The UK's dismal recent summers can be blamed on a substantial warming of the North Atlantic Ocean in the late 1990s, according to new scientific research. The shift has resulted in rain-soaked weather systems being driven into northern Europe, increasing summer rainfall by about a third.
The pattern is likely to revert to drier summers and may do so suddenly, according to Prof Rowan Sutton, at the University of Reading, who led the work. "I can't guarantee it but it is likely," he said. "However we are not sure of the timing, which is what every one wants to know – but we are working on this now." Sutton added that when the switch occurs, it could happen as rapidly as over two to three years.
The summer of 2012 was the wettest in a century and follows a series of above average years for summer rainfall. Sutton's team, who published their study in Nature Geoscience, examined over a century of data and found that the temperature of the North Atlantic remains above or below the long term average for decades at a time. The periods of warmer temperature, the latest of which started in the late 1990s, were found to correlate with wet summers in Northern Europe and hotter, drier summers in the Mediterranean. The team used existing detailed climate simulations to demonstrate a causal link between the warmer oceans and the change in the weather.
Sutton said these shifts have been occurring for many hundreds of years, but that global warming was also having an impact. "It is not now purely natural or purely a manifestation of human-induced climate change," he said. "There is lot of evidence to show that climate change is changing the timing and amplitude of the temperature changes." For example, he said, the cooler period from the 1960s to the 1980s occurred when soot and other pollution from dirty power stations cooled the planet.
The previous North Atlantic warm phase, which ran from the 1930s to the 1950s, also saw a run of wet summers in the UK, including severe flooding in August 1948, which closed the east coast mainline railway for three months, and the Lynmouth floods in August 1952 in which 34 people died.
The warming of the North Atlantic has been one reason for the record low in Arctic sea ice this summer. It is possible that the shrinking of the sea ice is also contributing to poor summers in the UK, as the exposed ocean waters warm in the sun. However, Sutton said that this remains to be proven by scientific work that is now underway.
The warm and cold swings in the North Atlantic affect temperatures, rain and winds across Europe, Africa and North and South America, and previous research indicates they are related to changes in ocean circulation. Other research at Reading University has suggested that it may in future be possible to predict the warming and cooling cycles some years ahead.