Assigning historical responsibilities for extreme weather events

Combining the new science of extreme event attribution and assessments of historic emissions from individual countries and regions in the world, scientists with the University of Oxford’s Environmental Change Institute (ECI) and CICERO in Oslo, Norway demonstrate that it is possible to assign individual countries responsibilities for some types of extreme weather events.

Quantification of these country specific contributions rest on data and science but also depend on value based judgements.

“We found that it is scientifically possible to quantify historical responsibility of individual countries/regions for specific extreme events,” said Dr Friederike Otto, deputy director of the ECI and lead author of the study." The fact that it is possible to provide such quantification will advance the possibility of an informed discussion,” Otto added. “The aim of the study was to explore what science could contribute to the debate of climate justice.”

The team applied two different statistical methodologies to assign contributions of individual countries’ emissions to an extreme weather event, using the example of the Argentinian heatwave of 2013/14. While a previously published attribution study found that anthropogenic climate change overall made the event approximately five times more likely, the new analysis showed that when accounting for all historic emissions from 1850 onwards large emitters like the U.S. and the EU made the event approximately 28% and 37%, respectively, more likely. The differences between the different methodologies were small compared to the overall responsibility assigned to the individual region.

The results strongly depend on two important choices that need to be made prior to assessing the individual countries responsibilities for the increase in an extreme weather event: (1) the definition of the extreme event itself in this case the choice was seasonal mean temperatures over an area around Buenos Aires and (2) the time period over which historical emissions are assessed; in this case, the decision was to use emissions since 1850.

Dr. Ragnhild Skeie, a research scientist at CICERO and co-author of the study said that these are only two among other important choices the team made. “Our results also depend on which climate gases to include in the assessment. If we only take fossil fuel CO2 emissions into account, the numbers would be higher than if we take all emissions, including cooling agents, into account,” said Skeie.

“Overall, we find that choices about how to do the calculations that are not only scientific but also moral and political determine the quantitative results, said Dr. Jan Fuglestvedt, research director at CICERO.

The manuscript is published in Nature Climate Change