"In the UK, summer 2012 was the wettest for over 100 years, with frequent occurrences of flooding that caused damage to property and some fatalities; and profound impacts on local farming and tourism," said Screen. "At the same time Arctic sea ice was very low. The six summers from 2007 to 2012 witnessed the six lowest amounts of sea ice on record, with summer 2012 the all-time low."

In order to check whether there's a link between melting Arctic sea ice and wet UK summers, Screen changed the amount of sea ice in a climate model – the UK Met Office Unified Model – whilst keeping constant other factors that affect European weather. Decreasing the amount of sea ice cover in the model caused a shift towards wetter summers over the UK and north-west Europe.

"The pattern of rainfall anomalies in the model looked very similar to the pattern of rainfall anomalies in recent years," said Screen. "This led to the conclusion that the loss of Arctic sea ice is one factor that has likely contributed to increased rainfall in recent summers."

Melting Arctic sea ice causes the jet stream (currents of strong winds roughly 10 km up in the atmosphere) to shift further south than normal, Screen found, increasing the frequency of cloudy, cool and wet summers over north-west Europe.



 "Normally in summer the jet stream lies between Scotland and Iceland, and weather systems pass north of the UK," Screen said. "When the jet stream shifts south in summer, it brings unseasonable wet weather to the UK and north-west Europe."

Previous research has suggested that European summer weather is influenced by the Atlantic Multidecadal Oscillation, a slow natural cycle of ocean warming and cooling in the North Atlantic. "Since about the late-1990s the North Atlantic has been in the warm phase of that cycle, which is believed to increase the risk of wet summers," said Screen. "This study suggests that the loss of Arctic sea ice is another important factor. Together, these two factors help to explain the frequent occurrence of extremely wet summers over the last few years."

The wet summers have caused flooding, landslides, damage to property and loss of life. "Some crops failed or were damaged by the wet conditions and more people took their holidays abroad," Screen said, "both of which had significant negative impacts on the UK economy. Local wildlife was also affected."

In contrast, reducing sea ice in the model caused drier conditions in Spain, southern France, Italy and Croatia. The jet stream also shifted over North America.

Screen, who reported his results in Environmental Research Letters (ERL), says the next step is to use estimates of future sea ice loss to predict how further melting could influence summer rainfall in Europe. He also plans to investigate whether using current sea ice conditions in weather forecast models can improve summer rainfall predictions.

"Future weather will be dependent on many factors, as well as the amount of sea ice," he said. "The results do suggest, however, that if sea ice loss continues as it has over recent decades, the risk of wet summers will increase."

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