Winter wonderland

Climate cranks are keen to paint the last northern hemisphere (boreal) winter as unusually cold – a clear sign, they say, that “global warming is over”, and that global cooling has begun. Every crank’s at it: Bob Carter at Muriel’s place, Gerrit van der Lingen in an article in a Christchurch magazine and Vincent Gray in a submission to the select committee looking into the Emissions Trading Bill. It’s nonsense. The winter was cooler than many recent ones – but still 16th warmest, according to NOAA. A strong La Niña is cooling the tropical Pacific, and dragging the global average down, the precise converse of the strong El Niño that made 1998 so hot. In other words it’s weather noise, not long term change, as Stu Ostro explains at the Weather Channel. However, the cranks are right about one thing: last winter was unusual, but not for the reasons they think. In this post, I want to explore some of the reasons why this winter was out of the ordinary, and why I think it may demonstrate that rapid climate change is happening now. It’s an expanded version of how I began my last two talks…

Back in January, I posted some musings on how the rapid loss of Arctic sea ice changes the way we have to look at the future of the planet’s climate system. What had been seen as a (relatively) gradual process playing out over the coming century now looks as though it’s happening in unexpected ways – and sooner rather than later. This is the important bit:

As the Arctic Ocean warms, the climate of the entire northern hemisphere will change – and in ways that no-one can project, because none of the models get the current ice loss – and therefore Arctic heat budget – right.

As the Arctic enters its winter-long night, the sea cools and ice reforms. A lot of heat is lost to space, but a lot also goes into the atmosphere. At the same time, having open ocean where there used to be year-round ice allows for a lot of evaporation. Warmer air, holding more water vapour, instead of colder, drier air. Here’s NOAA’s analysis of the global temperature anomalies over the winter:

There are a few very striking things about this map. The cool La Niña in the Pacific is obvious, and the large cold anomalies over central Asia, spreading in an arc from the Sahara to Vietnam. You can see some red dots recording New Zealand’s warm summer. But above those cold blue dots in Asia there are significant warm anomalies spreading from western Europe through Scandinavia and northern Russia into Siberia and down to Korea and Japan. From NOAA’s climate summary:

…temperatures across Europe were 2-5°C (4-9°F) above average. These warm anomalous conditions resulted in the warmest winter in Finland since records began more than a century ago. During December 2007 – February 2008, Southern Finland experienced a total of 20-30 days of below zero mean temperatures, while Central Finland had only 45-55 days. This was 30-40 and 25-35 days less than typical for Southern and Central Finland, respectively. These conditions also led to a reduction of number of days that had at least 1 cm (0.4 inches) of snow on the ground, with Southern and Southwestern Finland experiencing a total of 20-30 days while Central Finland had 50-60 days. Typically, snow covers the ground more than 70 days during the winter. According to the Finnish Meteorology Institute (FMI), the second mildest winter occurred in 1924-1925. According to reports (released on 26 February 2008), Sweden might have experienced its warmest winter since records began in 1756. The previous record was set during the winter of 1972-1973 when the mean temperature for winter was 1.9°C (3.4°F). Since the report was released, the data available at the time suggested the winter 2007-2008 mean temperature average would be near 2.0-2.2°C (3.6-4°F)

Equally interesting, if not so widely reported, is the fact that Norway had its third warmest winter – but record snowfalls (Reuters), while sea ice in the Baltic was at historic lows. Look at the NOAA map again. All those big red anomalies are… next to the Arctic Ocean – and the warmest part of that ocean. And there was a lot of snow, including some record and near record late season amounts in Canada and the northern USA. If the Arctic air is warmer than normal, and is carrying more water vapour, what would we expect to see? Warm anomalies around the ocean, and more precipitation – which, because “warmer” just means warmer than average, and average in the high Arctic is well below freezing, means more snow.

To give some idea how large this effect might be, take a look at this map released by the National Snow and Ice Data Centre (NSIDC) at a press conference last month:

This shows the age of the Arctic sea ice (purple is 6+ years, red is first year ice) for the February average in 1985-2000, and on the right, the situation in Feb 2008. The oldest ice has all but disappeared, and the first year ice covers the majority of the ocean – including the North Pole. Jeff Masters has an interesting post on what this might mean for the coming summer, but the message is clear enough. The Arctic has all but lost its multi-year, permanent ice. That leaves a lot of ocean to melt back in spring, and refreeze in autumn – and a lot of heat and water vapour to cycle round the northern hemisphere. Unless the Arctic heat budget changes dramatically, this is now part of the annual climate cycle, and it will affect weather all round the Arctic.

But why, when you look at the NOAA temperature anomalies do you get all those cold blue dots in central Asia? The answer is that you get “poles of cold” in the continental interiors. Just as the middle of big land masses are hotter in summer because they’re a long way from the moderating influence of the oceans, so they get colder in winter. In NZ, we can see that in Central Otago. It’s not really a long way from the sea, but it gets hotter in summer and colder in winter than any other part of our islands. In the northern hemisphere, the polar ocean is surrounded by large landmasses. Before the sea ice declined, the Arctic acted as a large reservoir of cooling, feeding into the continental coldness seen in northern Russia, Siberia and Canada. With a warmer Arctic, the middle of Asia still gets cold, but it gets shifted south by the warm anomaly to the north – and the contrast between the warm and cold air masses drives very active weather systems. In winter that means snowstorms. The same sort of thing seems to happen in North America, based round a “pole of cold” in northern Canada.

Climate change is usually expressed in terms of movements in averages and changes in the frequency of extreme weather events. Climate is average weather, so when the pattern of weather – the movement of weather systems and their behaviour (see Stu Ostro’s earlier blogs) changes, climate changes with it. The key word is “pattern” – the way weather systems move over the surface of the planet. The changing heat budget of the high Arctic, the loss of summer sea ice and rapid winter freeze-up, is driving changes in the behaviour of weather systems over a wide swathe of the northern hemisphere. The global average temperature may not have changed much – might even fall slightly this year (before rebounding when the La Niña declines) – but the shape of the weather patterns have, and that’s bringing rapid climate change to parts of the northern hemisphere. I’m willing to wager that Scandinavia’s climate has permanently changed and the climate of the regions around the Arctic Ocean has warmed significantly. The climate system, driven by changes in the Arctic heat budget, has moved towards a new state. That might mean an accelerating loss of permafrost, and faster carbon cycle feedbacks. It could increase the rate of melting of the Greenland ice sheet, and therefore the rate of sea level rise. It will have impacts on weather, and therefore climate, beyond the Arctic – perhaps, through a process meteorologists and climate scientists call teleconnections, even south of the equator.

I am not a meteorologist or an atmospheric physicist, so my view has to remain speculative – but I don’t think I’m a million miles away from the truth. Reading back through Jeff Masters and Stu Ostro’s thoughts, I’m sure they suspect something’s going on – and they are far better qualified than me to work out the details. But the big picture isn’t encouraging. Even if I am only half right, the uncertainty about the future pace of change has to alter the view we take of the risks flowing from climate change. We may not have as much time as we thought to adapt, or decades to get our mitigation policies worked out. We need to get serious, now. That makes the cries of “global cooling” not just silly, but downright stupid.

And for this boreal summer, my advice remains the same as last year. Watch the ice. It’s telling us a very important story.