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Wet & Cold: The Shape Of British Summers To Come?

The Guardian, 8 August 2012

It’s been a dull, damp few months and some scientists think we need to get used to it. Melting ice in Greenland could be bringing permanent changes to our climate

[…] A series of unusually wet and cold summers has afflicted the UK for several years. Remember the devastating floods of 2007, when some areas received double their normal rainfall for June? Or the predictions of a “barbecue summer” in 2009 that backfired badly on the Met Office as the (correctly anticipated) high temperatures were accompanied by heavy clouds and rainstorms? The impression that many Britons have had that summer weather has been getting worse in recent years is borne out by the data – five out of the last six years (2007-2012), have shown below-average sunshine from June to August, and in some cases well below average. All have had above-average rainfall – in some cases more than 50% above the long-term average. “It is not just a perception – we have had a run of relatively poor summers,” says Stott.

This year has been the worst so far. April was the wettest on record, and so was the period from April to June. The sun was missing too – June was the second dullest recorded. Hopes that August might bring more settled weather were dashed when the first few days brought floods as far apart as Scotland and Somerset, forcing scores of people from their homes. The unseasonally wet and miserable summer may have failed to dampen the Olympic spirit but it has brought misery to thousands.

Nor has the UK been alone in suffering extreme weather. In the US, the eastern seaboard has been hit by heatwaves and storms but even worse has been the “dustbowl effect” in Texas and across much of the nation’s agricultural heartland. India’s monsoon failed to appear on schedule, leaving millions of farmers in the subcontinent facing destitution. Floods in Beijing, after the heaviest rainfall in 60 years, caused devastation to millions.

The consequences across the world have been and will be dire. A food crisis is now all but inevitable, according to the US agriculture secretary. Emergency plans are being discussed in India, while in China the clear-up is accompanied by concerns that environmental degradation may be making the country’s problems worse.

Attributing any single weather event, or short pattern of events, however extreme, to climate change is always tricky. Extreme weather events occur, in the scientists’ term, stochastically – they happen by themselves, unpredictably, owing to the natural variations of the weather.

But the science of climate change has progressed rapidly in recent years. Last month, the Met Office and NOAA, the US National Oceanic and Atmospheric Administration, published a groundbreaking report that showed recent events could be attributed to human causes. Last year’s unseasonally warm November in the UK – the second hottest since records began in 1659 – was shown to be at least 60 times more likely to have happened because of climate change than because of natural variations in the earth’s weather systems.

Stott says: “We are much more confident about attributing [weather effects] to climate change. This is all adding up to a stronger picture of human influence on the climate.”

For the British Isles, the melting Arctic could hold the key to whether the weather is changing under human impacts. Recent poor summers have been strongly linked by scientists to a change in the usual position of the jet stream, a weather system that normally lies in high latitudes during the northern hemisphere summer.

Earlier this year, two US scientists published a paper in the journal Geophysical Research Letters, suggesting that the meandering of the jet stream could be linked to the reduction in sea ice. Edward Hanna, reader in climate science at the University of Sheffield, who is taking part in similar research, explains: “The last six summers since 2007, while often rather cool and wet over the UK, have brought Greenland unusually high air pressure, mild southerly winds, record-breaking temperatures and melting of the land ice.” The link, he believes, is that Arctic sea-ice losses and the release of heat over the Arctic Ocean have tended to weaken the jet stream and make it more meandering. This has brought more low pressures over Britain, less stable conditions, more cloud cover and rain-bearing weather systems from the Atlantic.

This year, the jet stream moved much more than usual, passing south of the UK. It also persisted in this position for an unusually long time. If this pushing of the jet stream southward is indeed linked to less sea ice over the Arctic circle, as Hanna suspects, then the signs are that we will see many more of these wet summers in future.

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