It's cold again and the M.CO team have been investing in ice grips in anticipation of another Arctic winter. There has been much speculation in the papers as to why Arctic weather has been visiting our shores recently.

Firstly there is the sun spots theory, which correlates recent lack of solar activity with colder weather (see William Reville's article here, and Dick Ahlstrom's piece here). Sunspots are caused by strong solar magnetic activity, and cause a temporary increase in the Sun's brightness. Ahlstrom notes that 'low solar activity causes high altitude jet stream winds to twist back on themselves during winter months. This channels bitterly cold Arctic air and frigid winds from the Russian Steppes across northern Europe and on to Ireland'.

Sunspot activity runs in 11-year cycles, however many researchers now believe this cycle is shutting down and that we are heading into a period of lower solar activity, on a par with what was seen in the seventeenth century (the period known as the little ice-age). The sun has been uncharacteristically quiet for the last 2 years, even though it is currently due to reach a peak of solar activity. Much of the research on which these predictions are based was published by Michael Lockwood and collaborators in two recent papers ( Environmental Research Letters 2010, 5, 024001 and 2011, 6, 034004 – see iopscience.iop.org).

In ‘The Long Thaw’ (2009), David Archer sets out a history of climate variations in recent history, noting that the paucity of sunspots between 1300-1800 caused what is known as the little ice-age, which had turbulent and inconsistent weather. But what is surprising is that he says the little ice age was only on average (over the globe) 1 degree colder.

Next is the reducing arctic sea ice extent theory. This theory is more controversial and inconvenient as it is a direct result of an anthropogenically enhanced greenhouse effect.

It makes logical sense – Irish weather is dependent on the North Atlantic Oscillation (NAO), which is created by high pressure over the Azores and low pressure over Iceland. This results in strong west to east winds and brings mild, wet weather from the Atlantic Ocean to northwest Europe.

When sea ice extent is reduced, more energy is absorbed, not reflected, by the arctic ocean, which creates high pressure over Iceland. The NAO west to east jet stream is increasingly unreliable, and we get the Warm Arctic-Cold Continents pattern (Overland et al, 2010), which compromises Northern Europe’s protection from Arctic air.

Mason notes that the NAO is not the only factor dictating colder winters in northern Europe, and it is not certain as to whether this pattern forms a new qualitative norm.

Hansen et al (2010) do not fully agree that climate ‘remembers’ low Arctic sea-ice the following autumn, because the years previous to 2009-10 did not produce exceptionally cold winters in Europe, despite low Arctic sea-ice extent.

However Honda et al (2009) record anomalous low temperatures and exceptional extreme snowfall in Japan (2005-06) and southern China (2007-08), citing similar causes. It might therefore be interpreted that a threshold is being met beyond which the polar jet streams cannot operate so effectively or consistently.

If this is a reasonable reading of why these temperature and pressure anomalies are occurring, it means that developed countries, who are the greatest contributors to anthropogenically induced climate change, are experiencing severe and tangible climate change impacts, and therefore this weather presents a (previously unanticipated) more globally equitable spread of negative impacts.

If widely understood this may present some interesting opportunities in terms of communication, particularly in terms of what Lenton et al (2008) describe as the tipping element of ‘human socioeconomic systems’, which could precipitate a transition to a low carbon society. In other words the effects of our actions are impacting on us directly, so we might be motivated to change. No doubt there are several contributing factors, including both theories above.

If you are wondering how low the sea ice extent was this year, have a look at nsidc.org - it was the second lowest on record after 2007. The ice grips might be a good investment.