And today's forecast is . . . chaotic: Bill Burroughs explains why the weather gets stuck in a mood, and so defies prediction

In case you hadn't noticed, it has been raining a lot recently. Total rainfall from September to November was 34 per cent above the long-term average for England and Wales. December was wetter still - 57 per cent above average.

Every winter the weather 'gets stuck' in one of a limited number of well-defined patterns. Here in Britain the most recognisable of these is the mild and wet westerly conditions associated with a deep depression close to Iceland. In late November, however, the pattern changed to the cold easterly weather that occurs when high pressure lurks over Scandinavia.

For around three-quarters of each winter the atmospheric circulation falls in one of four or five of these patterns. Once established, a given regime may persist for several days or longer. During such a quasi-stationary situation the weather behaves in a more predictable manner. But when the regime breaks down, the change is often rapid and unexpected.

This relative stability punctuated by sudden and less predictable changes has profound implications for both day-to- day weather forecasting and predicting the nature of future change. It means the accuracy of weather forecasts will vary substantially with the changing global circulation patterns. It also influences how the climate responds to perturbations, such as the build-up of carbon dioxide due to the burning of fossil fuels - responses that may be more subtle than computer models suggest.

The response of the atmosphere to changes in its physical properties is non- linear. So as one parameter changes, others alter in a way that is not in direct proportion to this change. This non-linear behaviour is central to Chaos Theory, and the problems of weather forecasting and climate prediction are classic examples of this discipline.

In the case of weather forecasting, this means that the quality of the forecasts is sometimes highly sensitive to the uncertainties in measuring the initial state of the atmosphere. This variation in performance tends to reflect whether the atmosphere is in transition between quasi-stationary states or is stuck in one mode. But because of uncertainty about the switch between the states it is impossible to tell by inspection whether a change will occur during the forecast period and hence whether the forecast will be good or bad.

One way to tackle this problem is to see how the predictions behave when slightly different starting conditions are used to reflect the uncertainty about the current state of the atmosphere. If, with a subtle range of starting conditions, the ensemble of forecasts look remarkably similar up to 10 days ahead, then there is a good chance that they are on the right track. If, however, each forecast diverges significantly after a few days, then clearly the atmosphere is in a less predictable mood.

At the European Centre for Medium- Range Weather Forecasts near Reading, Tim Palmer has been researching ensemble forecasting for many years. The centre now produces blocks of 33 ten-day forecasts each weekend. These show that it is possible to get a much better fix on when the weather is in a predictable regime, and this will place increasing pressure on forecasters to provide a statement on the quality of their output.

When modelling climatic change, the effects of non-linearity are best examined in a different way. If the climate is subjected to a small perturbation - say the result of natural fluctuations in ocean currents or the build-up of atmospheric carbon dioxide - its impact will vary depending on the state of the atmosphere. When it is stuck in a well-defined regime it may be of little consequence, but when in transition it could have quite an effect on which state the atmosphere next settles into. So even if the different regimes basically remain unaltered by the perturbation, the proportion of time each quasi-stationary state lasts could shift substantially and with it the global climate.

The consequence of this interpretation, as Dr Palmer expounded recently in the magazine Weather, is to suggest that the impact of a given increase in carbon dioxide may not necessarily be a proportionate global warming. Intuition suggests this perturbation would make warmer regimes more probable. In northern winters this perturbation effectively tilts the climate in favour of spending more time in the mild westerly regime. But this is not a foregone conclusion, and the reverse, in which the colder blocked pattern prevails, cannot be ruled out.

The test for the global computer models (which predict that increased carbon dioxide leads to global warming) is whether they can stimulate the statistics of different quasi-stationary patterns. In practice, they do not rise to this challenge well. So there is a suspicion that they are producing an over-simplified incremental response that may not reflect the real consequence of the non-linearity of the climate.

This conclusion is not merely a theoretical hypothesis. Recent results from new ice cores drilled in Greenland have suggested increasing evidence of chaotic behaviour in the climate. In particular they have produced dramatic insights into climatic variations during the warm period before the last Ice Age.

Because the temperature during this interglacial period, known as the Eemian, was higher than at present, it had been assumed that it might be a good model for predicting the consequences of global warming. What seems to have emerged from the new observations was wholly unexpected. Instead of a relatively stable warm climate, there appear to have been three different climatic states. Shifts between these states took place suddenly, within a decade or so, sometimes involving average temperature changes of 10C or more, and lasted anything from 70 to 5,000 years.

Measurements of parallel variations in dust level in the ice cores suggest that the fluctuations in climate were associated with significant switches in atmospheric circulation patterns. It is postulated that these shifts may have been driven by sudden alterations in the circulation of the oceans. But, whatever the explanation, they provide a chilling reminder of how non-linear responses to climatic perturbations can be amplified by the changing prevalence of atmospheric circulation regimes. They also indicate that, while computer models remain the most sophisticated analysis we have, it is not safe to assume that the response of the global climate to the build-up of carbon dioxide will be gradual, nor that it is absolutely certain to lead to warming.

(Photograph omitted)

Start your day with The Independent, sign up for daily news emails
Life and Style
healthMeet the volunteer users helping to see if the banned drug can help cure depression and addiction
Arts and Entertainment
tvDick Clement and Ian La Frenais are back for the first time in a decade
Life and Style
tech
News
i100
News
Foo Fighters lead man Dave Grohl talks about the band's forthcoming HBO documentary series
people
News
i100
News
ebooksAn unforgettable anthology of contemporary reportage
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Direct Mail Machine Operative

£13500 - £15000 per annum: Recruitment Genius: This is an opportunity for an i...

Recruitment Genius: Customer Accounts Executive

£14000 - £18000 per annum: Recruitment Genius: This is an opportunity for the ...

Recruitment Genius: Team Administrator / Secretary - South East

£14000 - £17000 per annum: Recruitment Genius: Full time Administrator/Secreta...

Recruitment Genius: Parts Advisor

£16500 - £18500 per annum: Recruitment Genius: One of the leading Mercedes-Ben...

Day In a Page

Thousands of teenage girls enduring debilitating illnesses after routine school cancer vaccination

Health fears over school cancer jab

Shock new Freedom of Information figures show how thousands of girls have suffered serious symptoms after routine HPV injection
Fifa President Sepp Blatter warns his opponents: 'I forgive everyone, but I don't forget'

'I forgive everyone, but I don't forget'

Fifa president Sepp Blatter issues defiant warning to opponents
Extreme summer temperatures will soon cause deaths of up to 1,700 more Britons a year, says government report

Weather warning

Extreme summer temperatures will soon cause deaths of up to 1,700 more Britons a year, says government report
LSD: Speaking to volunteer users of the drug as trials get underway to see if it cures depression and addiction

High hopes for LSD

Meet the volunteer users helping to see if it cures depression and addiction
Why the cost of parenting has become so expensive

Why the cost of parenting has become so expensive

Today's pre-school child costs £35,000, according to Aviva. And that's but the tip of an iceberg, says DJ Taylor
Fifa corruption: The officials are caught in the web of US legal imperialism - where double standards don't get in the way

Caught in the web of legal imperialism

The Fifa officials ensnared by America's extraterritorial authority are only the latest examples of this fearsome power, says Rupert Cornwell
Bruce Robinson: Creator of Withnail and I on his new book about Jack the Ripper

'Jack the Ripper has accrued a heroic aura. But I'm going after the bastard'

The deaths of London prostitutes are commonly pinned on a toff in a top hat. But Bruce Robinson, creator of Withnail and I, has a new theory about the killer's identity
Simon Stephens interview: The playwright on red-blooded rehearsals, disappointing his children - and why plays are like turtles

Simon Stephens interview

The playwright on red-blooded rehearsals, disappointing his children - and why plays are like turtles
Holidaying with a bike nut: Cycling obsessive Rob Penn convinces his wife to saddle up

Holidaying with a bike nut

Cycling obsessive Rob Penn convinces his wife to saddle up
Bill Granger recipes: Our chef pays homage to South-east Asia's palate-refreshing desserts

Bill Granger's fruity Asian desserts

Our chef's refreshing desserts are a perfect ending to a spicy, soy-rich meal
Fifa presidential election: What is the best way to see off Sepp Blatter and end this farce?

Michael Calvin's Last Word

What is the best way to see off Sepp Blatter and end this farce?
Mediterranean migrant crisis: 'If Europe thinks bombing boats will stop smuggling, it will not. We will defend ourselves,' says Tripoli PM

Exclusive interview with Tripoli PM Khalifa al-Ghweil

'If Europe thinks bombing boats will stop smuggling, it will not. We will defend ourselves'
Raymond Chandler's Los Angeles: How the author foretold the Californian water crisis

Raymond Chandler's Los Angeles

How the author foretold the Californian water crisis
Chinese artist who posted funny image of President Xi Jinping facing five years in prison as authorities crackdown on dissent in the arts

Art attack

Chinese artist who posted funny image of President Xi Jinping facing five years in prison