Climate Attribution and the Buchan Caves Reserve Boxing Day Flood

A State Emergency Services flood guide stated that on Boxing Day 2023, “a severe thunderstorm resulted in localised intense rainfall causing significant stormwater run-off across Buchan, and severe flash flooding in Spring and Fairy Creeks impacting the Buchan Caves Reserve. During this event over 110 millimetres fell at Buchan with 69.4 millimetres in the 30 minutes to 4:48pm and 87.8 millimetres in the one hour to 6:18pm. The December 2023 rainfall amounts were extreme and more than a 1 in 2000 AEP rainfall event.” Referring to last figure the Flood Guide noted that the “Annual Exceedance Probability (AEP) is a term used to the express the percentage of likelihood of a flood of a given size or larger, occurring in a given year. If a flood has an AEP of 1%, it has a one in 100 likelihood of occurring in any given year.”

Previously I have commented on these floods and noted “on Boxing Day we had our own flash flood in the Buchan reserve with floating cars, caravans and two fatalities”. The Guardian reported on them in detail (image) and noted widespread damage. Local Councillor Mendy Urie referred to the floods as “I think people’s understanding of climate change is changing, even over the past six months, 12 months…But I think we need to be even more aware.” Whilst statistically this is an exceedingly rare event and obviously strongly influenced by climate change, it remains difficult to prove this link beyond doubt.

Here the new science of climate attribution may eventually help. It “is the study of whether [or how much] …human influence…contributed to extreme climate or weather event. Scientists can now estimate whether human activities have influenced extreme weather or climate events and how likely they are to occur.” Using computer models this event is compared with a pre warming state for the same location and then estimates can be made of the probability of the event occurring with human caused global warming.

The atmosphere holds 7% more moisture for each degree of warming and a recent study has confirmed that the moisture in the atmosphere that results in flash floods may be much more than this. A “hotter atmosphere has the capacity to hold more moisture. But the condensation of water vapour to make rain droplets releases heat. This, in turn, can fuel stronger convection in thunderstorms, which can then dump substantially more rain. This means that the intensity of extreme rainfall could increase by much more than 7% per degree of warming. What we’re seeing is that thunderstorms can likely dump about double or triple that rate – around 14–21% more rain for each degree of warming”.

The Buchan flood was not an aberration but entirely predictable and another warning that more events of a similar nature, and much worse, are on the way. Attributing events such as this clearly with global warming is becoming easier, and with litigation, perhaps one or two of the many responsible may eventually be brought to account.