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Saturday, February 08, 2014

Turned Out Wet

Climate scientists talk of extreme weather in terms of 3-sigma or even 5-sigma events. In a distribution of possibilities 3-sigma refers to the probability of something happening that lies at least 3 standard deviations away from the norm. For a normal distribution that’s a chance of some 1 in 370. A 5-sigma event has a probability of about 1 in 1.7 million, so you really should not be holding your breath waiting for it to happen.

A plot of a normal distribution (or bell-shaped curve) where each band has a width of 1 standard deviation Source: Wikipedia

The real world is not quite so simple. The probabilities of weather events are not distributed evenly about the mean – a dry day can’t get any drier but a wet day could be a lot wetter. Skewed or fat-tailed distributions are common. But to get a qualitative handle on things for practical purposes, such as whether it’s worth spending money on a particular flood defence, one might consider a 3-sigma event as very rare, maybe having occurred during the historical record just once or twice or not at all. That’s the sort of probability that the Environment Agency seriously considers planning for and often spends big money on defending against. There will always be some who say the money should not be spent, or should be spent differently, but that’s all part of the normal cut and thrust of public policy making and spending.
A 5-sigma event, even with fat-tailed weather event distributions, is so rare it’s probably never been experienced and may never have happened at least during much of the last few thousand years of the Holocene climate regime. Most people in a democracy would baulk at policies and public spending to protect against 5-sigma events. And if there are downsides to a policy that protects against such an event then it is soon and sensibly ruled out.
However, such thinking presupposes that the climate is stable, that what were 3-sigma events haven’t become a whole lot more common and that the previously 5-sigma events have not now slipped into the 3-sigma category. But that is exactly what climate science tells us to expect in a warming planet. The global warming caused by our emission of carbon dioxide and other greenhouse gasses is producing climate changes that are shifting the probabilities. For the British Isles more stormy weather with higher rainfall totals falling with greater intensity, interspersed with occasional severe droughts, is the future we need to expect.
The winter of 2013/4 has seen a 5-sigma event in southern Britain. Rainfall has been the highest in the record and the number, frequency and intensity of Atlantic depressions has surpassed previous knowledge. One might justifiably wonder whether there has been a similar period of stormy weather since the Atlantic Period, since the Neolithic settlers built the Sweet Track across the Somerset Levels.
Ian Liddell-Grainger, MP for Bridgwater and West Somerset, asked, rhetorically, “Was it climate change or incompetence?” and stated, “These floods were predictable”. His opposition to windfarms and poor voting record in Parliament on climate related issues, makes one think that here is a man who does not understand climate science and does not take climate change seriously. But then he is a parliamentarian who, presumably, feels the need to represent the views of his constituents. Opinion surveys have shown that a large proportion of the British public do not accept the scientific consensus on climate change, so perhaps Mr Liddell-Grainger is not unusual. But he has now gone beyond the pale, not only failing to inform himself of the science and provide leadership towards sound policy, but has stooped to abuse of civil servants with a better grasp of reality. He has called the Chairman of the Environment Agency, Lord Smith (who took a double first and a PhD at Cambridge) “a little git” and “a coward” and said, "If I just have to stick his head down the loo and flush, I will." That may win him popularity amongst some flooded residents but it is hardly a scientifically defensible position. It diminishes the office of Member of Parliament.
No, Mr Liddell-Grainger, these floods were, quite reasonably, not predicted, because they are a 5-sigma event. If the Environment Agency were to ask for money to protect against 5-sigma events up and down the land, it would be MPs like Mr Liddell-Grainger who would be voting the requests down. But these are just the sort of requests that will have to be made, and accepted, as 5-sigma turns to 3-sigma as our planet warms. If the MP for Bridgewater and west Somerset were to lead the calls for climate mitigation, reducing our greenhouse gas emissions, and adaptation, building resilience in the face of already committed climate change, then he would be worthy of his post.
There are times when insults have a place.  Mr Liddell-Grainger seems to support Prince Charles's intervention in the Somerset flood problem, but it was just a week earlier that the Prince of Wales berated those he called the brigade of headless chicken climate deniers.  His speech is well worth reading.

The Ecologist has just published a useful piece about 'Vision 2030', the long term plan for the Somerset Levels and Moors.  Natural England also write abut it and here's a useful contribution from Mark Avery.
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Update 9th Feb 2014 Lord Chris Smith, Chair of the Environment Agency, has now spoken out in this piece for the Guardian. He says "Over the past two and a half months, Britain has faced the most extreme series of weather events we have ever experienced.... The surge down the east coast of England in early December was the biggest in 60 years, and in some cases even higher than in the tragedy of 1953. The storms over Christmas and new year were unprecedented, and they have since been followed by the wettest January in the south since records began. Last week the highest waves ever recorded in Britain were crashing against the south-west coast. Serious flooding has resulted, in many different parts of the country.

In other words, this has been a 5-sigma event, a concept that Mr Pickles clearly failed to grasp, while the Met Office's Chief Scientist said Dame Julia Slingo said "all the evidence suggests there is a link to climate change".


There's a sequel to this piece on my next blog here.

3 comments:

  1. Anonymous5:50 am

    excellent post. thankyou

    ReplyDelete
  2. This post prompted a debate on twitter, which you may or may not be aware of, including comments from Zoe Williams the Guardian writer. I have storified it here (http://storify.com/snapey1979/the-5-sigma-debate) - it concludes with an apparent refutation of your point that we have witnessed a 5-sigma event. I wanted to give you the right of reply, hence posting this comment. Please don't hesitate to tweet me, G+, or email me (just google and you'll find my address) if you want to make any changes or dispute anything I've said.

    Best Regards,

    Richard

    ReplyDelete
  3. Thanks Richard. That's a great thing you put together. I'll be in touch and meanwhile recommend folk to read the http://storify.com/snapey1979/the-5-sigma-debate

    ReplyDelete