Extreme weather: an attribution perspective

Extreme weather

13 November 2012

A wealth of observations collected over the globe has shown convincingly that the climate is changing. But although climate change is expected to lead to more extremes of weather including more heatwaves, droughts and floods, does this mean that recent events like the Texas drought or Thailand floods of 2011 can be blamed on human emissions of greenhouse gases? Or were they actually natural disasters that could have happened anyway?

Such questions are increasingly being asked, prompted by the severe human and financial consequences of extreme weather. Identifying a need for better information on the links between extreme weather and climate change, Met Office scientists are leading the way in developing operational attribution services.

One analogy of the effects of climate change on extreme weather is with a cricketer who starts taking steroids and finds he hits twice as many sixes in the new season than he did during the previous one. For any one of the sixes, a spectator near the boundary rope would not know for sure whether the fact the ball was looping over his head into the stands was due to steroids.

But it might be possible to attribute the increased number of sixes to steroids. And, given that steroids have resulted in doubling the number of sixes, you would be able to make an attribution statement; that, all other things being equal (such as the size and state of the pitch or weather conditions), the use of steroids had doubled the probability of that particular six.

As Dr Peter Stott, Head of Climate Monitoring and Attribution at the Met Office, explains, "The job of attribution of extreme weather events is to determine whether human induced climate change or natural climate variability, such as that due to changing solar output or variations in Pacific Ocean temperatures associated with El Niño, have altered the probability of a particular extreme weather event."

The idea of attributing recent extreme weather has been trialled in a new publication published in the Bulletin of the American Meteorological Society (BAMS) and co-edited by Dr Peter Stott of the Met Office and Thomas Peterson of the US National Oceanic and Atmospheric Administration. The report, published in July 2012, examines six extreme weather events of 2011, including the Texas drought and Thailand floods, to put them into a climate perspective.

The report showed that the Texas drought, which led to extensive crop damage and livestock losses, was associated with La Niña conditions in the Pacific Ocean but that human induced climate change had also increased its probability. "Such a conclusion is important because while it underlines the need for societies to deal with natural climate variability, it also shows the risks associated with such natural climate oscillations may be greater as a result of warming of the ocean and atmosphere," says Peter.

The BAMS publication generated a great deal of media interest which indicates there is a large demand for this kind of attribution information. But it was only possible to examine six weather events of 2011 in time for the publication, showing that there is a long way to go before it is possible to provide scientifically robust information for the full range of extreme weather events that occur over the globe in any one year.

This is what Met Office scientists are endeavouring to address by developing an operational attribution system. This will be a continuous process of combining observations and computer models of the climate system. It will provide regular updates on the extent to which the changing odds of recent extreme weather events can be attributed to human-induced climate change or naturally occurring climate variability.

While sounding straightforward, this presents a huge scientific and computational challenge at the forefront of climate science and Met Office scientists are collaborating with other attribution experts from around the world.

But, as Peter says, "If we don't do this, we risk people making poor decisions based on past weather statistics which are no longer a reliable guide to current weather risks with a changing climate. By attributing the changing odds of extreme weather accurately, we hope we can avoid people blaming extreme weather on the wrong causes and help them improve their resilience to the vagaries the weather might throw at them."

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