6 august 2019

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Climate change: how the jet stream is changing your weather

Northern Atlantic current is shifting course — with implications for crops and sea levels

Leslie Hook in Ilulissat, Greenland, Financial Times, August 6, 2019

At the summit of the Greenland ice cap the temperature rarely rises above zero degrees centigrade — the elevation is 3,200m and the ice below is more than a mile thick.

But last Friday, as the sun beat down, a small weather station laden with sensors captured something highly unusual: the temperature crept past zero and up to 3.6C — the highest since records began three decades ago. As temperatures rose across the massive ice sheet, which blankets an area five times the size of Germany, around 60 per cent of the surface started to melt, one of the largest ever recorded.

Scientists know of only three prior occasions in the past 800 years when there has been melting at the very top of the ice cap, which is kept chilled by the large volume of ice beneath. But this seems to be getting more frequent — it is now the second time this decade it has happened.

“The last time we saw melting at the summit, in 2012, we thought it was the extreme of the extremes, and wouldn’t happen again so quickly,” says Konrad Steffen, a professor of climate and cryosphere at ETH Zurich, who operates a network of 18 monitoring stations across the ice sheet. “But now we are facing more of these extremes.”

Prof Steffen’s data shows that between July 30 and August 2 a heatwave in Greenland produced several record highs across the ice sheet, including at East Grip, the second highest monitoring station. “If you start melting at the top of the ice sheet, we are going to lose [the] Greenland ice sheet long-term,” he adds.

The immediate trigger for the heatwave was a shift in atmospheric currents high above the earth’s surface: the North Atlantic Jet Stream, a fast current of wind that blows from west to east, had formed a buckle that was trapping warm air over Greenland. The same pattern had caused a record-setting heatwave in Europe a few days earlier, before shifting over to sit on top of the Greenland ice sheet.

It’s not just Greenland’s weather that is governed by the jet stream. Across Europe and North America, it controls extreme weather conditions of all kinds, from winter cold snaps, to heatwaves, to storms.

As the world gets warmer, the behaviour of the jet stream is one of the most studied and hotly debated mysteries of climate change. Scientists are racing to understand how this current of air is changing as the planet heats up, and whether it will get stronger or weaker — which holds big implications for weather patterns, crop yields and rising sea levels.


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