The Attribution Question

Whenever an extreme weather event happens that impacts society the “attribution question” whether human induced-induced climate change played a role is asked.

Scientists are now able to answer this question for many different types of extreme weather events. However, the answer depends on how the attribution question is framed. Different framings and differing definitions of the extreme event in question lead to different answers.

In a new commentary published in Nature Climate Change today CPDN researchers investigates how different studies pose the attribution question highlighting that it is paramount for every extreme event attribution study to clearly define the event and state the framing of the attribution question being asked

Watch CPDNs Fredi Otto explaining the paper.

Every extreme weather event has multiple causes and ultimately defined unique. The same impacts of the event that occurred however are usually not unique but can be caused by a certain type or class of extreme events, e.g. a summer heat wave of a certain temporal and spatial extend with temperatures above a critical threshold. If the purpose of an attribution study is thus to make planning decisions in disaster recovery the harm caused and not the exact state of the atmosphere are the defining characteristics.

Human induced climate change can alter the weather in different ways and there are two basic effects:

Firstly, with raising temperatures on a global average we expect more heat waves on average. Warmer air can also hold more water vapour so we expect more heave downpours.

However, the increased concentration of greenhouse gases in the atmosphere can also alter the atmospheric circulation. This is the second, so-called dynamic, effect. Locally changes in the circulation can counteract or increase the thermodynamic effects and thus for example decrease the risk of extreme rainfall in a particular region and season despite the globally observed increase.

Attribution studies rely on climate models which are necessarily imperfect in particular with respect to representing the atmospheric circulation. For that reason some scientists have suggested to only focus on the thermodynamic effect and ask the attribution question:

Given the state of the atmosphere at the time of the event how did the overall warming influence the event.

This is scientifically interesting and might answer a very specific stakeholder question but the answer to this question does not say anything about how likely the atmosphere is ever going to be in that state again. And thus what the overall risk of the event occurring is and how it changes. From the perspective of a stakeholder seeking information to inform disaster risk reduction strategies, it can however be unhelpful to ask the question in this conditional framing as it does not assess whether the event that has caused the harm was a case of bad luck and is unlikely to happen again or whether this type of event is what society needs to get used to.