The main science-related event of the past fortnight, while I’ve been on holiday, has been the “big freeze” affecting the normally temperate regions of the northern hemisphere.
Some climate change sceptics have seized on this with glee, like children handed a surprise ice lolly in a heatwave.
But there is an opposite way of looking at the cold, which supports the case for global warming.
The immediate cause of the freeze is the Arctic Oscillation, a change in the atmospheric pressure distribution of the northern hemisphere between the mid-latitudes and polar regions.
When the AO is positive, the combination of high pressure in the mid-latitudes and low pressure further north blocks the outflow of extremely cold air from the arctic. A more “negative” AO allows the cold to slip further south.
Adam Scaife, seasonal forcasting chief at the UK Met Office, told the FT that this December’s oscillation over the Atlantic was the most negative for more than 100 years. Pressure was actually higher over Iceland than over the Azores – normally it is much lower.
The extremely negative AO has indeed allowed very cold air to flow down to normally temperate regions, leaving Alaska, Greenland and other areas around the Arctic Ocean up to 10degC milder than usual.
However the intensity of the cold has been moderated by man-made global warming. If an identical pressure distribution had arisen in the mid-20th century or earlier, before human activities had added so much carbon dioxide and other greenhouse gases to the atmosphere, the current freeze would probably be a degree or so more severe.
It feels cold in northern Europe at the moment, because we have become so used to mild weather, but veterans of the 1962/63 and 1946/47 winters have seen much worse.