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What caused the flood disaster? Is the climate crisis to blame?

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On Saturday I was at Maribyrnong Community Centre. Some people were there for a community meeting; others had chosen to return home and start sweeping the thick, foul-smelling mud out of their bedrooms and bathrooms.

Some people at the meeting still couldn’t quite wrap their heads around what had happened. Thursday night’s rain wasn’t that heavy – and then, under the cover of darkness, the Maribyrnong River jumped its banks and muddy water swallowed their homes.

The aftermath of the flooding in Maribyrnong.Credit:Scott McNaughton

The answer: an interaction of short-term weather events and long-term weather cycles. Laid over the top: the insidious role of a warming planet.

“The climate system is on steroids,” says Professor David Karoly, a leading climate scientist at the Climate Council. “Now is exactly the right time to talk about it. Because now we can see the impacts.”

Why did this happen?

Three medium-term trends are making Australia’s climate much wetter than usual right now: La Nina, a negative Indian Ocean Dipole, and a positive Southern Annular Mode. My colleague Miki Perkins does a great job of unpacking them here.

These trends led to widespread and devastating flooding across Queensland and north-east NSW earlier this year, inundating tens of thousands of homes and killing 22 people. On Thursday, it was south-east Australia’s turn.

On Wednesday morning a huge low-pressure system was spinning over the Great Australian Bight.

At the same time as that system was building, a tropical low was sitting in the Indian Ocean to the west of Australia. The Indian Ocean is particularly warm right now, allowing a very large amount of moisture to build above it.

The low-pressure system grabbed this cloud and helped pull it thousands of kilometres east across Australia; at one point this band of cloud was measured at more than 5000 kilometres in length, says Ben Domensino, meteorologist at Weatherzone.

The low-pressure system was moving unusually slowly. This meant the cloud band it had tugged across was also moving slowly – allowing it extra time to dump rain on Victoria, Tasmania and NSW.

But that heavy rain may not have caused disaster in Victoria had the ground not already been saturated.

Scenes from around Melbourne after the rain poured down.Credit:The Age

August and September were unusually wet and cold, with some parts of the state receiving double the usual rainfall, and moderate flooding in several water catchments. It was the wettest August in 12 years, while September saw 11 sites record their highest rainfall levels in at least 20 years. Cold weather means less water than usual evaporates, leaving the ground sodden.

The state’s dams offer a second line of defence against flooding, absorbing some excess rainfall.

Not this time. Several years of heavy rainfall, including two wet summers, left Melbourne’s water-storage 95 per cent full; several dams were already making controlled releases.

All that primed the state for floods when heavy rain came.

Did the climate crisis play a role?

Overall, the climate crisis increases the risk of flooding in Australia.

Warm air can hold more water than cold air. When that moisture turns to rain, it’s more likely to fall as an intense burst. In recent decades the intensity of extreme rainfall events has increased by about 10 per cent in some parts of Australia, according to the Climate Council.

“It makes some extreme events, like extreme temperatures and rainfall, more extreme,” says the Climate Council’s Karoly.

The climate crisis does not directly cause a flooding event like this, but it can make one more likely – or more severe.

Whether it played a role this time is now up to scientists to determine. A new field called “event attribution” is blossoming as scientists work to link disasters to a warming climate. For heat extremes, this is fairly straightforward but determining “the effect on any one extreme rainfall event and the influences leading to floods, such as wet soils – it is more complicated,” says Michael Grose, a climate scientist at the CSIRO.

Does that matter? I don’t think so. What the floods expose is that large parts of Australia are very vulnerable to flooding. The Maribyrnong River winds its way through Melbourne’s north-east; when rivers flood, those bends become a straight line. The evidence clearly shows flooding will become more likely in the future. It is time for us to start living as though the impacts of the climate crisis are here – because it is getting worse.

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