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One in 1,000 years? Old flood probabilities no longer hold water

TJ Ryan Foundation Research Associate, John Quiggin, writes in The Conversation (10.3.22) about the recent devastating floods to hit southeast Queensland and northern New South Wales. The author reminds that, while 'one in 1,000 years'-style descriptions apply to probabilities that don’t change, the likelihood of severe floods is in fact increasing.

'Australia’s catastrophic east coast floods have been described by the NSW premier as a “one in 1,000-year event, a term that has created a great deal of confusion. Lengthy explanations that these terms are not the same as "occurring 1,000 years apart” or “once every 1,000 years” have only added to the confusion.

'The simplest explanation is that the actual meaning of “one in 1,000 years” is “having a probability of 0.1 per cent in any given year” (1 in 1,000), which raises the question: why don’t people simply say that?

'The main reason is that these terms date back to a time when most people didn’t think in terms of probabilities, and even those who did were confused about how they worked. These days we interact with probabilities all the time.

'... But the probability of a severe flood changes over time as the relationship between the components that make up the weather system change. Whether a flood has occurred gives us evidence about that change. This makes it no longer helpful to refer to a severe flood as “one in x years” event.'


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