Explaining Extreme Weather Events of 2013 from a Climate Perspective

The annual Bulletin of the American Meteorological Society (BAMS) special issue on the attribution of last year’s extreme weather events is published today. This year’s issue “explaining extreme events of 2013 – from a climate perspective” includes two papers led by researchers from our climateprediction.net team. bams_eee_2013_cover

This is a highly-cited and influential annual publication coined in 2012 asking whether and to what extent anthropogenic climate change altered the risk of major extreme weather events of the past year to occur.

The first of these papers, led by Dr Nathalie Schaller, looked at the heavy rainfall last summer in the Upper Danube and Elbe Basins in central Germany.

Nathalie explains her research: “Using the weather@home project, we performed two types of experiments to investigate the effect of human influence on the heavy precipitation event that occurred in May-June 2013 in Central Europe and led to floods along the Elbe and Danube rivers. Comparing extreme rainfall amounts in the Elbe and Danube catchments in simulations of the ‘world as it happened’ and of the ‘world that might have been’ shows that human influence did not affect the risk of such an event happening.

An attribution study that was based only on observations came to the same conclusion. This paper shows that despite the fact that in a warming world we do expect and observe more extreme precipitation on average, this is not true for all regions and all types of events.”

The second paper, by Dr Juan Añel Cabanelas and colleagues, looked at the extreme snow in the western Spanish Pyrenees during the winter and spring of 2013.

Juan explains his research: “We analyzed a phenomenon of extreme snow accumulation in the Pyrenees for several months of 2013 using different techniques. The phenomenon was extreme and rare. However results from weather@Home simulations were not able to find a conclusive fingerprint of climate change on it. If anything, the results suggest a slight decrease of the likelihood of such an accumulation of snow occurring in a warming world but this decrease is not scientifically significant.”

The special issue comprises of 22 studies of 16 events that occurred in 2013 all over the world. A particular focus of 5 studies was the extreme heat in Australia, which forced the local meteorological services to design a new colour for the weather maps to display unprecedented heat. All found a strong increase in the risk of such record-breaking heat waves occurring in a warming world, with Knutson et al. showing that the annual mean temperatures in Australia in 2013 are impossible to simulate without global warming.

The studies of extreme precipitation and cyclones show that our understanding of how the probability of their occurrence changes in a warming world is less complete, however, as the report concludes, a “failure to find anthropogenic signals for several events examined in this report does not prove anthropogenic climate change had no role to play. Rather, an anthropogenic contribution to these events that is distinguishable from natural climate variability could not be detected by these analyses. Thus, there may have been an anthropogenic role, but these particular analyses did not find one.”

In compiling different methods to answer the same research question and highlighting the strengths and weaknesses in these methods and gaps in our understanding these annual reports add considerably to the body of evidence of climate change.

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