What not to say about the bad weather

Reduction of flood and associated extreme weather costs is the primary benefit of climate change mitigation. (Photo credit: Wikipedia)

The last six weeks has seen the UK hit by a number of storms and surely climate change is to blame? Well, no, you cannot say that.

Not because climate change is not to blame. But because we just cannot definitively pin down a weather event to a change in climate. (Of course, this does not bother many on the other side of the argument, but those who defend science should judge themselves by higher standards.)

But we can, if the resources are available, calculate the statistical probability that specific weather events are linked to climate effects.

This is the abstract of a 2011 paper published in Nature:

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 (17 February 2011)

doi:10.1038/nature09762

Received: 30 March 2010

Accepted: 10 December 2010

Published online: 16 February 2011

Interest in attributing the risk of damaging weather-related events to anthropogenic climate change is increasing¹. Yet climate models used to study the attribution problem typically do not resolve the weather systems associated with damaging events² such as the UK floods of October and November 2000. Occurring during the wettest autumn in England and Wales since records began in 1766^{3, 4}, these floods damaged nearly 10,000 properties across that region, disrupted services severely, and caused insured losses estimated at £1.3 billion (refs 5, 6). Although the flooding was deemed a ‘wake-up call’ to the impacts of climate change at the time⁷, such claims are typically supported only by general thermodynamic arguments that suggest increased extreme precipitation under global warming, but fail^{8, 9} to account fully for the complex hydrometeorology^{4, 10} associated with flooding. Here we present a multi-step, physically based ‘probabilistic event attribution’ framework showing that it is very likely that global anthropogenic greenhouse gas emissions substantially increased the risk of flood occurrence in England and Wales in autumn 2000. Using publicly volunteered distributed computing^{11, 12}, we generate several thousand seasonal-forecast-resolution climate model simulations of autumn 2000 weather, both under realistic conditions, and under conditions as they might have been had these greenhouse gas emissions and the resulting large-scale warming never occurred. Results are fed into a precipitation-runoff model that is used to simulate severe daily river runoff events in England and Wales (proxy indicators of flood events). The precise magnitude of the anthropogenic contribution remains uncertain, but in nine out of ten cases our model results indicate that twentieth-century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%, and in two out of three cases by more than 90%.

At the time of those floods I was a special adviser to Paul Murphy, the Secretary of State for Wales, and accompanied him when he visited stricken areas of North Wales. It would be an exaggeration to say we saw Biblical levels of distress and destruction, but  they were unlike anything I had seen at first hand.

As the abstract makes clear the paper certainly does not say “floods were caused by climate change” – indeed it goes so far as to say that “observed UK fluvial-flood and high-flow trends for recent decades suggest no clear evidence for any change above that of natural variability^{17, 18}, mirroring the mixed picture in observed precipitation changes^{19, 20}.”

And, moreover, the authors state that the risk of some flooding events may actually be reduced by the factors they say increased the flooding risk from heavy rainfall: climate change is complex, something else those who oppose the science that underlies it are often unwilling to concede as they focus on populist arguments about this week’s weather or short-term trends.

The computing for the project was organised through http://climateprediction.net .

You can read more about all of this in a news item on Nature’s website.

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