On Sunday I returned to York – by road, since the railway was still cut off – to find a city under siege. The sense of crisis was heightened by crowds of sightseers, and by TV crews giving breathless live reports in front of the still-rising river. Brown water lapped around the foot of the castle mound, and in many riverfront streets pumps were fighting to extract water from basements, multiple hosepipes discharging back out into the road. On the west side of the river sandbags had been piled high along the tops of walls.
As I walked around, leaks were springing up everywhere, leading in some places to mini-waterfalls cascading down the bank of sandbags. On the other side of this fragile barrier thousands of tonnes of river water were perilously close to escape – a fact that seemed lost on all the people who, like myself, wandered around carelessly below river level to take pictures. Somewhere on the other side of the defences was a riverside park, though only trees and the tops of bushes testified to its continued existence.
York escaped complete disaster by a mere five centimetres that day, although a thousand properties had already been submerged. It was the worst flooding in four hundred years of records. The River Ouse had peaked five and a half metres higher than normal, and the city centre was only saved by the round-the-clock efforts of police, soldiers and firemen shifting 65,000 sandbags to hold back the water.
And York was far from alone – in Shropshire, Shrewsbury was also being hit hard, as was Bewdley in Worcestershire. Downstream from York a lake the size of Windermere, the largest in the English Lake District, had formed: as my train travelled south back down to Oxford, passengers gawped at the new inland sea (complete with large white-capped waves) which had obliterated fields for miles on both sides of the raised track.
As people cast around for a cause, different theories were advanced. Some blamed new housing developments on river floodplains, whilst others claimed that new farming practices meant that water ran off ploughed fields too quickly. But although both may have played a role, they were far from being the whole story. The truth was much more straightforward: Britain had simply been deluged with a staggering amount of rain.
More than three times the normal monthly average of rain fell in parts of the southeast and Yorkshire. Whilst in most places the deluges were judged to be a once in a century event, rainfall totals were sometimes so extreme that they far outweighed previously-observed natural variability. Plumpton in East Sussex recorded 144 millimetres in a single twenty-four-hour period, something that would be expected only once every 300 years,3 whilst the River Uck catchment in the same county had a thirty-day rainfall total that should only occur once every 650 years.4
Most floods come and are gone again in a few days at most. But in October and November 2000 storm relentlessly followed storm, leaving no time for the water to drain away. In England and Wales the September to December 2000 rainfall total was the highest since records began in 1766. In major river basins the floods were the most extreme of any ninety-day period on record, and for shorter time periods were only outranked by the March 1947 ‘Great Floods’ – which had been generated by rapid snowmelt and rain running off still-frozen ground, and thus were much briefer.5
But even very extreme events – which happen only once in a lifetime or even less – can still be part of the natural variability of the climate. A single flood, however dramatic and destructive, isn’t enough to convince a scientist that global warming is to blame. In order to be able to identify more clearly humanity’s telltale fingerprint on the climate, there has to be a trend – evidence of a definite longer and wider change for which other causal factors can be confidently ruled out.
THE ‘SMOKING GUN’?
As it happens, such evidence is indeed available for the UK. To find out more, I went to visit the climatologist Dr Tim Osborn at the Climatic Research Unit (CRU), part of the University of East Anglia. It was almost two years to the day after the start of the autumn 2000 disaster, and there was a hint of winter in the air as the London train sped through the Essex town of Colchester, past the River Stour saltmarshes and on through the flat Norfolk fenlands to Norwich.
As so often, Osborn confounded my expectations. No white lab coat for him: instead, a youngish, fair-haired man in shorts, trainers and a red golfing T-shirt was leaning over the balcony three floors above as I arrived at the round, glass-fronted CRU building.
‘Hello! Come on up,’ he shouted as I climbed the stairs. His room was strewn with back copies of the International Journal of Climatology and meteorology books, as well as sheafs of paper – many covered with impenetrable algebraic scribbles.
‘Sorry about the mess,’ he said as I sat down on a free chair. Then he swivelled his own chair round to face the computer screen. ‘Now, have a look at this.’
Osborn has spent years analysing nearly half a century of rainfall statistics. From a damp day in 1960s Blackpool to a torrential summer downpour in 1990s Devon, all these records were fed into his number-crunching computer. When spat out the other end into a series of graphs, these statistics – rather than just showing the usual random vagaries of the British weather – showed that something very unusual was going on. In fact the trend was so clear that even Osborn himself was ‘surprised’ by what it revealed.
What Osborn discovered was that over recent decades heavy winter downpours have indeed increased dramatically. ‘Over the period from the 1960s to the mid-1990s there was a doubling of the amount of rain that came in the “heavy category” in winter,’ he explained. ‘So in the 1960s something like seven or eight per cent of each winter’s rainfall came from what we call the “heavy” events, whilst by the mid-1990s that had increased to about fifteen per cent.’6
With more rain falling in a short time, river systems were unable to cope – and floods were the inevitable result. What’s more, this heavier winter rainfall was directly related to rising atmospheric temperatures.
Straightforward atmospheric physics suggests this could be the global warming ‘smoking gun’. The relationship between temperature and the air’s capacity to hold water vapour is not linear – in fact the air can hold proportionally more water as temperature rises.7 So in a given ‘precipitation event’, whether it is snow, hail or rain, more water is available to fall out of the sky over the same short period of time.
This is exactly what seems to be happening in Britain: as a result of global warming, more warm, saturated air rushes in from the Atlantic, causing stronger storms and heavier rainfall. As a result, the probability of heavy rainfall has doubled over the last thirty-five to forty years in southeast England, according to observations and analysis conducted by Osborn and his CRU colleague Mike Hulme.8
These aren’t one-off downpours, either. The frequency of prolonged five-day heavy rainfall events has also been increasing. In Scotland floods have been getting far more frequent over the last few decades, whilst in England and Wales there have been four major floods in the last twelve winters: 1989/90, 1993/94, 1994/95 and, of course, 2000/01.9 The match for 2000 isn’t perfect because the worst flooding came during the autumn – but the floods also lasted right through until January, just as the trend would suggest.
Osborn’s work also coincides with evidence from other parts of the world. Study after study has come to the same conclusion: that throughout Earth’s
