Umbrellas at the ready: Can the UK really expect washout summers for a decade?
If this morning’s news is anything to go by the UK’s set to see rainy, grim summers for the next 10 years – all because of natural changes in the Atlantic Ocean. Following all the recent talk of whether melting Arctic sea ice could be to blame for our weird weather, are humans are off the hook after all? Seems not. As ever, it’s far from a simple story.
“Unusual seasons”
The UK’s climate is usually described as temperate. But this year began with an extremely cold and prolonged winter, ending up as the second coldest March since records began in 1910.
As if that wasn’t bad enough, it followed a particularly disappointing summer in 2012, which saw the wettest June for over a century. December in 2010 saw a mean temperature of – 0.18 degrees Celsius, the coldest on record.
As much of the UK press reported, the Met Office held a meeting yesterday “to discuss the recent run of unusual seasons here in the UK”. It released a summary of its findings last night.
Media attention
Most media reports have led with the Met Office’s suggestion that we may see poor summers for a few more years to come.
‘Stand by for another DECADE of wet summers’, reads the Independent’s headline, while the Guardian suggests: ‘Don’t worry, summer is on its way – but you might have to wait until 2023’.
But some media reports seem a bit lost on the Met Office’s conclusions about whether our weird seasons are naturally-generated or caused by human activity – and about the uncertainty that comes with weather forecasting.
A wobbly jet stream
According to the Met Office, scientists from the University of Reading presented some new evidence at the meeting, looking at how long-term Atlantic currents may be influencing UK summer weather.
The link might not be immediately obvious, but scientists think it all has to do with the jet stream – a river of fast-flowing air high up in the atmosphere.
How does the Jet Stream affect UK weather? Source: Met Office
Changes in the position of the jet stream mean it’s ending up in places we wouldn’t normally find it in a given season. As Professor Stephen Belcher, head of the Met Office Hadley Centre, explained in yesterday’s statement:
“Ultimately, what we’ve seen in each of these seasons is shifts in the position of the jet stream which impact our weather in certain ways at different times of year.”
So what’s driving the changes?
Ocean drivers
The new evidence discussed at the meeting suggests a natural cycle that affects temperatures in the Atlantic Ocean, known as the Atlantic Multidecadal Oscillation ( AMO), could be behind changes in the jet stream. And it could be why six out of the last seven UK summers have seen above average rainfall.
Professor Rowan Sutton from the University of Reading says in an ITV news report:
“We saw a rapid switch to a warmer North Atlantic in the 1990s and we think this is increasing the chances of wet summers over the UK and hot, dry summers around the Mediterranean – a situation that is likely to persist for as long as the North Atlantic remains in a warm phase.”
Scientists can’t say definitively how the AMO might affect UK summers in the next few years, but the cycle isn’t set to reverse for a while. So chances are we could be seeing more of the same in the near future. The Met Office says:
“These [long term changes in the Atlantic] are understood to operate on cycles of a decade or more, which suggests that we may see their influence on our summers for a few more years to come.”
Forecasting is not about saying that event will or won’t happen – it’s about working out how the probability of an event occurring might change. That means scientists aren’t saying for certain it’ll be bad news. The Met Office says:
“While these influence the odds of wet summers, it doesn’t rule out the possibility of decent summers over the next few years.”
In other words, the outlook may not be quite as pessimistic and soggy as some reports would have us believe. Of course, as is so often the case with UK weather, it gets even more complicated that that. The AMO could be influenced by climate change, as Belcher says in the Independent:
“Climate change may be intensifying the natural cycle and may prolong it, but it is too early to say for certain.”
Cold winters
The Met Office suggests shifts in the jet stream may be allowing cold Arctic air to push further south in winter, persisting for longer over mid-latitude countries like the UK. Belcher explains:
“The key question is, what is causing the jet stream to shift in this way? There is some research to say some parts of the natural system load the dice to influence certain states of the jet stream, but this loading may be further amplified by climate change.”
One much discussed question recently is whether the recent rapid decline in Arctic sea ice could contribute to a change in the UK’s weather. We wrote more about how this could work here.
Image - Met Office _Sea Ice Decline (note)
The decline in monthly area covered by sea ice in the Arctic (blue) and summer minimum in September (brown). Source:Met Office
Contrary to the Telegraph’s report that “the high level meeting concluded that climate change is a major factor in colder winters”, the Met Office statement says there is “some evidence” for this, but it’s only one factor in a “wide range of drivers” and needs more research.
A complicating factor could be that this winter also saw what’s called a Sudden Stratospheric Warming ( SSW) event, where winds in the stratosphere above the north pole reverse direction. This brought cold weather conditions to the UK.
Untangling the story
While scientists are making progress in working out how climate change could influence the odds of abnormal weather, it would be unwise to attribute a single event to a single cause. It’s definitely too soon, for example, to conclude that poor summers are down to natural causes but harsh winters are down to humans.
From what the Met Office has said, it seems likely our recent run of unusual weather is down to a combination of both – and untangling their relative importance will take time. Either way, best not place bets on a sunny Wimbledon fortnight just yet.