After this summer’s heat wave, the particularly cold and frosty start to 2013 seems like a distant memory. But scientists are still keen to know what caused it and whether we can expect more cold snaps in the future.
One suggestion from scientists is that rapidly diminishing sea ice in the Arctic could influence weather patterns much further south.
But a paper earlier this week seemed to put paid to that explanation, suggesting previous research had misinterpreted key data. We look at what the scientists involved are saying about it all.
An icy start to the year
March 2013 was the second coldest in the UK since records began. The average temperature was a full 3.3 Â°C below the 1981 to 2010 long term average. Everywhere was colder than average in April and May, too.
One topic under discussion was whether the recent rapid decline in Arctic sea ice was contributing to a change in UK weather patterns.
Arctic sea ice has been declining by about four per cent per decade, with the seasonal low in summer shrinking particularly quickly.
The decline in monthly area covered by sea ice in the Arctic (blue) and summer minimum in September (brown). Source: Met Office
How could sea ice affect our weather?
As sea ice retreats, sunlight that would have been reflected is absorbed by open water instead. It’s largely for this reason that the Arctic region is warming twice as fast as the rest of the planet, a phenomena known as Arctic amplification. This means the temperature difference between the frozen north and the more temperate mid-latitudes is getting smaller.
A lot of the discussion linking Arctic amplification and UK weather stems from research by Dr Jennifer Francis, polar scientist at Rutgers University. A paper by Francis and colleague Stephen Vavrus in 2012 suggested enhanced polar warming can weaken the jet stream – a river of fast-flowing air in the atmosphere.
An explanation of the Jet Stream and how it affects weather. From the Met Office.
The theory goes that as the jet stream slows down and meanders more, Arctic air reaches further south and sticks around for longer than usual, affecting the climate in places like Britain.
Or if you prefer your atmospheric physics in interpretative dance form, here’s a nice explanation of “stuck” weather patterns from PhD student Elizabeth Burakowski (watch from about 8m 50s).
Back in April, Professor Charles Greene from Cornell University told us:
“Global warming has increased the loss of summer sea ice in the Arctic, which has altered atmospheric conditions in a manner that stacks the deck in favor of more severe winter outbreaks”.
And the Met Office’s Julia Slingo told ITV news in May that scientists need to look into any possible link between climate change and cold winters in the UK “as a matter of urgency”. So the theory is clearly something scientists are taking seriously.
But new research published in the Journal Geophysical Research Letters analysed changes in the jet stream’s movements using a different method to Francis’s 2012 paper. The paper concluded there’s little evidence for a link between changes in Arctic atmospheric circulation and midlatitude weather patterns.
Looking at data over the period 1980 to 2011, author Elizabeth Barnes from Colorado State University found no evidence for jet stream waves getting slower or meandering more, or of an increase in weather systems getting stuck over midlatitudes. She says in the paper:
“We conclude that the mechanism put forth by previous studies â?¦ that amplified polar warming has lead to the increased occurrence of slow-moving weather patterns and blocking episodes, is unsupported by the observations.”
The difference between the two paper’s approaches gets into rather technical territory. It’s all to do with how scientists interpret changes in the position of jet stream waves. The new paper argues the patterns Francis’s earlier research describes are not real, instead labelling them “an artifact” of the way the data is analysed.
A few concerns
This is a very new area of research, so we should expect some degree of back and forth between scientists – that’s how hypotheses gain traction and scientific progress is made. But Jennifer Francis tells us she isn’t persuaded by the new findings:
“I am pleased that Dr. Barnes, a respected and talented atmospheric dynamicist, has taken an interest in the topic of linkages between the rapidly changing Arctic and large-scale circulation â?¦ [but] a very different interpretation of the results could be made.”
On top of some very technical concerns about Barnes’s interpretation, Francis tells us that because the new analysis looks at the last 30 years, it’s not surprising a link with Arctic Amplification doesn’t show up. The phenomenon has only become noticeable above natural fluctuations in the last 15 years or so, Francis explains.
But Barnes warns we probably shouldn’t draw conclusions from such a short period of time anyway:
“Atmospheric weather has a large amount of natural variability from year to year. A 15-year period may be too short to diagnose any significant trends in midlatitude weather due to Arctic Amplification, so if such a link has emerged over the last 15 years, it is likely we’ll have to wait… to be sure it is nothing more than natural variability.”
A complicated picture
Barnes tells us she doesn’t think her research necessarily shows there isn’t a link between Arctic sea ice loss and UK weather, just that it’s more complicated than previous research implies.
Dr Jeff Knight from the Met Office, an expert in northern hemisphere climate variability, tells us the difference between the two sets of results “highlights the complexities” of diagnosing changes in atmospheric circulation patterns. Different approaches to examining the problem may not diagnose the same things, he warns:
“The message I take from the Barnes paper is that it confirms that midlatitude circulation is complicated, with a range of potential influences beyond Arctic warming”.
On this same point of what else could be playing a role, Barnes adds:
“If there is a mechanistic link, we don’t know how large the influence of Arctic warming is compared to the many other factors that influence midlatitude weather. There is a lot more work to be done on this topic.”
Back and forth
Such debates add to the body of literature and should help build scientists’ understanding of the mechanisms that could be at play. As Knight puts it:
“[M]ore work is needed to understand trends, variability and their causes. Only this way will we be able to gradually unravel the complexity to gain a more solid view of what will happen in the future.”
With the British preoccupation with weather, the idea of having an answer one way or another is appealing. But it seems the jury is very much still out on this one.