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Climate change increased the odds of autumn 2000 UK floods

21 February 2011, by Tom Marshall

The odds of the extreme flooding that hit Britain in autumn 2000 were probably about doubled by greenhouse gases emitted by human activities, according to a new study.

Car in flood

The paper, published in Nature, involved running a widely-used Met Office climate model thousands of times with slightly different starting conditions, and in finer-than-normal detail, simulating both the actual weather patterns seen over 2000 and those the country would have experienced at that time if atmospheric carbon dioxide had stayed at 1900 levels.

We can never know for sure whether climate change caused any particular weather, but these results show it substantially increased the risk that the autumn 2000 floods – which damaged almost 10,000 properties and led to insured losses worth an estimated £1.3 billion – would happen.

In nine out of ten comparisons between the real climate and the hypothetical emissions-free climate, the presence of twentieth-century greenhouse gas emissions increased the risk of floods in England and Wales by 20 per cent or more. And in two thirds of cases, the increase was 90 per cent or more.

Until now scientists have only been able to suggest in general terms that climate change will bring more episodes of extreme weather, simply because a warmer atmosphere can hold more water. This is the first study to look in detail at how much greenhouse gas emissions increased the risk of flooding over a particular period.

'It's like rolling a die,' explains Dr Pardeep Pall, an atmospheric physicist at the University of Oxford and the paper's lead author. 'You might roll it once and get a six, but it's very difficult to say why this happened, or how likely it was. To start building up a picture of how probable a particular outcome is, you need to roll the dice again and again, and that's what we are effectively doing by running the climate model thousands of times.'

Comparing the range of results from the climate model representing the real world with those from the model they'd tweaked to represent the world with lower greenhouse gas levels allowed the team to understand the effect of each scenario on weather, and in particular on the general likelihood of rainfall.

Loading the dice with greenhouse gases

'This was like comparing the results of lots of rolls of a loaded dice with those of an unloaded dice,' Pall explains. 'First we did the experiment with the "unloaded" climate, and then we went back and did it with the climate "loaded" with greenhouse gas emissions.'

The resulting range of rainfall values was then fed into a model from Risk Management Solutions Ltd, a company that develops risk models for the insurance industry. This model simulates how much water drains off the land and into watercourses, in order to estimate of the risk of flooding.

The team drew on computing power supplied by volunteers all over the world, using the infrastructure of the climateprediction.net project, which seeks to use so-called 'distributed computing' to improve predictions of the future climate. This meant they didn't need to pay large amounts for time on a supercomputer.

Pall says the methods his project developed could be applied much more widely, and that he would be keen to see other teams try the methodology using different climate and flooding models, and to look at different weather episodes.

Climateprediction.net has now launched a follow-up project called weatherathome.net that will look at trends in the weather over many decades, not just the year 2000, and will focus on particular regions including Europe, northwestern America and southern Africa.

Climate models are only now starting to simulate the atmosphere at fine enough resolutions to allow this kind of regional simulation, and to let researchers link the results with other models to simulate the impact of weather, Pall explains.

One of his co-authors, Dr Peter Stott of the Met Office, is now looking at the possibility of running this kind of 'climate attribution' analysis constantly, like a weather forecast. This would give researchers a sense of how climate change is affecting the weather in near real time.

While working on the project, Pall was supported by a NERC CASE studentship in partnership with Risk Management Solutions Ltd.


Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000. Pardeep Pall, Tolu Aina, Dáithí A. Stone, Peter A. Stott, Toru Nozawa, Arno G. J. Hilberts, Dag Lohmann, Myles R. Allen. Nature 470, 382-385; doi:10.1038/nature09762


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Your comments

Its strange to read this report, because at the time of this flooding the MET Office said this was down to the Jet Stream. As It was in 2007/8 and nothing to do with Global Warming or Climate Change.
Nemo

Jon Nemo, Llanelli
Monday, 21 February 2011 - 17:47

Hello Nemo,

you're right that the immediate cause of the flooding was shifts in the jetstream - the paper is pretty open about this (eg it says "Autumn 2000 weather was characterized by a general eastwards displacement of the North Atlantic jet stream from its climatological position, bringing intense systems further into western Europe.") - I didn't want to go into too much detail about it in the article so I may not have explained the background as clearly as I should. The whole paper's accessible via the link at the right if you want more information.

The study is more about what factors led to these changes in the jet stream, and whether greenhouse gas emissions affected the likelihood of them happening in 2000 - it doesn't really make sense to ask if any particular weather episode was 'caused by' climate change but it's possible to look at probabilities, and in this case the model suggests the risk was higher with a century's worth of greenhouse gases in the air.

Tim Woolings goes into this area in a lot more detail in his recent feature .

Tom Marshall, Planet Earth
Tuesday, 22 February 2011 - 11:16

A cooling ball of molten rock floating in space. In the cooling process a crust forms on part of the ball whilst puddles of condensed steam form pools of water on the remainder. Life forms begin to grow both in the water and on the crust. Most of these life forms manage to adjust to their environment except for one species, Homo sapiens or man as he is known. Man is infected with a mental condition which impairs his judgement and for which there is no known cure. Because of the nature of his illness man is destroying all other forms of life on the cooling ball of earth. We are led to believe that the planet Earth has been in a similar situation before and solved the problem by total extinction of the troublesome species. Are we missing something?

Patrick Kenehan, Bristol UK
Monday, 28 February 2011 - 10:40