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A rapid near-time analysis of the UK’s record-breaking wet December in 2015 suggests that climate change increased the odds of the exceptionally high rainfall by 50-75 percent.

Carbon Brief takes a look at the research and quizzes of the experts on the science behind attributing extreme weather events to human-caused climate change.

Warmest and Wettest

Last week, the Met Office announced that December 2015 was the UK’s wettest on record, seeing more rain than any other month since 1910. It was also the UK’s warmest December over the same period.

A series of storms—first Desmond, then Eva and finally Frank—dumped 230mm of rain on the UK during December, triggering flooding across much of Scotland, northern England and Northern Ireland.

After some rapid number-crunching, scientists at the University of Oxford and the Royal Netherlands Meteorological Institute (KNMI) have assessed the role climate change played in last month’s extreme weather.

The preliminary results—from three different approaches—indicate the human impact on climate was as large or even larger, than the impact of natural fluctuations in the Atlantic and Pacific ocean—even during a strong El Niño event.

Climate change and ocean variability each made the record rainfall totals 50-75 percent more likely, the researchers said and doubled the chances of such a warm month. Random variability in weather also contributed to the severe conditions.

You can find more details on the findings, which haven’t yet been peer-reviewed, on the climateprediction.net website.

The researchers have used the three approaches in a number of attribution studies before, including earlier research into Storm Desmond—where they found the exceptional rainfall was 40 percent more likely because of climate change—and a paper on the recent Brazil drought, where they found that climate change had not made the dry spell more likely.

Following the announcement of these results, Carbon Brief spoke to two of the scientists behind the research: geosystem science professor Myles Allen and senior researcher Dr. Friederike Otto. Both are at the Environmental Change Institute (ECI) at the University of Oxford—a leading center for attribution research.

We begin with a question on how scientists choose which extreme weather events to study.

Carbon Brief: How do you decide which extreme events to look at? Is it more down to the resources available to you (time, computing power, etc) or the events themselves (location, severity, whether they’re in the news)?

Myles Allen: The World Weather Attribution project—led by Climate Central in the U.S., with key partners including ECI, KNMI in the Netherlands, the University of Melbourne and the Red Cross Red Crescent (RCRC)—tries to do what it says on the tin: world weather attribution. So we aim to select events on the basis of impact and get excellent input from RCRC on this, because they maintain global databases on the humanitarian and economic impacts of all kinds of natural disasters. That said, clearly we don’t have equal capabilities everywhere, although we are working with partners in vulnerable regions of the world to improve that, so right now there is obviously a bias towards our own back yards: Northwest Europe, Australia and New Zealand.

One thing we don’t do is select events on the basis of whether we think climate change made them more likely to occur. I think, as a group, we have probably published as many null or negative results as we have positive attribution statements, although there is probably a tendency for the positive statements to get more publicity. My personal view is that, in the long term, attribution should be a routine part of any package of climate services, so a quantitative assessment of how various external drivers may be making weather events more or less likely to occur should become just part of the job of the world’s meteorological services. We currently get a lot of qualitative hand waving about how various drivers may have contributed, leaving the public pretty much in the dark (or worse, guided by which papers or websites they read rather than the evidence) about which drivers are most important.

This last December is a case in point: Everyone who was prepared to listen probably got the message that both natural ocean variability, including variations in the Atlantic and the El Niño event in the Pacific Ocean and possibly human influence on climate, contributed to our warm, wet December. But which was more important? Was human influence a tiny effect compared to El Nino or a substantial one? Our preliminary results, released today [see above], suggest that the role of human influence on climate was as large or larger than the influence of these patterns of ocean variability, but that random and unpredictable atmospheric weather noise played an important role as well. That puts these influences into context and helps people understand what is important. This kind of quantitative assessment could be routine; I firmly believe it should be routine; and I’m happy to say that a lot of Met Offices, including our own in the UK, are moving fast in this direction.

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