Sydney’s disastrous flood wasn’t unprecedented: we’re about to enter a 50-year period of frequent, major floods


Tom Hubble, University of SydneyLast month’s flood in the Hawkesbury-Nepean River region of western Sydney peaked at a staggering 12.9 metres, with water engulfing road signs and reaching the tops of many houses.

There hasn’t been a major flood on the Hawkesbury-Nepean for more than 30 years, with the last comparable one occurring in 1990. Long-term Sydneysiders, however, will remember that 12 major floods occurred during the 40 years before 1990. Five of these were larger than last month’s flood.

So what’s going on? The long-term rainfall pattern in the region and corresponding river flow is cyclic in nature. This means 40 to 50 years of dry weather with infrequent small floods are followed by 40 to 50 years of wet weather with frequent major floods.

As river and floodplain residents take stock of the recent damage to their homes and plan necessary repairs, it’s vital they recognise more floods are on the way. Large, frequent floods can be expected to occur again within 10 or 20 years if — as expected — the historical pattern of rainfall and flooding repeats itself.

Living in a bathtub

Many of the 18,000 people who were evacuated live in and around a region known as the “Sackville Bathtub”. As the name suggests, this flat, low-lying section of the floodplain region was spectacularly affected.

The flooded Hawkesbury-Nepean River last month. Brown floodwater is evident between Penrith (right) and the Pacific Ocean (top left). The Sackville Bathtub is located left of centre.
Digital Earth Australia Map, Geoscience Australia, Tom Hubble

The Sackville Bathtub is located between Richmond and Sackville. It’s part of the Cumberland Plain area of Western Sydney and formed very slowly over 100 million years due to plate tectonic processes. The bathtub’s mudstone rock layers are folded into a broad, shallow, basin-shaped depression, which is surrounded by steep terrain.

Downstream of Sackville, the Hawkesbury-Nepean River flows through sandstone gorges and narrows in width. This creates a pinch-point that partially blocks the river channel.

Just as a bath plug sitting half-way over a plughole slows an emptying bath, the Sackville pinch-point causes the bathtub to fill during floods.

How the bathtub effect in the Hawkesbury-Nepean Valley causes floodwaters to back up and lead to deep and dangerous flooding.

Will raising the dam wall work?

The NSW state government is planning to raise the wall of the Warragamba Dam to help mitigate catastrophic floods in the region. But this may not be an effective solution.

Typically, somewhere between 40% and 60% of the floodwater that fills up the Sackville Bathtub comes from unimpeded, non-Warragamba sources. So, when the Hawkesbury-Nepean River floods, the bathtub is already quite full and causing significant problems before Warragamba begins to spill. The Warragamba water then raises the flood level, but often by only a couple of metres.

Raising Warragamba Dam’s wall as a mitigation measure will only control about half the floodwater, and won’t prevent major floods delivered by the Nepean and Grose rivers, which also feed into the region. This represents a small potential benefit for a very large cost.

The timing of observed flood peaks during the August 1986 Hawkesbury-Nepean flood, in relation to the time when Warragamba Dam began to spill. The arrival of Warragamba water in the Sackville Bathtub increased the flood depth only by about a metre above the floodwaters delivered earlier during the flood from the Grose and Nepean rivers.
Tom Hubble – Redrawn from data presented in Appendix One of the Hawkesbury-Nepean Flood Study; Infrastructure NSW 2019.

A long flooding period is on our doorstep

The idea of drought-dominated and flood-dominated periods for the Hawkesbury-Nepean River system was proposed in the mid-1970s by the University of Sydney’s Robin Warner. Since the late 1990’s, it hasn’t been the focus of much research.




Read more:
What is a 1 in 100 year weather event? And why do they keep happening so often?


He showed a century-long cycle of alternating periods of dry weather and small floods followed by wet weather and big floods is normal for Sydney. This means the March flood may not have come as a surprise to older residents of the Sackville Bathtub, who have a lived experience of the whole 40-50 year flooding cycle.

As a rough average, one major flood occurred every four years during the last wet-weather period between 1950 and 1990. The largest of this period occurred in November 1961. It filled the Sackville Bathtub to a depth of 15 metres and — like the June 1964 (14.6 metres) and March 1978 (14.5 metres) events — caused more widespread flooding than this year’s flood.

A photo of a flood that occured in Maitland in September 1950.
Sam Hood/NSW State Library/Flickr, CC BY

We’re currently 30 years into a dry period, which may be about to end. Conditions might stay dry for another 10 or 20 years.

These cycles are likely caused by natural, long-term “climate drivers” — long-term climatic fluctuations such as El Niño and La Niña, the Pacific Decadal Oscillation and the Indian Ocean Dipole, which are driven by oceanic current circulations. These global phenomena bring both benevolent weather and destructive weather to Australia.

Eastern Australia experiences decades-long periods of wetter weather when these climate drivers sync up with each other. When they’re out of sync, we get dry weather periods.




Read more:
A rare natural phenomenon brings severe drought to Australia. Climate change is making it more common


These long-term cycles are natural and have been operating for thousands of years, but climate change is amplifying and accelerating them. Dry periods are getting drier, wet periods are getting wetter.

The good news and bad news

The bad news is that 12-plus metre floods at Hawkesbury River (Windsor Bridge) are not all that unusual. There have been 24, 12-plus metre floods at Windsor Bridge since 1799.

The good news is meteorological forecasters are excellent at predicting when the storms that generate moderate, large and catastrophic floods are coming. We can expect several days’ to a week’s notice of the next big flood.

We can also prepare our individual and communal responses for more large and frequent floods on the Hawkesbury-Nepean. Residents of the area need to think about how they might live near the river as individuals. Decide what is precious and what you will fit into a car and trailer. Practice evacuating.

As a community, we must ensure the transport infrastructure and evacuation protocols minimise disruption to river and floodplain residents while maximising their safety. It’s particularly important we set up inclusive infrastructure to ensure disadvantaged people, who are disproportionately affected by disasters, also have a fighting chance to evacuate and survive.




Read more:
Not ‘if’, but ‘when’: city planners need to design for flooding. These examples show the way


Upgrading the escape routes that enable people to evacuate efficiently is absolutely vital. As is rethinking whether we should continue urban expansion in the Sackville Bathtub.

So remember, the next major flood is going to occur sooner than we would like. If you live in this region, you must start preparing. Or as a wise elder once said, “Live on a floodplain, own a boat!”


This story is part of a series The Conversation is running on the nexus between disaster, disadvantage and resilience. Read the rest of the stories here.The Conversation

Tom Hubble, Associate Professor, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Floodplains aren’t separate to a river — they’re an extension of it. It’s time to change how we connect with them


Melissa Parsons, University of New England and Martin Thoms, University of New EnglandDramatic scenes of flood damage to homes, infrastructure and livelihoods have been with us on the nightly news in recent weeks. Many will be feeling the pain for years to come, as they contend with property damage, financial catastrophe and trauma.

But what if, for a moment, we removed the humans and their structures from these tragic images — what would we see?

We would see a natural process of river expansion and contraction, of rivers doing exactly what they’re supposed to do from time to time. We’d see them exceeding what we humans have deemed to be their boundaries and depositing sediment across their floodplains. We’d see reproductive opportunities for fish, frogs, birds and trees. The floods would also enrich the soils. Floods can be catastrophic for humans, but they are a natural part of an ecosystem from which we benefit.

These scenes clearly depict the intersection of humans and nature, and it’s not working out well for either side.




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We must envision a new way of interacting with floodplains – these brilliant social-ecological systems that are not separate to rivers but rather part of the riverine landscape.

Humans can live on and with floodplains — but the way we do that has to change.

What is a floodplain?

Floodplains are relatively flat stretches of land located next to rivers. It helps to think of them as an extension of the river; it is natural and normal for a river to flood their adjacent plains.

Floodplains are composed of sediment the river has transported and then deposited, which makes them incredibly fertile. Flow and sediment regimes interacting over decades — or millennia — determine the physical and ecological character of floodplains, and the way they flood.

There are more than 15 generic floodplain types in Australia. Each harbours a unique set of evolutionary properties, physical features and ecosystems.

These influence the way floodwaters traverse floodplains, how long water remains on a floodplain, the velocity, turbulence and depth of floodwaters, and ecosystem responses to flooding. Floodplains are complex and highly variable.

Floodplains are also dynamic and ever-changing — and we should expect them to change even more in the coming years. Australian rivers have experienced regular periods of increased flood activity in the past 100 years.

And climate change is predicted to increase flood activity.




Read more:
5 ways the government can clean up the Murray-Darling Basin Plan


Humans benefit from floodplains

Floodplains are among the most productive ecosystems on the planet – they are biodiversity hotspots.

That’s in large part due to periodic flooding between different parts of a river-floodplain system; flooding is crucial to the function of floodplains. Without floods, these floodplains wouldn’t “work” — they would not be able to deliver the ecosystem services we benefit from. Those benefits include, but are not limited to:

  • food grown in these fertile soils
  • regulation of a balanced ecosystem
  • cultural heritage
  • transportation (as floodplains are easy to build roads on)
  • the supply of good quality drinking water
  • recreation.

The economic value of floodplain ecosystem services exceed US$25,681 per hectare. Roughly 25% of global terrestrial ecosystem services come from floodplains.

Humans are drawn to live on floodplains because of their productivity. In Australia, the floodplains of the Murray Darling Basin, heavily developed for agriculture, yield more than A$10 billion annually. These floodplain ecosystems provide an estimated A$187 billion per annum from their various ecosystem services.

However, the more we interrupt floodplain processes with development, the more we diminish the supply of ecosystem services.

The perils of living on floodplains

Putting the people back into the news footage reveals a social picture that is costly, traumatic and disruptive. The events of the past weeks have now brought into focus the perils of living on floodplains.

Humans have come up with ways to contend with this peril. Dams and levees. Land use planning. Building codes. Engineered floodscapes. Insurance. Emergency preparation systems and community engagement.

But if floodplains are a social-ecological system, where society gains great benefits but is also periodically placed at risk, which side should get the greatest policy attention? The humans or the ecosystem?

The answer is: both. But they also need to be better integrated.

Balancing the social with the ecological

Balancing the social and ecological aspects of floodplains requires a mindset change. We must combine community participation with research, resilience and adaptation to make long-term decisions about the future of these complex social-ecological systems.

If society wants to continue to derive the billions of dollars of benefits from floodplains, we need to ensure that flooding continues to occur on floodplains, and adapt to risk in imaginative and innovative ways that also protect the benefits.

Business as usual is not an option. The limitations of technocratic controls such as dams and levees should now be obvious. Time and time again, these have increased flood risk and failed to flood-proof the floodplain.

Rarely do such linear solutions solve complex problems in social-ecological systems. Linear solutions often exacerbate a problem or simply move it on to other parts of the system, creating social inequality, environmental decline and future risk.

The Australian government’s 2018 National Disaster Risk Reduction Framework sets the challenge to join up the built, social, economic and natural environments to address disaster risk in Australia.

Accepting the challenge requires a broader focus on balancing the social-ecological sides of Australia’s vast floodplains. Complexity, not linear thinking, must be embedded in the way we reimagine policy about floodplains and floods.

This requires transformative collaborations between government departments, researchers, business, and community stakeholders.


If this article has raised issues for you, or if you’re concerned about someone
you know, call Lifeline on 13 11 14. This story is part of a series The Conversation is running on the nexus between disaster, disadvantage and resilience. You can read the rest of the stories here.
The Conversation

Melissa Parsons, Senior Lecturer, Geography and Planning, University of New England and Martin Thoms, Professor of Physical Geography, University of New England

This article is republished from The Conversation under a Creative Commons license. Read the original article.

After the floods, stand by for spiders, slugs and millipedes – but think twice before reaching for the bug spray


Lukas Koch / AAP

Caitlyn Forster, University of Sydney; Dieter Hochuli, University of Sydney, and Eliza Middleton, University of SydneyRecord-breaking rain has destroyed properties across New South Wales, forcing thousands of people to evacuate and leaving hundreds homeless.

Humans aren’t the only ones in trouble. Many of the animals that live with and around us are also heading for higher ground as the floodwaters rise.

Often small creatures — especially invertebrates like spiders, cockroaches and millipedes — will seek refuge in the relatively dry and safe environments of people’s houses. While this can be a problem for the human inhabitants of the house, it’s important to make sure we don’t add to the ecological impact of the flood with an overzealous response to these uninvited guests.

Warragamba Dam in southwestern Sydney has been spilling a Sydney Harbour’s worth of water each day during the rains.
Eliza Middleton, Author provided

What floods do to ecosystems

Floods can have a huge impact on ecosystems, triggering landslides, increasing erosion, and introducing pollutants and soil into waterways. One immediate effect is to force burrowing animals out of their homes, as they retreat to safer and drier locations. Insects and other invertebrates living in grass or leaf litter around our homes are also displaced.

Burrowing invertebrates come to the surface during floods, providing food for opportunistic birds.
Dieter Hochuli, Author provided

Snakes have reportedly been “invading” homes in the wake of the current floods. Spiders too have fled the rising waters. Heavy rain can flood the burrows of the Australian funnelweb, one of the world’s most venomous spiders.

Some invertebrates will boom; others may plummet

Rain increases greenery, which can support breeding booms of animals such as mosquitoes, locusts, and snails.

Even species that don’t thrive after floods are likely to become more visible as they flock to our houses for refuge. But an apparent short-term increase in numbers may conceal a longer story of decline.




Read more:
After the floods come the mosquitoes – but the disease risk is more difficult to predict


After periods of flooding, the abundance of invertebrates can fall by more than 90% and the number of different species in an area significantly drops. This has important implications for the recovery of an ecosystem, as many of the ground dwelling invertebrates displaced by floods are needed for soil cycling and decomposition.

So before you reach for the bug spray, consider the important role these animals play in our ecosystem.

What to do with the extra house guests?

If your house has been flooded, uninvited creatures taking shelter in your house are probably one of the smaller issues you are facing.

Once the rain subsides, cleaning in and around your property will help reduce unwanted visitors. Inside your house, you may see an increase in cockroaches, which flourish in humid environments. Ventilating the house to dry out any wet surfaces can help get rid of cockroach infestations, and filling crevices can also deter unwanted visitors.




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In the garden, you may see an increase in flies in the coming weeks and months as they lay eggs in rotting plants. Consider removing any fruit and vegetables in the garden that may rot.

Mosquitoes are also one to watch as they lay eggs in standing water. Some species pose a risk of diseases such as Ross River virus. To prevent unwanted mozzies, make sure to empty things that have filled with rainwater, such as buckets and birdbaths.

If you do encounter one of our more dangerous animals in your home, such as venomous snakes and spiders, do not handle them yourself. If you find an injured or distressed snake, or are concerned about snakes in your house, call your local wildlife group who will be able to relocate them for you.

Just like the floods, which will subside as the water moves on, the uninvited gathering of animals is a temporary event. Most visitors will quickly disperse back to more appropriate habitat when the weather dries, and their usual homes are available again.

You may see an increase in slugs in your local area after rainy conditions.
Eliza Middleton @smiley_lize

Don’t sweat the small stuff

While many of the impacts of floods are our own making, through poor planning and development in flood-prone areas, effective design of cities and backyards can mitigate the risks of floods. Vegetation acts as a “sponge” for stormwater, and appropriate drainage allows water to flow through more effectively. Increasing backyard vegetation also provides extra habitat for important invertebrate species, including pollinators and decomposers.




Read more:
Not ‘if’, but ‘when’: city planners need to design for flooding. These examples show the way


With severe weather events on the rise, it is important to understand how ecosystems respond to, and recover from natural disasters. If invertebrates are unable to perform vital ecosystem functions, such as soil cycling, decomposition, and pollination, ecosystems may struggle to return to their pre-flood state. If the ecosystems don’t recover, we may see prolonged booms of nuisance pests such as mosquitoes.

A few temporary visitors are are a minor inconvenience in comparison to the impacts floods have on the environment, infrastructure and the health and well-being of people impacted. So while it may seem like a bit of a creepy inconvenience, maybe we should let our house guests stay until the flood waters go down.The Conversation

Caitlyn Forster, PhD Candidate, School of Life and Environmental Sciences, University of Sydney; Dieter Hochuli, Professor, School of Life and Environmental Sciences, University of Sydney, and Eliza Middleton, Laboratory Manager, School of Life and Environmental Sciences, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Managed retreat of settlements remains a tough call even as homes flood and coasts erode


Tayanah O’Donnell, Australian National UniversityIt is no joke that New South Wales residents are in the midst of their fourth “one in 100 year” event since January 2020. Much of the Australian east coast continues to experience heavy rainfall, strong winds and abnormally high tides. All will make the current floods worse.

As climate tipping points are reached and the Earth’s systems begin to buckle under the strain, the need for considered adaptation strategies is overwhelmingly clear. One of these strategies is for human settlements to retreat from areas most at risk, whether from floods or bushfires. While something needs to be done to ensure future generations do not suffer catastrophic consequences, managed retreat is a complex tool.

These strategic decisions in the next five to ten years will be challenging. And these decisions really matter: where and how do we build residential areas that can cope with a climate-changed world?




Read more:
Yes, Australia is a land of flooding rains. But climate change could be making it worse


What is managed retreat?

Managed retreat can be defined as “purposeful, co-ordinated movement of people and assets out of harm’s way”. Managed retreat more often refers to the retreat of existing development out of harm’s way. Planned retreat is usually the preferred phrasing for new development that is planned for possible future relocation.

Both planned and managed retreat are focused on the permanent relocation of people and assets, as opposed to the evacuations we are seeing now.

Managed retreat is experiencing a resurgence in scientific literature as the impacts of climate change become increasingly frequent, severe and more obvious. These impacts bring with them a recognition of the need for adaptation even as we urgently reduce greenhouse gas emissions.

Of course, relocating away from high-risk locations is not an entirely new concept. However, managed retreat in response to a changing climate is not only complex, but also has a lot of political baggage. The complexity spans legal, financial, cultural and logistical factors among others: the political baggage seemingly associated with effective climate action in Australia often hinders governments’ abilities to respond properly.

Societies around the world need to grapple with the reality that managed retreat will become a suite of tools to respond to crisis. Insurers will not always be available, and the costs to governments (and therefore to you, the taxpayer) of responding to increasing rates of disasters, irrespective of insurance, will continue to grow exponentially.




Read more:
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Responding to events after the fact is an unsustainable model of adaptation. There is, too, a need to acknowledge settlement needs and historical built environment legacies that have put significant state infrastructure in harm’s way.

Managing difficult trade-offs

We know trade-offs need to be made between what we protect and what we let go in suburban floodplain areas.

Legal machanisms to force people and assets to move can and must be thoughtful. The implementation of managed retreat in urbanised areas faces multiple hurdles. These include:




Read more:
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It is wrong to see managed retreat as the panacea for climate risk and development in vulnerable locations. In many cases, once development is in place, it can be more appealing to some to protect an at-risk area rather than work towards managed retreat. Even where managed retreat has been successful, as in the case of the flood-prone township of Grantham, it was not without pain.

There are also other, more basic needs, such as having land available where people can relocate.

Working out highest and best use of land

There are ways that land can be used for its highest and best use at a point in time. For example, tools like easements can enable vulnerable land to be used, subject to event-based or time-based trigger-point thresholds. Once these thresholds are reached, the land is put to some other use. The advantage of these machanisms, especially for new development, is that owners are clear about the risks from the start.

This still leaves us with hard decisions about responding to at-risk current developments. Putting off these hard decisions and leaving them for future decision-makers will result in a huge injustice, because there will be catastrophe as Earth’s tipping points are passed. Development decisions made now will determine the impacts on our children and grandchildren.

Urban development decisions for both new and existing development in this coming decade demand courage and leadership. If we accept that Australian cities will continue to expand and increase in density, then we have some serious questions to ask ourselves. What kind of future do we want?

Some areas should simply not be developed.




Read more:
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There is a risk that an over-reliance on managed retreat will over-simplify the challenge of working out what to do about development in at-risk locations. There is a clear need to separate out what to do about current and past developments, and how to approach new developments.

The latter is easy – do not rebuild residential homes in at-risk areas. Governments should repurpose these zones for uses that permit nature-based solutions to the need to adapt to climate change.

Current development is much more complex. In some cases, managed retreat – done thoughtfully and considerately – will be the only option.The Conversation

Tayanah O’Donnell, Honorary Senior Lecturer, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

A staggering 1.8 million hectares burned in ‘high-severity’ fires during Australia’s Black Summer


Shutterstock

Ross Bradstock, University of Wollongong; Hamish Clarke, University of Wollongong; Luke Collins, La Trobe University; Michael Clarke, La Trobe University; Rachael Helene Nolan, Western Sydney University, and Trent Penman, The University of MelbourneIn the aftermath of Australia’s devastating Black Summer fires, research has begun to clarify the role of climate change.

We already know climate change contributed to the record-breaking drought and fire weather conditions, leading to the bushfires’ unprecedented range across Australia.

Our new research looks at whether bushfires are becoming more “severe” (an indicator of how intensely the vegetation burned) as a result of climate change.

Our findings were unexpected, as we learned the proportion of high-severity fires generally hasn’t increased in recent decades. However, the sheer breadth of the Black Summer fires meant an unprecedented 1.8 million hectares across southeast Australia were exposed to high-severity fires. This has dire consequences for the people and wildlife who call the forests home.

What is fire severity?

Two measurements in fire science are pertinent to our research: fire severity and fire intensity.

Fire severity refers to how high the flames and the plume of hot air reach, as measured by the resulting damage to vegetation (vertical profile of scorch and consumption of leaves and twigs). Fire intensity refers to the energy released from the fire — how hot and destructive the flames are.

Scientists can estimate severity using using satellite imagery, by contrasting differences in the cover and condition of vegetation before and after fires.

In forests, “high-severity” fires occur when the crowns of dominant trees are fully burnt or scorched. High-severity fires are lethal to tree-dwelling mammals in forests, such as possums, gliders and koalas. They also pose a big risk to nearby homes and buildings.

“Low-severity” fires, on the other hand, may be confined to the leaf litter and ground cover plants beneath the forest canopy, and can even leave entirely unburnt patches in forests.

Are high severity fires becoming more common?

To determine if high-severity bushfires are becoming more common, we looked at satellite data for bushfires from 1988 to 2020. The data covered more than 130,000 square kilometres of forest, woodland and shrubland ecosystems in southeast Australia.

If fires were becoming more intense in recent decades, we would have expected the proportion of vegetation subjected to high-severity fire to have increased.

Instead, we found the average proportion of high-severity wildfire remained constant in dry forest — the dominant vegetation across this region. There was, however, evidence of an increase in the average proportion of high-severity fire in wet forests and rainforests, along with woodlands.




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Nonetheless, the main conclusion was clear: across the bulk of the study area, the average proportion of high-severity fires has not changed in recent decades, despite an increase in the area burned during the Black Summer bushfires.

Why the Black Summer bushfires were exceptional

While the proportion of high-severity fires hasn’t changed, the enormous range of the 2019-2020 bushfires meant 44% of the total area burned by high-severity fire since 1988 occurred in that one summer alone.

This means 1.8 million hectares of the forest and woodland regions of southeastern Australia — an enormous proportion — was exposed to intense and severe fire. In this regard, the Black Summer bushfires were exceptional.

As Australians remember all too clearly, this had a devastating effect on the environment. An estimated three billion animals were killed or displaced, vulnerable rainforests burned and 3,000 homes were destroyed.

A firefighter runs through a burning forest
Firestorms could become more common under a changing climate.
AAP Image/Dean Lewins

The 2019-20 fire season also involved a record number of “firestorms”, particularly during the latter part of the season in January and early February. This occurs when fires create their own weather.

These fires can burn at exceptional intensity. And research from 2019 indicates such firestorms could become more common under climate change.

This means we can’t rule out a future change in the proportion of bushfires that burn at the highest levels of intensity and severity.

Ecosystems in jeopardy

The results of our study underline one of the likely consequences of future climate change.

The sheer scale of the area burned in the 2019-20 fire season exceeded not only historical records for forested ecosystems of southern Australia, but also outstripped projections for the late 21st century under strong scenarios of climate change.




Read more:
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As bushfires become larger in the future, the area exposed to intense and severe fires is likely to increase commensurately. As a result, the future of our wetter forest types, which have not evolved to cope with frequent and severe fires, is in jeopardy.

So, as the area exposed to intense fires is likely to increase in the future, we’ll see major challenges to the long-term viability of our forested ecosystems, the services they provide and the people who reside in and around them.The Conversation

Ross Bradstock, Emeritus professor, University of Wollongong; Hamish Clarke, Research Fellow, University of Wollongong; Luke Collins, Research scientist, La Trobe University; Michael Clarke, Emeritus professor, La Trobe University; Rachael Helene Nolan, Postdoctoral research fellow, Western Sydney University, and Trent Penman, Professor, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Yes, Australia is a land of flooding rains. But climate change could be making it worse


Etching of the 1867 flood in the Hawkesbury-Nepean Valley, depicting the Eather family.
illustrated Sydney News/author provided

Joelle Gergis, Australian National UniversityOver 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.

People watch swollen river
Flooding is likely to become more severe as the planet warms.
AAP

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.

A warmer atmosphere can hold more moisture which can lead to more extreme rainfall events.
Climate Council

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.

NSW rainfall total, week ending March 22, 2021
NSW rainfall totals for the week ending March 22, 2021.
Bureau of Meteorology

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.

Sea surface temperature anomalies along the NSW coast.
Bureau of Meteorology

A nation exposed

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’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.The Conversation

Joelle Gergis, Senior Lecturer in Climate Science, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

5 remarkable stories of flora and fauna in the aftermath of Australia’s horror bushfire season



hamiltonphillipa/iNaturalist, CC BY-NC-SA

Will Cornwell, UNSW; Casey Kirchhoff, UNSW, and Mark Ooi, UNSW

Around one year ago, Australia’s Black Summer bushfire season ended, leaving more than 8 million hectares across south-east Australia a mix of charcoal, ash and smoke. An estimated three billion animals were killed or displaced, not including invertebrates.

The impact of the fires on biodiversity was too vast for professional scientists alone to collect data. So in the face of this massive challenge, we set up a community (citizen) science project through the iNaturalist website to help paint a more complete picture of which species are bouncing back — and which are not.

Almost 400 community scientists living near or travelling across the firegrounds have recorded their observations of flora and fauna in the aftermath, from finding fresh wombat droppings in blackened forests, to hearing the croaks of healthy tree frogs in a dam choked with debris and ash.

Each observation is a story of survival against the odds, or of tragedy. Here are five we consider particularly remarkable.

Greater gliders after Australia’s largest ever fire

The Gospers Mountain fire in New South Wales was the biggest forest fire in Australian history, razing an area seven times the size of Singapore. This meant there nothing in history scientists could draw from to predict the animals’ response.

So it came as a huge surprise when a community scientist observed greater gliders deep within the heart of the Gospers Mountain firegrounds in Wollemi National Park, far from unburned habitat. Greater gliders are listed as “vulnerable” under national environment law. They’re nocturnal and live in hollow-bearing trees.

A greater glider with shining eyes at night
A citizen scientist snapped this photo of a greater glider in the heart of the the Gospers Mountain firegrounds.
Mike Letnic/iNaturalist, CC BY-NC

How gliders survived the fire is still unknown. Could they have hidden in deep hollows of trees where the temperature is relatively cooler while the fire front passed? And what would they have eaten afterwards? Greater gliders usually feed on young leaves and flowers, but these foods are very rare in the post-fire environment.

Finding these gliders shows how there’s still so much to learn about the resilience of species in the face of even the most devastating fires, especially as bushfires are forecast to become more frequent.

Rare pink flowers burnishing the firegrounds

The giant scale of the 2019-20 fires means post-fire flowering is on display in grand and gorgeous fashion. This is a feature of many native plant species which need fire to stimulate growth.

Excitingly, community scientists recorded a long-dormant species, the pink flannel flower (Actinotus forsythii), that’s now turning vast areas of the Blue Mountains pink.

Pink flannel flowers are bushfire ephemerals, which means their seeds only germinate after fire.
Margaret Sky/iNaturalist, CC BY-NC

Pink flannel flowers are not considered threatened, but they are very rarely seen.

Individuals of this species spend most of their life as a seed in the soil. Seeds require a chemical found in bushfire smoke, and the right seasonal temperatures, to germinate.

Rediscovering the midge orchid

Much of Australia’s amazing biodiversity is extremely local. Some species, particularly plants, exist only in a single valley or ridge. The Black Summer fires destroyed the entire range of 100 Australian plant species, incinerating the above-ground parts of every individual. How well a species regenerates after fire determines whether it recovers, or is rendered extinct.

The midge orchid.
Nick Lambert/iNaturalist, CC BY-NC

One of these is a species of midge orchid, which grows in a small area of Gibraltar Range National Park, NSW.

All of the midge orchid’s known sites are thought to have burned in late 2019. The species fate was unknown until two separate community scientists photographed it at five sites in January 2021, showing its recovery.

Like many of Australia’s terrestrial orchids, this species has an underground tuber (storage organ) which may have helped part of it avoid the flames’ lethal heat.




Read more:
After last summer’s fires, the bell tolls for Australia’s endangered mountain bells


Don’t forget about insects

Despite their incredible diversity and tremendous value to society, insects tend to be the forgotten victims of bushfires and other environmental disasters.

Many trillions of invertebrates would have been killed in the fires of last summer. A common sight during and after the bushfire season was a deposit of dead insects washed ashore. Some died from the flames and heat, while others died having drowned trying to escape.

Dead insects washed up on the beach was a common sight in the fire aftermath.
BlueBowerStudio/iNaturalist, CC BY-NC

One dead insect deposit — one of hundreds that washed up near Bermagui, NSW on Christmas Eve — included a range of species that have critical interactions with other organisms.

This includes orchid dupe wasps (Lissopimpla excelsa), the only known pollinator of the orchid genus Cryptostylis. Transverse ladybirds (Coccinella transversalis), an important predator of agricultural pests such as aphids, also washed up. As did metallic shield bugs (Scutiphora pedicellata), spectacular iridescent jewel bugs that come in green and blue hues.

Some insects died from the flames and heat, while others died having drowned trying to escape the flames.
BlueBowerStudio/iNaturalist, CC BY-NC

The unlikely survival of the Kaputar slug

Creatures such as kangaroos or birds have a chance to flee bushfires, but smaller, less mobile species such as native slugs and snails have a much tougher time of surviving.

The 2019-2020 bushfire season significantly threatened the brilliantly coloured Mount Kaputar pink slug, found only on the slopes of Mount Kaputar, NSW. When fires ripped through the national park in October and November 2019, conservationists feared the slug may have been entirely wiped out.




Read more:
Photos from the field: zooming in on Australia’s hidden world of exquisite mites, snails and beetles


But park ranger surveys in January 2020 found at least 60 individuals managed to survive, likely by sheltering in damp rock crevices. Community scientists have spotted more individuals since then, such as the one pictured here found in September 2020.

But the slug isn’t out of the woods yet, and more monitoring is required to ensure the population is not declining.

Bright pink slug
A community scientist spotted this rare slug in firegrounds.
Taylor/iNaturalist, CC BY-NC

Continuing this work

While community scientists have been documenting amazing stories of recovery all across Australia, there are still many species which haven’t been observed since the fires. Many more have been observed only at a single site.

The Snowy River westringia (Westringia cremnophila), for instance, is a rare flowering shrub found on cliffs in Snowy River National Park, Victoria. No one has reported observing it since the fire.

So far these community scientist observations have contributed to one scientific paper, and three more documenting the ability for species to recover post-fire are in process.

Recovery from Black Summer is likely to take decades, and preparing a body of scientific data on post-fire recovery is vital to inform conservation efforts after this and future fires. We need more observations to continue this important work.




Read more:
Summer bushfires: how are the plant and animal survivors 6 months on? We mapped their recovery


The Conversation


Will Cornwell, Associate Professor in Ecology and Evolution, UNSW; Casey Kirchhoff, PhD Candidate, UNSW, and Mark Ooi, Senior Research Fellow, UNSW

This article is republished from The Conversation under a Creative Commons license. Read the original article.