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What caused the flooding?

By Tom Clarke

Updated on 23 July 2007

We may understand the events that produced the floods. But does climate change mean we will see more in the future?

Unprecedented, says the government - no-one could have predicted such intense rainfall, over so short a time.

The river Severn was already some five metres above its normal height, and then the downpours over the last few days sent a surge of water down the Thames.

The root cause is the jet stream. A band of fast-moving air in the upper atmosphere has been pushed further south than usual, keeping drier conditions away from the UK.

May and June were the wettest on record. And just when you thought it couldn't rain any more, 16 centimetres of rain fell 24 hours. The true extent the flooding is still unfolding.

But where did all this rain come from? And is this something we will see more of in future?

History suggests the rains are not unprecedented. The Severn flooded severely in 1258 and 1483 and 1770.

This rain has a lot to do with the high-altitude air flow of the jet stream. Normally it runs north of the UK and pushes much of the summer rain north of Britain.

But recently it has been looping south. That means rainfall has been carried right across the country for weeks on end.

Worse for the Severn and Avon valley, the weekend's rain was carried up along the river valley, filling the river from estuary to head.

It meant that rainfall entering the already full river had nowhere to go - and the Severn flooded. Because rain was falling on already waterlogged soil, there was nowhere for it to go but downriver.

Experts think this week's flooding as at least as bad as the 1947 floods.

The rains that are flooding the Severn and the Avon, and that threaten the Thames valley, are rare. But are they unprecedented?

History suggests otherwise. The Severn flooded severely in 1258 and 1483 and 1770.

But recent history may be best reflected in Tewkesbury, hit hard because it is where the Severn and Avon rivers meet. In February 1990 its two rivers overflowed.

The worst for a long, long time was 1947, when melting snow across Britain caused the worst flooding in living memory. Experts think this week's flooding is at least as bad.

But the £2 billion question - if insurers' estimates are to be believed - is: will we see more of this in future because of climate change?

It has been raining more in northern latitudes in since 1925. And today in the journal Nature, Canadian and British scientists published the first research actually linking that trend to man-made climate change. Man may account for 50-85 per cent of the extra rain.

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