The New South Wales premier, Dominic Perrottet, made headlines earlier this week when he described the record-breaking rain that swamped the north of the state before moving south to Sydney as a “one-in-1,000-year event”. The mayor of Ballina shire council, Sharon Cadwallader, told the ABC that town was expecting a one-in-500-year flood before it was inundated on Wednesday.
To the layperson, these descriptions might be interpreted as suggesting floods would only occur once in a millennium, or half a millennium. But this isn’t the case, and scientists say such phrases risk misleading the public about the likelihood of disasters of this magnitude being repeated.
Where does the one-in-1,000 event description come from?
According to the premier’s office, it was drawn from a 2014 Lismore Floodplain Risk Management Plan that included data on the expected “average recurrence interval” for floods of different degrees of severity.
There is no direct mention of a one-in-1,000-year flood level in the report, but it does set out the expected frequency of various flood peaks, as measured against a gauge at the city’s rowing club.
The highest is 16 metres above sea level, which is the “probable maximum flood” level for a notional one-in-100,000-year flood. A one-in-500-year flood is set at 13.4 metres; a one-in-100 year flood at 12.4 metres; a one-in-10 year flood at 10.9 metres.
What is known as “danger height” – the level at which local landholders are alerted – was set at less than half that – just 5.2 metres above sea level.
This week’s flood in Lismore reached 14.4 metres, placing it somewhere in the vast gulf between the once-in-500-year and once-in-100,000 year markers – hence, the back-of-the-envelope description of it as a one-in-1,000-year event.
Does any of this mean anything?
Yes, but not what you might think. Describing a flood as a once-in-a-millennium event actually has a specific meaning.
But it’s not, as many people might assume, that the planet won’t see another one until some time around the year 3000.
When employed in a technical report, it means there is a 0.1% chance of a flood of that severity happening in a given location in any given year. (For a one-in-100-year flood it’s a 1% chance, for a 1-in-10 year flood it’s a 10% chance, and so on.)
Dr Tom Mortlock, an adjunct fellow at Macquarie University and senior catastrophe analyst with reinsurance broker Aon, says when translated to lay use, describing the current flood as a one-in-1,000-year event is misleading.
“It implies we would be waiting another 1,000 years before we see another flood of this magnitude again,” he said. “The fact we’ve had a one-in-1,000-year flood a year after we’re supposed to have had a one-in-100-year flood [in Sydney last March] basically refutes that straight away.”
Mortlock says the chance of a catastrophic flood occurring somewhere in NSW, across all the areas at risk of flood in any given year, is much greater than 0.1%.
It is higher still when there is a La Niña weather pattern. A La Niña pushes warm surface water towards Asia and the seas north of Australia, which in turn leads to increased rainfall across northern and eastern Australia. Mortlock says La Niña has “loaded the dice in favour of flooding over the past two years”.
He says one issue with saying a flood on this scale only occurs once in a millennium is the suggestion that it was not foreseeable, and that there is no need to plan for the next catastrophic storm when in reality it could come much, much sooner.
As we have seen this week, failing to properly plan can have terrible consequences for people’s lives and livelihoods.
Some people had rebuilt homes and businesses after previous floods, believing it was unlikely that a similar catastrophe would strike them in the near future. They were hit again, to devastating effect. A key question is: would a more realistic expression of the probability of a flood reoccurring have helped limit the impact?
Does global heating change the equation?
Yes. No flood can be blamed directly on the climate crisis, but it also loads the dice. The scientific rule of thumb is that 1C of heating allows the atmosphere to hold about 7% more moisture. The planet has already warmed more than 1C since pre-industrial times due to increased atmospheric concentration of greenhouse gases. It increases the risk.
Dr Andrew King, a climate scientist at the University of Melbourne, agrees with Mortlock: saying a flood is a one-in-1,000-year event is “definitely a misleading statistic”, he says, particularly when applied to several different places at once.
“People don’t understand that it doesn’t mean an event happens every 1,000 years, and in a changing climate the probability of these events will change as well.”
A key issue here is that the likelihood of an event recurring changes retrospectively as more data is collected. Put another way, it can be seen in hindsight that the chance of an extreme event like a flood happening was greater than previously understood.
An example of this is the flood in Queensland’s Lockyer Valley in 2011. A flood of that scale was initially deemed to have a one-in-2,000 chance of happening in any given year. After another flood came along two years later, and with five more years of data, that was recalculated to a one-in-90 chance – a substantial upgrade in likelihood.
King also gives the example of the Covid-19 pandemic, which has been described as a 1-in-100 year event based purely on there having been only one other global pandemic in recent history – the 1918 Spanish flu pandemic.
He says this misunderstands an important thing about recurrence statistics – that as you add more data, “you change them a lot”.
