the weather system causing flooding in Queensland will become more likely with climate change
Severe flooding in southeast Queensland this week forced hundreds of residents to flee the town of Gympie and cut off major roads, after intense rains battered the state for several days. The rain is expected to continue today and move south into New South Wales.
We are looking for a weather system called “atmospheric rivers”, which is causing this flood. Indeed, atmospheric rivers triggered many floods around the world in 2021, including the devastating floods in eastern Australia in March which killed two people and saw 24,000 evacuated.
We found that if atmospheric rivers bring much-needed precipitation to the agriculturally important Murray-Darling Basin, their potential to cause devastating floods will become more likely in a warmer world under climate change.
What are atmospheric rivers?
Atmospheric rivers are like highways of water vapor between the tropics and the poles, located in the first one to three kilometers of the atmosphere. They are responsible for about 90% water vapor moving from north to south of the planet, although it only covers 10% of the globe.
When atmospheric rivers crash into mountain ranges or interact with cold fronts, they rain down this water with potentially disastrous impacts. Mountains and fronts lift water vapor into the atmosphere where it cools and condenses into bands of giant clouds forming a liquid. Intense thunderstorms can also form in atmospheric rivers.
Last year, three atmospheric rivers were particularly devastating.
In November, British Columbia, Canada was battered by record rainfall that isolated Vancouver from the rest of the country.
And in March, eastern Australia suffered a flood that led to widespread flooding and A$652 million damage value. All states and territories on the mainland except WA faced simultaneous weather warnings.
What we found
Our recently published research provides the first quantitative summary of atmospheric rivers over Australia. It’s not all bad news – mostly atmospheric rivers bring beneficial rains to Australia. About 30% of southeast Australia’s rainfall comes from atmospheric rivers, including the Murray-Darling Basin.
Precipitation is vital for this region. The Murray-Darling Basin supports over 500 species of birds, reptiles and fish, and around 30,000 wetlands. Agriculture in the Murray-Darling Basin contributes 24 billion Australian dollars to the Australian economy.
Read more: How an ‘atmospheric river’ flooded British Columbia and caused flooding and mudslides
However, we also found that 30-40% of the heaviest rainy days in the northern Murray-Darling Basin, where cities such as Tamworth, Dubbo and Orange are located, were associated with atmospheric rivers.
A heavy downpour in Australia’s breadbasket might make farmers happier during a dry spell, but after a wet summer – like La Niña – those days are less welcome.
La Niña saturates the ground
La Niña can play a significant role in flooding because it exacerbates the damage caused by atmospheric rivers.
A La Niña was declared in the spring of 2020 and died out in March 2021. A second La Niña arrived in the summer of 2021 and 2022.
During a La Niña episode, the winds that blow from east to west near the equator increase. This leads to cold, deep ocean waters rising to the surface in the eastern Pacific near South America, and warm ocean waters forming near Australia.
Read more: Back so soon, La Niña? Here’s why we copy two soggy summers in a row
Warm sea surface temperatures favor rainfall, which is why La Niña is associated with more rainy weather over much of Australia.
The floor is like a kitchen sponge. It absorbs water, but once saturated, it can no longer absorb it. That’s what happened in eastern Australia in the months before the March floods – and when the record rain fell, the ground was inundated.
Our recent research has found that on March 17-24 last year, NSW experienced an almost constant flow of high atmospheric water vapor over both an atmospheric river that originated in the Indian Ocean and of a high pressure system in the Tasman Sea.
On March 23, more than 800 kg of water vapor passed over Sydney every second, or 9.6 Sydney water ports in one day.
Read more: Sydney’s disastrous flooding was not unprecedented: we are about to enter a 50-year period of frequent and major flooding
Similarly, soil moisture in southeast Queensland has been above average since last October. Last November was Australia the wettest Record November, with South East Queensland receiving well above average rainfall.
This meant that the ground was already soggy. So when the heavy rains fell this week, Queensland was inundated.
What is the role of climate change?
We also calculated the probability of future atmospheric rivers as large as the one in March 2021 flowing over Sydney using the last generation climate models.
Earth is currently on track to 2.7℃ warming at the turn of the century. In this scenario, we found that the probability of a weather event similar to the March floods becomes 80% more likely. This means we are on track for more extreme rainfall and flooding in Sydney.
We also know that climate change will increase the frequency of atmospheric rivers across the planet, but further research is needed to determine how often we can expect these damaging events to occur, including in south-east Queensland. .
However, this path is not definitive. There is still time to change the outcome if we urgently cut emissions to stop global warming beyond 1.5℃ this century. Every little bit we do to limit carbon emissions could mean one less flood and one less person to rebuild.
Read more: Floods leave a legacy of mental health issues – and the disadvantaged are often the hardest hit