Over the past three years, Ive been working on theforthcoming reportby the United NationsIntergovernmental Panel on Climate Change. Im a climate scientist who contributed to the chapter on globalwater cycle changes. Its 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.

Australias 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 increasesby 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, its helpful to return to the theory.

From year to year, Australias climate is subject to natural variability generated by the surrounding Pacific,IndianandSouthernoceans. 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 ofdrought and flooding rains.

However, Australias climate variability is no longer influenced by natural factors alone. Australias climate has warmed by1.4 since national records began in 1910, with most of the warming occurring since 1970. Human-caused greenhouse emissions have influenced Australian temperatures in our regionsince 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 thewinter of 2020, Australia has been influenced by the La Nia phase of the El NioSouthern Oscillation (ENSO). Historically, sustainedLa Nia conditions, sometimes with the help of awarmer 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 theWalker Circulation a giant seesaw of atmospheric pressure that influences the distribution of warm ocean waters across the Pacific Ocean.

The last La Nia occurred in20102012. It led to widespread flooding across eastern Australia, with particularly devastating effects inQueensland. The event caused thewettest two-year periodin the Australian rainfall record, ending the 19972009Millennium Drought.

Oceanographers from UNSW studied the exceptional event.They demonstratedhow 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 longterm ocean warming can modify rain-producing systems, increasing the probability of extreme rainfall during La Nia 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 byboth factors, it will take researchers time to tease everything out.

So what about climate change?

The theoretical changes to the global water cycle arewell 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 hasalready emerged.

Right now, La Nia conditions aredecayingin the Pacific Ocean. As expected, the 20202021 La Nia 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.

Whats interesting about the 20202021 La Nia is that it wasweakcompared with historical events. The relationship between La Nia and rainfall is generally weaker in coastal NSW than further inland. However, its concerning that this weak La Nia caused flooding comparable to the iconic floods of the1950s and 1970s.

The rainfall totals for the current floods are yet to beanalysed. 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.

Its also worth noting the current extreme rainfall in NSW was associated with acoastal trough, not anEast Coast Low. Many of the regionstorrential rainfall eventsin 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 HawkesburyNepean 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 mayhelp improvefuture climate change risk assessment. For example, past floods in the HawkesburyNepean have been a lot worse than the current disaster. In 1867, the Hawkesbury River at Windsor reached19.7 metresabove normal, and in 1961 peaked at14.5 metres. This is worse than the 13.12 metres above normal recorded atFreemans Reach on March 23.

Its sobering to think the Hawkesbury River once peaked 6 metres higher than what were seeing right now. Imagine the potential future flooding caused by an East Coast Low during strong La Nia conditions.

It will take time before scientists can provide a detailed analysis of the 20202021 La Nia event. But its 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.