In the 1950s, flood and rain records were set in New South Wales that stood unchallenged for a lifetime. Until now.
Condoblin's biggest flood was 1952.
Until this year.
Sydney's wettest year was 1950.
Until this year.
And New South Wales' wettest year ever was also 1950.
The state needs less than 100 millimetres more rain by New Year's Day to beat that record.
"There was a period from about 1949 to about 1956 in which pretty well every river in New South Wales had a genuinely big flood. And some of them had records which haven't been beaten since," said Chas Keys, a former deputy director-general of the New South Wales SES.
"For example, the Hunter River near Singleton and Maitland had their worst ever floods in 1955, and they had bad floods pretty well every year for the previous five or six years," Dr Keys said.
"The Lachlan Valley had bad floods in 52. The Macleay had bad floods in 49, followed by one almost as bad in 1950. The Clarence had really bad flooding in 50 and 54. And it goes on and on."
So what drove the seven-year deluge?
In short, the same forces behind the current floods — climate drivers.
The climate drivers behind Australia's wettest years on record
"The 11 wettest years on record in eastern Australia were all influenced by La Niña," wrote The Bureau of Meteorology and CSIRO in the recent State of the Climate report.
"The impact of multiple flood events is particularly pronounced when La Niña events occur in multiple successive years, as occurred in 2020-22, and previously in periods such as 1954-56 and 1973-76," they added.
While attention has focused on La Niña's role driving eastern Australia's wettest years, there's intriguing evidence that the Indian Ocean Dipole or IOD was also in its rain-promoting phase in some of the wettest years of the 1950s.
"The IOD is a relatively recent phenomenon. It was only discovered in 1999 by Japanese scientists," said Andrea Taschetto, a climatologist from the University of New South Wales.
"So the measurements before 1960 are a little bit uncertain because at the time we needed to have measurements taken by either buoys or ships at sea," Dr Taschetto added.
"But looking back in time in the index, we can see that there were a number of negative Indian Ocean Dipole events in the 1950s; 1954 to 1956 is considered a negative Indian Ocean Dipole event."
The Indian Ocean Dipole was in its negative, rain-promoting phase for both 2021 and 2022, just like the mid-1950s.
Australia's climate driven by oceans that surround it
Flood risk in Australia changes dramatically from year to year, driven by changes in the big climate drivers such as La Niña, according to Anthony Kiem, a hydroclimatologist from the University of Newcastle.
Dr Kiem said flood risk right now was much higher than normal, for similar reasons the 1950s were so wet.
"We know that when it's a La Niña, that the chance of that 'one-in-100-year' or '1 per cent' flood event occurring is not 1 per cent. It's significantly greater than that. It's about four or five times more likely," he said.
"We also know that when a La Niña occurs back-to-back with another La Niña, that chance increases again.
"The chance of flooding after back-to-back La Niñas is always going to be elevated because if you receive significant rainfall and your reservoirs are full and your catchments already saturated, that rain has to run off somewhere."
Is there a super-climate-driver behind Australia's super-flood cycles?
Dr Kiem said there was another factor that existed on top of back-to-back La Niñas that might be driving east coast Australia's flood super-cycles in the 1950s, 1970s and today.
He said it was a lesser-known climate driver called the Interdecadal Pacific Oscillation.
Think of it as a massive La Niña or El Niño, that can last for decades.
"The Interdecadal Pacific Oscillation is the warming and cooling of the Pacific Ocean on a 10, 20, 30-year timescale," Dr Kiem explained.
"So when you've got an IPO negative phase, the chance of a La Niña year event is significantly increased. And we know when we get a La Niña, you get increased chance of flooding."
There was a prolonged period of negative IPO and La Niña events in the 1950s, just as there is today.
As the forces driving flooding pile up, so too does the flood risk, according to Dr Kiem.
"When it's IPO negative, the chance of the 1 per cent flood event goes up to something like 12 per cent — so a 12 times greater chance of a one-in-100 year-flood," he said.
La Niña or IPO?
Decadal oscillations are a hotly debated topic in climate science, according to Mike McPhaden from the US National Oceanic and Atmospheric Administration.
"It's a big chicken or the egg question," Dr McPhaden said.
"Does La Niña and El Niño drive the IPO, or vice versa? Or is IPO just a name for periods where you have more La Niñas than El Niños?
"Whatever you call it, whether it's the underlying IPO, or whether it's a sequence of La Niña events, in some sense that's no longer relevant because those conditions are going to have a climatic effect in places like Australia, bringing prolonged seasons of unusually heavy rains."
None of this was known in the 1950s.
The legacy of the 1950s floods
What was known was that something had to be done to protect lives and property, after so many years of flood disasters.
"The 1955 Hunter flood was the straw that broke the camel's back," Dr Keys said.
"The state Labor government of the time said, 'We've got to do something different from what we've been doing to get on top of this problem'.
"Government did two things really. They formed the SES. And they spent decades building levees and flood bypasses, spillways, to try to manage and to a degree control floodwaters."
The next triple La Niña phase of the 1970s brought a new wave of record floods, though this time centred on Queensland.
1974 was Australia's wettest year on record.
In the wake of the 1974 disasters, a new weapon was trialled in the fight against flooding.
"That's when we started saying, let's not build on the lowest parts of the flood plain. Let's only build above the level reached by the 1-per-cent flood," Dr Keys said.
"That was another move in the right direction in terms of making people free from flooding. Now, it didn't apply, of course, to buildings built before then.
"In the case of the Hawkesbury, there are still 5,000 homes below the currently allowable standard, which is you don't build below the 1 per cent, the so called 100-year flood line. And they're mostly from before the standard was established in the 1970s," said the former emergency chief.
Fast-forward to 2022 and the IPO is once again negative, La Niñas and negative IODs have lined up back-to-back again, and New South Wales towns have disappeared beneath unimaginable floods.
Now, the climate change wildcard
And there's a new wildcard in calculating flood risk — climate change.
"It is expected that extreme rainfall will increase in Australia with climate change," wrote The Bureau of Meteorology and CSIRO in the recent "State of the Climate" report.
But while extreme rainfall overall is expected to increase, how climate change will impact on weather patterns in individual locations is still uncertain, according to the BoM and CSIRO.
In other words, some locations may experience more flooding, others less, depending on how climate change affects local weather patterns.
The report states regions in southern Australia that are already experiencing a long-term drying trend may still experience increases in extreme rainfall.
What will the legacy of these floods be?
If the legacy of the 1950s floods was engineered flood mitigation, and the legacy of the 1970s was regulation, what will the legacy of the current terrible phase of flooding be in Australia?
For Dr Kiem, it's critical that people understand that flood risk is neither static, nor random, rising exponentially when climate drivers align in their flood promoting phases.
"We're thinking that the risk of that 100-year flood is 1 per cent. But this year, the chances of that are actually 10 plus times greater.
"I don't know many people that would go to a casino and play a game where they thought the risk was 1 per cent [would] … be happy to find out later that actually they had a 12 per cent chance of losing."
"Would you still play the game in the same way? And that's exactly what we're doing with flood risk."
For Dr Keys, one the lessons of these latest floods is that mitigation has its limits, and those at greatest risk need to move to higher ground.
"We need to be educating people better about floods and how to manage them," he said.
"That would start with people being able to understand that they live on flood plains. That sounds simple. But some people don't even know they live on a flood plain.
"We need to be buying back some of the past mistakes we've made with regard to how we've settled our country, the locations on which we have grown large populations.
"We need to be buying those people out to the extent possible. And it's a terribly expensive thing. But it's also terribly expensive to be subsidising their existence in those locations, time after time after time, as we have over the last couple of years."
