Over the past three years, I’ve been working on the forthcoming report by the United Nations’ Intergovernmental Panel on Climate Change. I’m a climate scientist who contributed to the chapter on global water cycle changes. It’s concerning to think some theoretical impacts described in this report may be coming to life – yet again – in Australia.
The recent flooding in New South Wales is consistent with what we might expect as climate change continues.
Australia’s natural rainfall patterns are highly variable. This means the influence climate change has on any single weather event is difficult to determine; the signal is buried in the background of a lot of climatic “noise”.
But as our planet warms, the water-holding capacity of the lower atmosphere increases by around 7% for every 1℃ of warming. This can cause heavier rainfall, which in turn increases flood risk.
The oceans are also warming, especially at the surface. This drives up both evaporation rates and the transport of moisture into weather systems. This makes wet seasons and wet events wetter than usual.
So while Australia has always experienced floods, disasters like the one unfolding in NSW are likely to become more frequent and intense as climate change continues.
Understanding the basics
To understand how a warming world is influencing the water cycle, it’s helpful to return to the theory.
From year to year, Australia’s climate is subject to natural variability generated by the surrounding Pacific, Indian and Southern oceans. The dominant drivers for a given year set up the background climate conditions that influence rainfall and temperature.
It is a combination of these natural climate drivers that makes Australia the land of drought and flooding rains.
However, Australia’s climate variability is no longer influenced by natural factors alone. Australia’s climate has warmed by 1.4℃ since national records began in 1910, with most of the warming occurring since 1970. Human-caused greenhouse emissions have influenced Australian temperatures in our region since 1950.
This warming trend influences the background conditions under which both extremes of the rainfall cycle will operate as the planet continues to warm. A warmer atmosphere can hold more moisture (higher water vapour content), which can lead to more extreme rainfall events.
Since the winter of 2020, Australia has been influenced by the La Niña phase of the El Niño–Southern Oscillation (ENSO). Historically, sustained La Niña conditions, sometimes with the help of a warmer than average Indian Ocean, have set the scene for severe flooding in eastern Australia.
During these events, easterly winds intensify and oceans around Australia warm. This is associated with the Walker Circulation – a giant seesaw of atmospheric pressure that influences the distribution of warm ocean waters across the Pacific Ocean.
The last La Niña occurred in 2010–2012. It led to widespread flooding across eastern Australia, with particularly devastating effects in Queensland. The event caused the wettest two-year period in the Australian rainfall record, ending the 1997–2009 Millennium Drought.
Oceanographers from UNSW studied the exceptional event. They demonstrated how a warmer ocean increased the likelihood of extreme rain during that event, primarily through increased transport of moist air along the coast.
Their analysis highlighted how long‐term ocean warming can modify rain-producing systems, increasing the probability of extreme rainfall during La Niña events.
It is important to point out that changes in large-scale atmospheric circulation patterns are still not as well understood as fundamental changes in thermodynamics. However, because regional rainfall changes will be influenced by both factors, it will take researchers time to tease everything out.
So what about climate change?
The theoretical changes to the global water cycle are well understood. However, determining the contribution of natural and human influences on climate variability and extremes – known as “attribution” – is still an emerging science.
More studies are needed to distinguish natural or “background” rainfall variability from recent human-caused changes to the water cycle. This is particularly the case in a country like Australia, which has very high yearly rainfall variability. This contrasts with some regions of the Northern Hemisphere with less variable rainfall, where a clear climate change signal has already emerged.
Right now, La Niña conditions are decaying in the Pacific Ocean. As expected, the 2020–2021 La Niña has brought above-average rainfall to much of eastern Australia. This helped ease the severe drought conditions across eastern Australia since 2017, particularly in NSW.
What’s interesting about the 2020–2021 La Niña is that it was weak compared with historical events. The relationship between La Niña and rainfall is generally weaker in coastal NSW than further inland. However, it’s concerning that this weak La Niña caused flooding comparable to the iconic floods of the 1950s and 1970s.
The rainfall totals for the current floods are yet to be analysed. However, early figures reveal the enormity of the downpours. For example, over the week to March 23, the town of Comboyne, southwest of Port Macquarie, recorded an extraordinary 935mm of rainfall. This included three successive days with more than 200mm.
The NSW coast is no stranger to extreme rainfall – there have been five events in the past decade with daily totals exceeding 400mm. However, the current event is unusual because of its duration and geographic extent.
It’s also worth noting the current extreme rainfall in NSW was associated with a coastal trough, not an East Coast Low. Many of the region’s torrential rainfall events in the past have resulted from East Coast Lows, although their rainfall is normally more localised than has been the case in this widespread event.
Remember that as the air warms, its water-holding capacity increases, particularly over the oceans. Current ocean temperatures around eastern and northern Australia are about 1℃ warmer than the long-term average, and closer to 1.5℃ warmer than average off the NSW coast. These warmer conditions are likely to be fuelling the systems driving the extreme rainfall and associated flooding in NSW.
A nation exposed
Weather and climate are not the only influences on extreme flood events. Other factors include the shape and size of water catchments, the presence of hard surfaces in urban areas (which can’t absorb water), and the density of human settlement in flood-prone areas.
The Hawkesbury–Nepean region in Western Sydney, currently experiencing major flooding, is a prime example. Five major tributaries, including the Warragamba and Nepean Rivers, flow into this extensively urbanised valley.
Improving our understanding of historical weather data may help improve future climate change risk assessment. For example, past floods in the Hawkesbury–Nepean have been a lot worse than the current disaster. In 1867, the Hawkesbury River at Windsor reached 19.7 metres above normal, and in 1961 peaked at 14.5 metres. This is worse than the 13.12 metres above normal recorded at Freemans Reach on March 23.
It’s sobering to think the Hawkesbury River once peaked 6 metres higher than what we’re seeing right now. Imagine the potential future flooding caused by an East Coast Low during strong La Niña conditions.
It will take time before scientists can provide a detailed analysis of the 2020–2021 La Niña event. But it’s crystal clear that Australia is very exposed to damage caused by extreme rainfall. Our theoretical understanding of water cycle changes tells us these events will only become more intense as our planet continues to warm.
This article was written by Senior Lecturer in Climate Science at ANU, Dr Joelle Gergis, and has been republished from The Conversation 24 March 2021. Click here to read the original article.
The Conversation is a network of not-for-profit media outlets that publish news stories on the Internet that are written by academics and researchers, under a Creative Commons — Attribution/No derivatives license.
Comments
3 responses to “Yes, Australia is a land of flooding rains. But climate change could be making it worse”
The peak of Hawkesbury River at Windsor in 1867 was pre-Warragamba dam times, so it was the result of the free flow of the Nepean and Hawkesbury Rivers natural catchment areas before the dam held back much of the water.
In 1960 when it was completed and later opened, the wall was five metres lower than today. In other words more water had to be released earlier due to the rainfall, and that may be the reason for the peak of 14.5 meters in 1961.
“When rain and flood event studies showed that Warragamba Dam could experience floods much larger than it was originally designed for, the height of the dam wall was increased by five metres. This was the first of two steps to ensure the dam met modern international dam safety standards. The wall was raised and strengthened using post-tensioned steel cables to tie the upper wall to its base.”
https://www.waternsw.com.au/supply/heritage/timeline/timeline/1987-89
Now with a 5 metre height extension and the damn being able to store more water we just witnessed 13.12 at Freemans Reach.
Arguments about raising the dam wall continue, but if extreme rainfall events continue to increase at latitudes further South in Australia as predicted due to climate change, raising the wall may be good for times of drought so more water can be saved for a growing Sydney (the other problem), but to do this is also a question of causing further environmental damage, and destruction of indigenous sacred sites. In any case what we have seen with one wall raise of 5 meters is that it was not enough, and that is only some 30 years ago since it was built.
At some stage whether the wall is raised or not or further spillways added to release water faster, it will still have to jettison water if there are excessive volumes of rain falling. No one also knows whether this was really a 100 year or 1000 year event as yet either. Adding such an event to climate change would create even greater levels of water having to be held back. Some water can be released slowly during convenient periods when down stream water levels are lower, but predicting such high rainfalls in advance as we have just seen is very difficult to do. The only solution in the long run is either to move thousands of houses and businesses to higher levels above the flood river levels factoring in maximum potential rises first, or construct enormous levee banks on each side of the river systems as is done in other countries to protect the urban, business, and farming communities.
I don’t think the NSW government has planned anything for this and the cost would be monumental.
So much for the cost cutting benefits of inaction on climate change.
George,
I completely agree with your remarks.Dr Gergis has outlined in great detail the factors which seem to have contributed to this event, not just in Western Sydney, but along much of the coast north into South East Queensland. I have been involved in monitoring weather and climate for over 50 years. I admit to having been gobsmacked by the reported rainfalls . The two separate weather patterns which contributed to the event are unparalleled in my experience.
The issue of continued urbanization of the Nepean Hawkesbury flood plain has been a contentious issue for many years, even before Warragamba Dam was constructed. The early explorers noted flood debris many metres up in trees along the river as early as 1789, so the risks have been known for the whole period of European settlement. It baffles me why our planning authorities have failed to regulate idiotic urban developments, on not just this flood plain, but most flood plains throughout New South Wales and Queensland . Leeve banks have been constructed along rivers to protect urban centres over the years, with varying success .Unfortunately they can give a false sense of security with disastrous results should they break or be over topped .
Unlike the move of Clermont in central Queensland to high ground following the massive 1914 flood, the cost of relocating the affected suburbs in Western Sydney would be astronomical .However, I do wonder whether with the impact of climate change increasing the risk and severity of floods , such a decision may have to be made. Raising the wall of Warragamba Dam would at best be temporary and ultimately a futile solution to this recurring crisis .
Gavin A. O’Brien, FRMetS.
Climate Watch Australia
Thanks Gavin. Your experience is stunning and so typical of many that work in the areas associated with climate change and it’s offspring of observing and monitoring extreme weather events. I can see that it is an extensive area of knowledge where so many still want to reject many scientists’ remarkable expertise, and sometimes it must feel very frustrating. How to get the message across? Our experts these days are frequently the last people to be consulted about what is going on. I can see that with Dr Andrew Glikson who writes on here from time to time as well who is also impeccably qualified, yet someone who kindly sent me a number of his and other scientists’ publications that are very sobering read to say the least.
Studying science originally in the early 1980s (and kept up with that interest), I was convinced of the effects of greenhouse gases and the potential for catastrophes way back then. Yet since those times so much research has been done while the pathway of convincing human beings has been so convoluted. My experience with dams goes back to working on engineering models of the same that involved plans for making secondary spillways in some of our dams, and I’m well aware as to how they serve in controlling excessive water flows. I also recently exchanged views with a friend of mine who is a highly experienced international hydrologist and he came to the same conclusions about the ‘bathtub’ as it is called downstream from Warragamba. A massive problem. And it is only one location in Australia likely to be affected.
As you confirm, the cost for doing something about it would be astronomical, and governments are likely to see it as beyond their short periods of election cycles to even bring the subject up. I noted a few days ago that the accent went onto blaming those who slowed plans of further raising of the dam wall, but that’s just a distraction.
From what I understand the tropics are extending South (as they are North) and so more heavier periods rainfall are only to be expected. In countries like Australia this latitudinal shift appears to be happening faster. It concerns me that predictions say that 60% of Australia could be affected, ruining agricultural production and making many places uninhabitable. That’s a latitudinal line down as far as Grafton from what I can estimate with effects even further South. It’s concerning that many Australians are so oblivious to this, the victims of false information and lack of government planning. I watched in shock last night when ABC that some people in flood (and often drought) stricken areas that seem to see much more frequent floods and more severe ones as well are now paying a premium on household insurance annually of between $14,000 and $16,000.
Last election the question on how much climate change would cost if nothing was done, was turned into how much it would cost to do anything. Australia has been characteristically called the “lucky country” but it still has no idea generally speaking, of where we are headed and the impending disasters we face. Given we have now gone past 415ppm CO2 in the atmosphere, when 350ppm was the tipping point of feedback mechanisms that are very difficult to control, we can only expect climate change to be exacerbated in speed and energy and just how long have we got to avoid serious problems will be far sooner than we think.
Thank you for your comment.