Why was last winter so cold? And is this a problem for climate change?

12 September 2010

Posted by vivienne

Barcelona snowfall on March 8, 2010 (picture by Adriano Agulló)

The winter of 2009/2010 was unusually cold across most of the Northern Hemisphere, leading to deaths and traffic chaos. Newspapers told lurid tales of planes sliding off icy runways, home-going revellers found frozen to death, and heavy snow and icy roads trapping motorists in their cars all night – and that was just in Britain.

Needless to say, some climate skeptics seized upon the record snowfalls and cold snaps as evidence the Earth’s climate wasn’t warming after all. But why was last winter so cold and is this a problem for the theory of climate change? A paper published yesterday in the journal Geophysical Research Letters claims the harsh winter weather is consistent with a warming world.

The paper blames the severe weather on warmer temperatures in the Northern Hemisphere caused by climate change. Warmer air means the atmosphere can hold more water. When continental Europe and northern America cool in autumn, the atmosphere can’t hold as much moisture and dumps it as snow.

During the first three weeks of October 2009, snow cover increased rapidly over Europe and persisted throughout winter. The cold and reflective snow cooled the atmosphere above it: the air in the mid-latitudes – around 45 degrees north – became colder and denser and began to sink towards the ground.

Scientists believe sinking air and lower temperatures at mid-latitudes help knock the Arctic Oscillation (AO) - a large-scale atmospheric pressure pattern that switches between two modes – into its “negative” state.

During a “negative” AO, air sinks at mid-latitudes and rises over the poles. The “positive” phase is the other way around – air is sinking over the poles and rising at mid-latitudes. The “negative” phase of the AO tends to bring colder weather to the US and northern Europe.

Effects of the positive phase (left) and negative phase (right) of the Arctic Oscillation (figures thanks to J Wallace, University of Washington)

The AO’s role in last winter’s bad weather isn’t a new idea. In January, the BBC explained the effect the AO was having on Britain’s weather. But the new paper, from a US team led by Dr Judah Cohen, charts how the widespread snow led to the “negative” AO forming and strengthening.

Widespread snow at mid-latitudes – the paper says – leads to a “negative” AO through a six-step process. During step one and two, air cools and sinks over the snow-covered mid-latitudes. As the air cools and descends, it causes the atmosphere above it – around five to 31 miles above the surface – to warm. This warming causes upper air turbulence, which – in step five and six – propagate to the surface as a negative AO.

Last year’s negative AO was unusually severe because the snow hung around throughout the winter and kept temperatures low. This triggered the six-step cycle twice in quick succession – once in late October/November and again in late January. Usually during a negative AO, the Northern Hemisphere only cycles through these steps once per winter.

Dr Cohen’s team aren’t shy about discussing the likely impact of their paper on the political debate about whether humans are causing climate change – a possibly worrying development in scientific research.

Pinning down the causes of “the harsh winter weather is critical to the debate of anthropogenic climate change”, they say as justification for their work. They argue the cold winter hit public confidence in climate science and reference February 2010 articles from the New York Times and Wall Street Journal as showing “public perceptions were clearly influenced – increasing skepticism towards global warming”.

MOP Marks

A middle-aged female Member Of the Public (MOP) reads all my blog posts and gives me a mark out of 10 for readability and excitement. She didn’t really understand this one (Outdoor Science fail), although she gave me good marks: “I will give you 9 for this blogpost, it was quite interesting. But I didn’t quite understand it. If I clear my head and put a lot of thought into it, I may understand.

I read it a few times, mostly because I kept thinking you meant altitude not latitude, not that I am sure what latitude I live in – I thought it was about 50 something. I must admit I cannot – at this moment with no thought – grasp what negative and positive actually does, besides in the former the cold air at 45 degrees drops and rolls over the Arctic and vice versa. So from a public point of view, reading lightly, which the public do, they are not scientists, I would think you meant altitude.

Maybe in southern Europe it was bad weather with snow, but – in UK – I have known worse weather. It was cold for a long time, but – then again – in around 1962, there was snow for months until April. I remember the roads piled with snow maybe six-feet high, it was like driving through a topless tunnel. I remember one year in maybe the 1980s when the temperature was so low, even the potatoes and onions in the coalbunker froze. I can go on and on, there have been many bad winters in my lifetime.”

Cohen, J., Foster, J., Barlow, M., Saito, K., & Jones, J. (2010). Winter 2009–2010: A case study of an extreme Arctic Oscillation event Geophysical Research Letters, 37 (17) DOI: 10.1029/2010GL044256