And the researchers now have support for their their suspicions: the jet streams that sweep the hemisphere in huge atmospheric waves, plunging between Arctic and tropics, bring changes of weather.
If they should stall, one region may be committed to long drought, dangerous hot weather (as in Russia in 2010 and Texas in 2011) and even forest fires as in California in 2015) – or, in some cases, catastrophic and sustained rainfall of the kind that flooded Pakistan in 2010.

Questions remain

No single extreme event could ever be satisfactorily and conclusively linked to a long-term trend like global warming. But once scientists register an increasing frequency of such events, they can start to use climate simulations to see if such events become more likely in a warming world.
“The more frequent persistent and meandering jet stream state seems to be a relatively recent phenomenon, which makes it even more relevant," said Dr Coumou. “We certainly need to further investigate this – there is some good evidence, but also many open questions.”
And Professor Mann said: “The warming of the Arctic, the polar amplification of warming, plays a key role here. The surface and lower atmosphere are warming more in the Arctic than anywhere else on the globe.
“That pattern projects onto the very temperature gradient profile that we identify as supporting atmospheric waveguide conditions.”

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