For thousands of people around Australia the Christmas of 2023 will be remembered not as a season of joy and goodwill, but for torrential storms and flash floods that claimed lives and tore through homes.
At least seven people died in storms in Queensland and Victoria from flood waters and falling debris, and three more were lost from a yacht in Brisbane’s Moreton Bay.
The Insurance Council of Australia saw more than 46,000 insurance claims linked to the extreme weather between 23 December and 3 January, mostly from Queensland, with about two-thirds relating to property damage.
The damage came after parts of New South Wales were also caught in flash floods from torrential downpours in November.
Much of the flooding in recent weeks has come from downpours over the course of a few hours, giving authorities and the public little time to prepare, and pushing storm drains beyond their design limits.
It is these types of downpours that climate scientists say are becoming increasingly intense in Australia because of a warming planet, and will probably intensify further as the planet warms.
But why?
More heat, more moisture
Australia’s landmass has warmed by almost 1.5C since records began in 1910 – a level of warming slightly higher than the global average.
Scientists have known for a long time that a warmer atmosphere is able to hold more water.
An oft-quoted figure is the “Clausius-Clapeyron equation”: for every 1C of warming, the atmosphere can hold about 7% more moisture.
One detailed study in 2018 found that for daily rainfall in Australia, amounts were increasing in line with expectations from warming – but were not outside what might be expected through natural climate variations.
But when the scientists analysed the intensity of hourly rainfall, the trend was much clearer.
Across the continent, the amount of rain falling over hourly periods was increasing at about double – and in some places triple – of that expected from the Clausius-Clapeyron equation.
A separate study has suggested the number of storms in some regions may be decreasing, but there is a trend for them to dump more rain. One study of 10-minute rain bursts over Sydney suggested they had intensified by 40% over the past two decades.
Dr Andrew Dowdy, an associate professor at the University of Melbourne and expert on extreme rainfall trends in Australia, says: “Many lines of scientific evidence show that sub-daily rainfall extremes have already increased in intensity as compared to previous decades, due to the human-caused global warming that has already occurred.”
He says many of the most extreme downpours ever recorded in Australia were from thunderstorms, but low-pressure systems and cyclones can also deliver sudden bursts of rain.
Before Christmas, ex-Tropical Cyclone Jasper caused flooding across vast areas of far north Queensland, with Bureau of Meteorology data showing it delivered Australia’s wettest December days on record. One weather station recorded 1.9 metres of rain over five days, and several non-bureau gauges reported totals in excess of 2 metres.
Dr Kimberley Reid, a climate scientist researching Australian rainfall at Monash university, says major downpours on timescales of less than a day are controlled by two key ingredients: the amount of moisture in the air and a weather system that can lift it up and condense it into rain.
“We know that climate change is likely increasing the thermodynamics so, when it rains, it pours,” she says. “We’re seeing clear trends in atmospheric water vapour.”
What is less certain, she says, is how global heating might be changing weather systems’ behaviour.
Dr Richard Matear, a senior scientist at the CSIRO with the Australian Climate Service, says studying human influences on rainfall is challenging because of the chaotic nature of weather systems and storms, and the relatively short record of observations. Extreme rainfall is also a natural feature of the Australian climate, particularly in the north.
Because of that, Matear says, many scientists working in the field are cautious about attributing specific events to global heating.
“The impacts of global warming will be felt in the extreme events,” he says. “So understanding how they respond to a future warmer world is a first-order question.”
There is also evidence global heating can influence rainfall extremes across longer timeframes.
A study on the La Niña of 2010 and 2011 that helped drive Australia’s wettest spring on record found global heating likely increased the amount of rainfall.
Ocean heating off the east coast, linked to climate change, made more moisture available for weather systems, the study found. Those systems dropped so much rain that global sea levels – also rising because of the burning of fossil fuels – briefly dipped.
Increased risk
The summer of 2023 was expected to be drier than usual because of an El Niño climate pattern. But some climate scientists have suggested one reason for the downpours could be a periodic shifting of westerly winds towards Antarctica, that tends to increase the flow of moisture from the ocean on to the eastern seaboard.
Some studies have suggested this shift of the so-called Southern Annular Mode is happening more often because of climate change.
Dr Tom Mortlock, a senior analyst at risk management consultancy Aon and adjunct fellow at the Climate Change Research Centre at the UNSW, says the increasing intensity of rainfall over short timeframes is a big concern.
“Sub-daily rain extremes lead to flash flooding – particularly in urban areas where the land surface is impervious, leading to dangerous overland flows. There is often little lead time and waters rise rapidly, leaving individuals and communities little time to prepare or evacuate.”
He says that in cities and urban areas, drainage networks and culverts were critical in moving water away from infrastructure, but were often “operating beyond their design capacity” aside from being blocked by debris.
“Rain rates responsible for flash flooding are increasing at a considerable pace with atmospheric warming.”
Several leading Australian experts on extreme rainfall, including Dowdy, are recommending Australian design standards for flooding allow for a 15% increase in the “intensity of sub-daily extreme rain” for every degree of global heating.
Mortlock says there is ongoing work being done by the federal government to update the Australian Rainfall and Runoff (ARR) guidelines that apply to major infrastructure projects, such as drainage and roads, as well as estimates of how extreme floods could get.
“While this is a very well-designed and implemented system, the ARR is built off rain observations [made] over the past several decades, which may no longer reflect the current environment we are in,” he says.
“For that reason, there is a concern that some existing urban drainage infrastructure may be ‘under-engineered’ to withstand the increase in rain rates expected with continued atmospheric warming.”
In a statement, the Bureau of Meteorology said analysing sub-daily rainfall records would take “considerable time” because amounts varied across regions.
How the Christmas storms related to long-term trends was a question “best answered by a peer-reviewed publication”.
