A white Christmas in a warming world

The answer to Wednesday's riddle ("what has six arms, can swallow farms and a million makes a man?") was snowflakes.

At the end of last month, I raised the question many people in the UK are asking at the moment: where did the global warming go? It started snowing in November and here in Edinburgh there have been numerous significant falls over the last three weeks, with snow continuously on the ground the whole time. I've lost count of the days that have dropped below -10ºC and only one or two have nudged above freezing. Transport in the UK has been thrown into chaos, with trains cancelled or delayed, roads blocked, airports disrupted. Heathrow airport, the largest in the world, was closed or running at reduced capacity for much of the last week.*
*This affected us personally since my sister, who was here visiting us until yesterday, was very unsure whether she would be home for Christmas with her husband and children. As it turned out, her flight from Edinburgh to Heathrow was cancelled, and the train which she took instead was delayed by three or four hours, but she managed to get home.

So far, it is shaping up to be the coldest UK December on record, running about 5ºC below average. The last three years have all been significantly colder than average. Prior to that, the trend was for warming winters, which was quite consistent with the widespread scientific understanding anthropogenic climate change, which predicts more warming at night, at the poles and during winters (all patterns evident in the temperature record and which rule out solar forcing as the main culprit). Indeed, only a couple of years ago, the MET Office infamously predicted, "Children just aren't going to know what snow is". So what happened?

I suggested back in November that a pattern of WACCy weather (Warm Arctic Cold Continents) might help us understand this phenomenon. Where did the warming go? The UK is five degrees colder than usual, but much of Greenland is up to 15 degrees warmer than the long term average, and large parts of the Arctic Ocean are more than 10 above average. Indeed, recent studies suggest a link between declining Arctic sea ice and patterns of cold air for the continents in northern latitudes. With less sea ice over the Arctic ocean, more heat escapes from the water (which is obviously no colder than 0ºC) to the atmosphere (which is, as one would expect during an Arctic winter, generally well below zero). This creates high pressure cells, which disrupt the usual wind patterns and lead to the much-warmer-than-usual-but-still-freezing Arctic air being pushed further south over the continents. When this freezing air coming down over the UK runs into moist air being brought from the Atlantic, we get significant (by UK standards) dumps of snow, bringing the country to a standstill.

All this has been mentioned numerous times in scientific papers and reports from NOAA and NASA (and again), but has barely rated a mention in most mainstream media (Monbiot is a notable exception).

Is this having one's cake and eating it if both warmer and colder winters are evidence of climate change? No, because climatologists have never claimed that every place would be affected in the same way at the same time. Global warming is (a) global and (b) only one of the effects of climate change. Climate change means climate disruption, increasingly dramatic shifts from the relative climatic stability of the Holocene that has nurtured the birth of agriculture and the rise of human civilisation over the last ten thousand years or so. Is using the term "climate change" in preference to "global warming" a con to save face? Not at all, since the terms mean slightly different things and both have been in use for decades. (Or if you'd prefer, here's a video response.)

It is not yet clear whether this pattern is likely to become the new normal UK winter as Arctic sea ice continues its apparent death spiral, or whether other factors will prove more significant. On that question hang billions of pounds in infrastructure decisions.

And so tomorrow in Edinburgh, there will probably be snow on the ground for Christmas: a white Christmas in a warming world.*
*I realise that, technically, bookmakers and the MET Office define a white Christmas as at least a single snowflake reaching the ground somewhere in the UK, but snow on the ground is good enough for this Aussie.