An Illustrated History of Extreme Weather Attribution
By Kai Kornhuber & Amy Howden-Chapman
Extreme Weather Attribution
Twenty thousand related deaths, vast losses of livestock and wildfires, water shortages, and the decimation of agricultural productivity across the European continent. This stock-take reads like a post-apocalyptic disaster scenario, but the event was far from fictional. The 2003 heatwave was the most fatal “natural” disaster in recent European history and the severity of the event was, to a large degree, fuelled by anthropogenic emissions of greenhouse gases. How can destruction be so firmly linked to human actions? The connection was made explicit in peer-reviewed articles such as those by Peter Stott, Myles Allen and colleagues. These concluded that it was now possible to estimate the degree to which human activities had increased the risk of such a heatwave, thus initiating a new sub-discipline within climate science: Extreme Weather Event Attribution – from here on, attribution.
Attribution requires climate modelling in which weather extremes are simulated in computer models based under two conditions, factual and counterfactual. Factual scenarios represent the world as it currently is – affected by an increased level of greenhouse gases and a corresponding level of global mean temperature increase. Counterfactual scenarios on the other hand assume a world that ‘would have been’ not affected by human influences in the otherwise identical models. Comparing those two allows for estimating the human contribution to a specific weather extreme. At present this method is most reliable for heat extremes, as models exhibit good skill in the relevant processes and the response of heat extremes to a global mean temperature increase is mostly unambiguous (see Peter Watson in Issue 1 of The Anthroposphere)
Now, fifteen years and 30 ppm of additional carbon dioxide later, the number of attribution studies has sharply increased, alongside the frequency of record-breaking extreme weather events due to largely unmitigated emissions. Today, climate attribution analysis exists in various forms ranging from the dedicated annual report issued by the Bulletin of the American Meteorological Society (BAMS) to the almost real-time attribution from the World Weather Attribution Project at ECI Oxford. Indeed, even the increased risk in human mortality during the aforementioned 2003 summer heat waves can now be reliably estimated.
Thus, the question repeatedly posed to scientists during an extreme event – ‘Was this heat wave fuelled by climate change?' – can be answered with increasing confidence. With extreme weather attribution, weather has lost its innocence.
Lived Experiences of Climate Change
Global climate change is challenging to comprehend as it exceeds the human imagination in both temporal and spatial scales. As climate change and its impacts are mostly felt via its extremes, attribution studies provide the all-important bridge between scientific knowledge and Ordinary Knowledge. The latter term, drawn from the works of Science and Technology Studies scholar Sheila Jasanoff, can be understood as characterized as awareness rooted in lived experience. Attribution humanizes and localizes data and scientific evidence, thereby going some distance to recognising global climate change and its devastating implications as a lived experience.
This localization of climate effects is key to motivating a political response to climate action. People need to see ‘their’ lived event linked to the wider phenomenon of climate change, as a prelude to taking action. As Elinor Ostrom and her colleagues concluded in their research on common pool resource management, it is communities in which a local environment is personally valued where community members are more inclined to take defensive action against perceived threats.
Through influencing the narrative of lived experiences, attribution can therefore transform how climate change is understood – and make a quantifiable link between an abstract concept and our tangible present.
Timing is crucial for making such connections between first-person knowledge and global climate systems.
Early attribution studies, such as that by Peter Stott and colleagues and those collected in the special issues of the Bulletin of the American Meteorological Society, were published with a significant time lag – at least a year – after the event in question occurred. In recent years, there has been a major reduction of the time between events themselves and the attribution of those events to human activities. Today, attribution studies are increasingly being produced during the extreme event they seek to reflect. This elimination of a time lag has been made possible through a combination of factors including a better understanding of climatic processes, more sophisticated models, and increased computing power. Primarily, this reduction is due to the emergence of dedicated research projects. For example, with projects like ECI Oxford’s weather@home, computing power is aggregated by members of the public who donate the processing power of their home computers to help run intensive climate models.
The links between anthropogenic climate change and hurricane Florence, which hit the United States in September 2018, were made in real time. Anthropogenic climate change manifested as warmer sea surface temperatures and additional moisture available in the atmosphere, which led to an estimated 50 percent increase in rainfall relative to the counterfactual world. The rains brought by Florence caused an estimated damage of $22 billion to the U.S. states of North and South Carolina, the wettest tropical cyclone recorded in that region.
Communicating Climate Change
Democratising discussions of the link between specific weather events and climate change has the potential to have climate knowledge disseminated