And today's forecast is...chaotic

Bill Burroughs explains why the weather gets stuck in a mood,and so defies prediction

Water,water everywhere in Marlow,Buckinghamshire on 5 January: unexpected weather variations predate the last Ice Age

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 the 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 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 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 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 in direct proportion to this change.This non-linear behaviour to Chaos Theory,and the problems of weather forecasting and the 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 Center for Medium Range Weather Forecasts near Reading,Tim Palmer has been researching ensemble forecasting for many years.The center 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 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 forgone 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 simulate the statistics of different quasi-stationary patterns.In practice,they do not rise to this challenge well.So there is 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 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,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 global warming.

Farmers who reap what they see in the stars Farmers often lament how clueless people are these days about where their food comes from. As a townie I must admit that I had no idea harvest festival was approaching until I heard how churchgoers in Gloucestershire had been banned from donating tins of baked beans because vicars just can't give them away. I haven't been to a harvest festival in years, but I recall finding the ceremony a bit quaint, with it's hymns and sermons expressing relief that all had been safely gathered in, ere the Winter storms begin. It seemed to hark back to an age when farmers were at the mercy of the elements. They still are to a large extent, of course: even satellites and computer forecasts cannot give farmers warning of weather months ahead. Even so, today's long-range forecasts are clearly better than all the mumbo jumbo that farmers relied on years ago: "The weather that comes in with the moon stays like it for a month", and similar nonsense. Farmers in the Andes of Peru and Bolivia still insist on planting their spuds only after consulting the heavens first. This agrarian astrology would be cause for a smug grin, were it not for the fact that it is stunningly effective, predicting rainfall months in advance more reliably than any scientific method. Or, rather, any other scientific method. According to Prof Benjamin Orlove, of the University of California, these Andean farmers have tapped into a genuine natural phenomenon with extraordinary predictive powers. In a fascinating paper in the current issue of American Scientist, Prof Orlove explains how he first heard of the stellar success of the potato farmers in 1973, while studying in Peru. During the festival of San Juan in late June each year, local farmers scrambled up to the top of the nearest mountain and scoured the horizon for the Pleiades, the famous star cluster in the constellation Taurus, sometimes called the Seven Sisters. After studying its appearance, the farmers would ponder the omens and return to their fields planting their potatoes according to what they had seen. According to folklore, if the Pleiades looked especially radiant, four months later there would be plentiful rainfall and a good harvest. If the Pleiades appeared lacklustre and faint, little rain would fall later in the year, making early planting essential. Prof Orlove and his colleagues quickly focused on one possible explanation: that the farmers had found a way to predict the effect of El Niño, the notorious warm current in the Pacific Ocean which triggers climatic upheaval worldwide. Studies of weather records in the area duly con-firmed that rainfall is much lower in those years when El Niño appears, threatening lower potato yields. Yet how could the appearance of a star duster 400 light years away be linked to an ocean current? A clue came from the farmers themselves, who placed great import on the brightness and number of stars in the cluster. This led Prof Orlove and his colleagues to wonder if the transparency of the night sky was somehow affected by El Niño. The most obvious culprit is increased cloud cover, but this idea quickly runs into trouble. First, as El Niño years bring less rain to the area, the Pleiades should surely be dearer when the ocean current strikes, not fainter as the folklore insists. More importantly, clouds are fickle, and could hardly be used to predict rainfall four months ahead. Less determined scientists - or more arrogant ones -might well have given up, dismissing astrological crop-planting as a silly myth. A discussion with a meteorologist at the University of Maryland, put the team back on track. Dr Gene Rasmussen suggested that the farmers might be witnessing the effects of high-altitude clouds - too thin to see, yet dense enough to affect starlight. It proved to be the key. After trawling through satellite data, Prof Orlove and his team found that during El Niño years, there was indeed an increase in high-altitude clouds in the region during late June - and in precisely the part of the sky where the Pleiades appears. Quite why such clouds are triggered by El Niño is not clear, but it is unlikely that the potato farmers fret about this lacuna in meteorological knowledge. What is clear that their centuries-old ritual works - and that scientists could do worse than leave their supercomputers for while, and see what else they can learn from those with first-hand knowledge of workings of nature. Robert Matthews [Sunday Telegraph Sep15 2002]

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Further Reading

Does God Play Dice? Ian Stewart
Chaos James Gleick
The New Scientists Guide to Chaos Nina Hall

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