{"id":15887,"date":"2021-03-24T21:03:44","date_gmt":"2021-03-24T08:03:44","guid":{"rendered":"https:\/\/www.greenpeace.org\/aotearoa\/?p=15887"},"modified":"2025-05-20T14:11:21","modified_gmt":"2025-05-20T02:11:21","slug":"yes-australia-is-a-land-of-flooding-rains-but-climate-change-could-be-making-it-worse","status":"publish","type":"post","link":"https:\/\/www.greenpeace.org\/aotearoa\/story\/yes-australia-is-a-land-of-flooding-rains-but-climate-change-could-be-making-it-worse\/","title":{"rendered":"Yes, Australia is a land of flooding rains. But climate change could be making it worse"},"content":{"rendered":"\n

Over the past three years, I\u2019ve been working on the forthcoming report<\/a> by the United Nations\u2019 Intergovernmental Panel on Climate Change<\/a>. I\u2019m a climate scientist who contributed to the chapter on global water cycle changes<\/a>. It\u2019s concerning to think some theoretical impacts described in this report may be coming to life \u2013 yet again \u2013 in Australia.<\/p>\n\n

The recent flooding in New South Wales is consistent with what we might expect as climate change continues.<\/p>\n\n

Australia\u2019s 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 \u201cnoise\u201d.<\/p>\n\n

But as our planet warms, the water-holding capacity of the lower atmosphere increases by around 7% for every 1\u2103 of warming<\/a>. This can cause heavier rainfall, which in turn increases flood risk.<\/p>\n\n

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.<\/p>\n\n

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. <\/p>\n\n

Understanding the basics<\/h2>\n\n

To understand how a warming world is influencing the water cycle, it\u2019s helpful to return to the theory.<\/p>\n\n

From year to year, Australia\u2019s climate is subject to natural variability generated by the surrounding Pacific<\/a>, Indian<\/a> and Southern<\/a> oceans. The dominant drivers for a given year set up the background climate conditions that influence rainfall and temperature.<\/p>\n\n

It is a combination of these natural climate drivers that makes Australia the land of drought and flooding rains<\/a>.<\/p>\n\n

However, Australia\u2019s climate variability is no longer influenced by natural factors alone. Australia\u2019s climate has warmed by 1.4\u2103 since national records began in 1910<\/a>, with most of the warming occurring since 1970. Human-caused greenhouse emissions have influenced Australian temperatures in our region since 1950<\/a>.<\/p>\n\n

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.<\/p>\n\n

\"\"\/
\n A warmer atmosphere can hold more moisture which can lead to more extreme rainfall events.<\/span>\n Climate Council<\/span><\/span>\n <\/figcaption><\/figure>\n\n

Since the winter of 2020<\/a>, Australia has been influenced by the La Ni\u00f1a phase of the El Ni\u00f1o\u2013Southern Oscillation (ENSO<\/a>). Historically, sustained La Ni\u00f1a conditions<\/a>, sometimes with the help of a warmer than average Indian Ocean<\/a>, have set the scene for severe flooding in eastern Australia.<\/p>\n\n

During these events, easterly winds intensify and oceans around Australia warm. This is associated with the Walker Circulation<\/a> \u2013 a giant seesaw of atmospheric pressure that influences the distribution of warm ocean waters across the Pacific Ocean. <\/p>\n\n

The last La Ni\u00f1a occurred in 2010\u20132012<\/a>. It led to widespread flooding across eastern Australia, with particularly devastating effects in Queensland<\/a>. The event caused the wettest two-year period<\/a> in the Australian rainfall record, ending the 1997\u20132009 Millennium Drought<\/a>.<\/p>\n\n

Oceanographers from UNSW studied the exceptional event. They demonstrated<\/a> how a warmer ocean increased the likelihood of extreme rain during that event, primarily through increased transport of moist air along the coast. <\/p>\n\n

Their analysis highlighted how long\u2010term ocean warming can modify rain-producing systems, increasing the probability of extreme rainfall during La Ni\u00f1a events.<\/p>\n\n

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<\/a>, it will take researchers time to tease everything out.<\/p>\n\n

So what about climate change?<\/h2>\n\n

The theoretical changes to the global water cycle are well understood<\/a>. However, determining the contribution of natural and human influences on climate variability and extremes \u2013 known as \u201cattribution<\/a>\u201d \u2013 is still an emerging science. <\/p>\n\n

More studies are needed to distinguish natural or \u201cbackground\u201d 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<\/a>.<\/p>\n\n

Right now, La Ni\u00f1a conditions are decaying<\/a> in the Pacific Ocean. As expected, the 2020\u20132021 La Ni\u00f1a 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.<\/p>\n\n

\"NSW
\n NSW rainfall totals for the week ending March 22, 2021.<\/span>\n Bureau of Meteorology<\/span><\/span>\n <\/figcaption><\/figure>\n\n

What\u2019s interesting about the 2020\u20132021 La Ni\u00f1a is that it was weak<\/a> compared with historical events. The relationship between La Ni\u00f1a and rainfall is generally weaker in coastal NSW than further inland. However, it\u2019s concerning that this weak La Ni\u00f1a caused flooding comparable to the iconic floods of the 1950s and 1970s<\/a>. <\/p>\n\n

The rainfall totals for the current floods are yet to be analysed<\/a>. 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.<\/p>\n\n

The NSW coast is no stranger to extreme rainfall \u2013 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. <\/p>\n\n

It\u2019s also worth noting the current extreme rainfall in NSW was associated with a coastal trough<\/a>, not an East Coast Low<\/a>. Many of the region\u2019s torrential rainfall events<\/a> 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.<\/p>\n\n

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\u2103 warmer than the long-term average, and closer to 1.5\u2103 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.<\/p>\n\n

\"\"\/
\n Sea surface temperature anomalies along the NSW coast.<\/span>\n Bureau of Meteorology<\/span><\/span>\n <\/figcaption><\/figure>\n\n

A nation exposed<\/h2>\n\n

Weather and climate are not the only influences on extreme flood events. Others factors include the shape and size of water catchments, the presence of hard surfaces in urban areas (which cant\u2019t absorb water), and the density of human settlement in flood-prone areas<\/a>.<\/p>\n\n

The Hawkesbury\u2013Nepean 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.<\/p>\n\n

\n