Thursday, February 13, 2014

Turned out Wet (and still raining)

My last blog-piece sparked off a twitter debate between Zoe Williams of the Guardian and one of the brigade of chicken headed deniers (See recent speech by Prince Charles). Richard Snape kindly recorded the conversation on this storify, and then went on to write a return blog-piece of his own. Thank you Richard, it’s very helpful and perhaps treats my piece with more gravitas than it deserved. The following is really a response so you’ll need to read it to make much sense of this post.
Importantly, mine is not a statistical analysis. The trouble is there aren't really any data to analyse. Rainfall records only go back ~250 years and I just made the wild assertion that this is the wettest such period since the Neolithic. That may or may not be true and probably we’ll never know.
It’s good not to cherry pick the data and I've not defined what period or events I'm talking about. Southern England December 2013, January 2014, into February 2014 and still counting, will do for time and space, rainfall, depression intensity and frequency, wind-speed, temperature, the whole gamut of weather will do for the 'event'. Sadly, we don't have the data set for all that lot going back to the Neolithic!
So is it 5-sigma? Well, frankly, I can't be certain; depending on where one draws the boundaries, it might be. Or not! You're quite right, Richard, about 'generic'. I'm wielding a broad brush.  So never mind the data, your three points of 'implied argument' are spot on:
1. If the observation is very unlikely, then the distribution must have changed.
2. The distribution changing implies climate change (and often the anthropogenic forcing element thereof).
3. If the climate is changing - that cannot be coped with using Business-as-usual methods.
I love Nick Taleb; if you haven't yet, read Black Swan.  Expect the unexpected.
But Richard, you say "climate science appears to indicate that warming would increase the amount of water that can be carried in the atmosphere". I’d put it more strongly. We know that global warming leads to more rainfall, (in as much as we know that apples fall downwards). And we’re not just dealing with Somerset. It’s reached Berkshire so must be important now. The bigger the unusual event is the more unusual it is, but of course there I go, cherry-picking boundaries.
“I think that Climate Change probably is happening." Good. But the IPCC put that more strongly. They use the word 'unequivocal' with reference not just to climate change but to the assertion that climate change is largely being caused by man-made greenhouse gas emissions. I agree with the IPCC position in this regard.
"Fundamentally, the rarity or otherwise of individual weather observations cannot, in my opinion, provide conclusive evidence for or against climate change." Very true, but the thing is, we've done and dusted that debate. We know, like we know that apples fall downwards, that our greenhouse gas emissions are causing global warming, which is, in turn, causing climate change (see my earlier piece on these phrases). There are plenty of details to research and argue about, which is what climate scientists do, (we're not quite sure which daisy the apple will roll onto) but all the scientists working in the field accept the basics. There is consensus. My political purpose is to get folks to realise this.
It's really very difficult for climate scientists to communicate what they really believe. Everyone, at least in public, has to be very careful, trying to please their employers and not risk their careers. (Be particularly wary of what some meteorologists say. Weathermen are not always climate scientists, failing to see the wood for the trees.)
One of the trickier areas is with the global climate computer models. The numerical models are largely built on the underlying assumption that the climate is stable and they do not sufficiently account for thresholds and tipping points. But don't be tempted to think that therefore the models are rubbish. That would be ignoring the vital rider that the probability space of error is all on the bad side. The models underestimate the likelihood of catastrophic change. If you are not frightened of serious mathematics you might like to dig into the work of Lenny Smith at the Grantham Research Institute on Climate Change and the Environment at the London School of Economics.

This diagram is for the comment below:

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.
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.