Thursday, January 06, 2011

After the cold December - is climate change real?

ice floes on the Dyfi Estuary, mid-Wales on 24 December 2010. Photo: Richard Collins
Many sectors of the media suggested last month that the exceptionally cold weather that gripped the UK and other parts of northern Europe challenged the science of climate change.

Sceptics, however, tend to look for easy, local, black-and-white answers to the problem of climate change and whether it is caused by human activity. Unfortunately, the climate is much more complex than this, and scientists are still struggling to adapt the models to the observed and collected data.

However, this does not mean that climate change is not happening: it is, and extreme weather events are part of the expected pattern.

Weather and climate

A basic mistake is to confuse weather with climate. The weather is not the same as the climate. A pattern of 'climate' in a region changes over a number of years. 'Weather' is local and changes by the hour.

There is no doubt than in general, the Earth is warming - global average annual temperatures are increasing. This is a view held by the vast majority of climate scientists.

The term 'climate change' is used because in the chaotic transition from one stable climate pattern (recent centuries) to whatever the future holds, there is and will be turbulence and unpredictability locally - an increased number of extreme weather events.

For example, last year saw also a Russian heat wave resulting fires that killed 56,000 people and the loss of 39% of the grain harvest. Furthermore, 15 nations around the world reported large scale coral bleaching events , as a result of record sea surface temperatures, including 94oF in the waters of the 'Coral Triangle'.

In fact, 2010 was one of the hottest on record around the world. The weather in north-west Europe is just one part of the global picture. Many variable factors affect it, meaning cold winters are perfectly possible in a warming world.

So what did cause the cold weather?

A big high centred on Greenland - one of the most intense ever, say meteorologists - spread south and blocked warm westerly winds from crossing the Atlantic. To fill the vacuum, bitterly cold air from the Arctic flowed down over Europe.

The culprit is widely blamed as the North Atlantic Oscillation (NAO).

The NAO has two phases: the positive phase when air pressure is low over Iceland, but high down south over the Azores islands off West Africa, driving strong westerly winds and weather fronts, whipping up storms and sometimes causing floods; and the negative phase as occurred in December.

According to science writer Fred Pearce, "the NAO has been in a generally positive phase for the past 25 years. As a result, winters have usually been mild since the late Eighties, encouraging one climatologist to predict an end to winter snow in Britain.

"But in the middle of summer 2009, it slipped back into a negative phase that has persisted month after month since, bringing us last winter's snow and now our current record-breaking December freeze."

What has caused the change? Well, fingers are being pointed at the thawing of the Arctic ice due to global warming. This has had two effects: the sea absorbs more heat from the sun than the white ice, which reflects the radiation back into space; and it also warms the air above the sea, which ice does not. It's a positive feedback loop for warming, and it has given rise to the high pressure.

The Arctic Dipole

But the climate is more complex even than this. According to climate-watcher John Mason, "a new atmospheric circulation pattern has been identified: the Arctic Dipole, which has become an increasingly-important feature of the Arctic climate during the first decade of the 21st Century."

He says that Arctic weather has until recently been driven by the NAO and its close relative, the Arctic Oscillation (AO), both of which broadly produce a circumpolar airflow from west to east. But the newly identified Arctic Dipole pattern features anomalously high and low pressure systems - they are occurring and persisting where previously they did not.

"Now, with the Dipole, they have competition and it is having some strange affects on the climate of the Arctic and further afield," says Mason.

The open water in the Barents-Kara seas reaches its maximum extent in mid-September: during the Autumn, the research has found, it returns some of that heat back to the lower atmosphere, driving up air temperatures and thereby affecting pressure and atmospheric circulation patterns, which in return go on to cause further excessive summer ice-loss in subsequent years.

This potential influence - the 'B-K Effect' - has been analysed using a global atmospheric circulation model by Vladimir Petoukhov of the Potsdam Institute for Climate Impact Research and Vladimir Semenov of the Leibniz Institute of Marine Sciences at Kiel University in a study submitted in November 2009 and recently published in the Journal of Geophysical Research.

They found that the model responded in a non-linear fashion: rather than resulting in a warming over adjacent continents as might have been expected, a strong regional cooling was generated  within a certain range of sea-ice cover.

In the abstract, they state: "Here we show that anomalous decrease of wintertime sea-ice concentration in the Barents-Kara (B-K) seas could bring about extreme cold events like winter 2005-2006."

Changing climate models

Climate models may need to be updated to account for the readings observed. Rasmus Benestad of the Norwegian Meteorological Institute, writing on the Realclimate blog on December 14th 2010, said that, while Petoukhov and Seminov's findings sound plausible, "There is a limit to what they are able to describe in terms of local regional details, and it is reasonable to ask whether the response to changes in regional sea-ice cover is beyond the limitation of the global model."

The extremity of the NAO is measured by an index from negative to positive. Whilst 2009-10 caused major problems in parts of the UK and had an index just under -4.0, it was not as cold as the 1962-63 winter, which had a lower NAO index of -4.0.

That winter had a Central England Temperature (CET) of -0.3C. The CET for the equivalent period in 2009-10 was 2.4C.

If the NAO was the only control-mechanism with respect to the severity of our winters, then by rights 2009-10 should have been colder than 1962-3. But it wasn't, and the difference might be due to overall average global temperatures increasing. The jury is still out.

Climate trends are multidecadal affairs and the research discussed above is relatively recent. The influence of open sea water in the Arctic, where at one time there was extensive sea-ice, is clearly crucial to watch in the coming years. As with most matters of science, the truth will come out in due course.

Other references:

> Budikova, D. (2009): Role of Arctic sea ice in global atmospheric circulation: A review. Global Planet. Change, 68(3), 149–163.
> Honda, M., J. Inoue, and S. Yamane (2009): Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters. Geophys. Res. Lett., 36, L08707, doi:10.1029/2008GL037079.
> Overland, J.E., and M. Wang (2010): Large-scale atmospheric circulation changes associated with the recent loss of Arctic sea ice. Tellus, 62A, 1–9.

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