Think of the chance that an extreme weather event occurs as a probability when rolling dice. In a world without any human-induced climate change (a counterfactual world), there is a certain probability that a climate variable, such as temperature, crosses a threshold point which would cause an extreme event. Take a look at the diagram underneath, where the blue line shows this climate variable’s distribution in the counterfactual world, and you’ll see that this probability is the blue area, P0.
Now, in the actual world, where the forcings from greenhouse gases and aerosols emitted by human activity on the climate system are included, the distribution of the climate variable changes to that represented by the red line. The red area, P1, is the probability of crossing the extreme event threshold in the actual world. As you can see, the additional forcings have in effect loaded the dice, increasing the probability of an extreme event happening.
So, how can we know whether an extreme event was caused by human-induced climate change causing a threshold to be crossed? By some pretty nifty maths, we can work out the Fraction Attributable Risk:
Fraction Attributable Risk=1-(P0/P1)
As P1 increases, the (P0/P1) part of the equation tends towards zero, so the overall Fraction Attributable Risk stays closer to the maximum probability of 1. Therefore, if the answer is around 1, we can say that the threshold could only have been crossed in a world where humans have an influence on the climate system.
For most extremes these changes in risk are more subtle, but in the highly complex climate system, with a lot of different variables and knock-on processes, known as feedbacks, this can have a profound effect on the weather experienced all over the planet. From ravaging droughts in East Africa to typhoons in the Philippines, the destruction of homes and livelihoods has affected millions worldwide, and the economic chaos as infrastructure is decimated and services grind to a halt costs billions of dollars. On the other hand, other weather events have become less likely to occur, such as in southern England, where flooding historically coincided with snowmelt, but as it snows less there now there is a decreased likelihood of flooding. It is therefore crucial to understand that human-induced climate change does not guarantee extreme weather events, but influences the probability that they happen.
After all, it’s all in the roll of a (loaded) dice.
This blog post is based on Dr. Friederike Otto (2017) Attribution of Weather and Climate Events, and Prof. Myles Allen’s lecture series Climate Change: A Summary for Policy Makers.
To find out more, and examples of weather events attributed, take a look at https://www.worldweatherattribution.org/analysis/cold-spells/
Thank you also to Dr. Friederike Otto for helping to edit this post.