How often do ‘once in a thousand year’ events happen?

Posted by christian — 22 November 2009 at 3:43pm - Comments

Flooding in the UK - The river Severn in 2000

I grew up in Cumbria, so I've been following the flooding there, described by the Environment Agency as ‘unprecedented', pretty closely. Electricity and gas supplies have cut out in parts of the area affected. Dozens of people have been rescued by the lifeboat service. People have died. Environment Secretary Hilary Benn called it a "one in a thousand year event".

It got me thinking. First, just so I can't be misunderstood: climate change does not ‘cause' extreme weather events, in the way that flicking a light switch ‘causes' the light to come on.

Weather is created by taking the processes of the planet's climate, which give you a rough idea what you're going to get, and filtering them through a sizeable amount of pure random chance. That means the occurrence of any weather event will ultimately be determined by chance. It's a bit like rolling a dice. You know it's possible to get a six, but each time you roll the dice, you have no idea what number you're actually going to get.

Where I used to live, in Wales, the valley would flood each year at about this time, and we'd have to take a twenty-mile detour to get to work. Again, we didn't know from one day to another whether the valley was going to flood that day, but we basically knew it was going to happen every year.

The flooding in Cumbria is clearly on another level entirely. And because ‘out of the ordinary' doesn't really tell you much, we talk about events like this by saying they occur ‘once every X years'. That's what Hilary Benn said - a bit like saying there's a dice with 1000 sides, and one of those sides represents the flooding in Cumbria.

Because random chance plays a big role in determining weather, weather forecasting quite difficult. (‘Barbeque summer', anyone?) But our ability to predict longer term climate trends is actually pretty good.

Why is that? Well, it's because ‘climate' is a shorthand word meaning ‘longer term trends in weather patterns' - things like average temperature, average rainfall, the frequency of extreme weather events. Average things out enough and they change much more slowly, become much more predictable. As the saying goes, ‘climate is what you expect, weather is what you get,' and to continue the analogy, although we might not know what side the dice is going to land on, but we know how many sides it has.

So what does our understanding of climate science tell us about extreme weather events? The IPCC produced this table of trends for extreme weather patterns as the climate changes, of which this is an excerpt:

IPCC Extreme weather table

(The original is here, on page 53.)

They say that as the climate changes, we should expect to see more extreme weather events. They conclude that for ‘Heavy precipitation events', it is ‘very likely' that we will see ‘Frequency increases over most areas'. (‘Very likely' in IPCC terms means ‘greater than 90% probability'.)

How about another example? Probably the most high-profile example of extreme weather damage we've had recently was the impact of Hurricane Katrina on New Orleans. Now, climate change didn't ‘cause' hurricane Katrina, but according to the IPCC, the warmer seas associated with climate change mean that it is more than two-thirds likely that tropical storms will become more destructive as the climate changes.

What does this tell us? Realclimate say:

...it is wrong to blame any one event such as Katrina specifically on global warming - Yet this is not the right way to frame the question... we can indeed draw some important conclusions about the links between hurricane activity and global warming in a statistical sense.

If we come back to the dice analogy. We don't know whether we'll get a six when we roll a dice. But we know we'll get a six, on average, one time in six. If we draw another ‘6' side onto the dice, we know we'll get a six twice as often. If we keep drawing ‘6's on the dice, then pretty soon we're rolling them all the time. That's what we're doing with the climate - changing the odds on a massive scale.

The US Army Corps of Engineers says Katrina was a ‘one in a hundred year' storm. The Cumbria river flow that caused the flooding might be one in a thousand. But as the climate changes, while we can't draw conclusions about any one event, we know that we are loading the dice against ourselves - making it more likely that events which used to be ‘once in a hundred years' will happen more frequently. And if you're caught up in that, wherever you are, that shift in frequency is going to have pretty real consequences.

About Christian Hunt

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Sea ice geek, former web editor at GP.

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