Rising sea levels in Australia may demand novel solutions
The State of the Climate 2020 report released this month delivered a clear message: climate change in Australia has arrived. While many need no reminding of the soaring temperatures of 2019 or the Black Summer bushfires that followed, another physical climate change, perhaps less noticeable, girds our shores. Sea levels are currently rising at 3.5 cm per decade, presenting an ongoing challenge for Australians now and in the decades ahead.
How sea level rise works
Global mean sea levels have already risen by around 25 cm since 1880. It’s what CSIRO Climate Science Centre Director Dr Jaci Brown refers to as a “confronting figure” particularly for the eight out of every ten or so Australians who live within 50 km of a coastline.
“There’s two parts to the story,” Dr Brown explains.
“One is the melting of ice around Antarctica, Greenland and the glaciers. The second part is thermal expansion. As the ocean warms, it expands. That contributes to sea level rise.”
Measurements taken from Argo floats and ocean arrays show that ocean heat content and ice loss from the Antarctic Ice Sheet have both accelerated in recent decades, causing the rate of sea level rise to also accelerate.
The result? Sea levels in Australia are currently rising at the rate of around 3.5 cm per decade.
To understand the consequences for the decades ahead, Dr Brown suggests grabbing a ruler and heading down to your local beach.
“Put your ruler at the edge of where you think high tide is and have a look at what another 25 or 30 cm might mean. It’s quite a lot, isn’t it?”
Long-term trend points to future sea level rise
Dr Kathy McInnes, CSIRO Climate Extremes and Projections Group Leader, says that the rate of increase in sea level rise is expected to ramp up in the second half of this century. However, this depends on the trajectory of future global greenhouse gas emissions. In other words, we’re going to need a bigger ruler.
“If countries fail to meet the goals of the Paris Agreement, we could be reaching the end of the century with a rate of increase similar to 10 or 11 mm per year. That’s quite considerable,” says Dr McInnes.
The delayed acceleration is explained by the long lifetime of heat-trapping greenhouse gases like carbon dioxide in the atmosphere and the slow response time of the ocean to those changes.
“The ocean is still responding today to the greenhouse gases put into the atmosphere last century, which means that some degree of ongoing sea level rise is now inevitable,” says Dr McInnes.
Where will we see the biggest levels of rise?
The rate of sea level rise is not uniform – it varies substantially across the world. That’s because cold and warm ocean currents play a big role in modulating the rate of rise, as do the Antarctic and Greenland ice sheets, and atmospheric processes and large-scale climate drivers including the El Niño-Southern Oscillation (ENSO).
During an El Niño year, for example, we typically see cooler ocean temperatures around the northern part of the continent. But La Niña results in warmer ocean temperatures around the tropics. This natural variability is reflected in the ‘wiggles’ of the sea level rise record.
In Australia, Dr McInnes says we can expect to see higher sea levels around the NSW coastline. This is a consequence of the East Australian Current bringing warm waters down along the eastern seaboard of Australia. In the southern part of Australia, the rate of sea level rise is closer to the global average.
Understanding past and future sea level rise
There are a considerable number of buildings at risk from sea level rise in Australia. It’s estimated that up to $63 billion (2008 value) existing residential buildings are potentially at risk of inundation from a 1.1 m rise.
The cost of not putting in place adaptative measures is more severe. A European study recently estimated 4 per cent in annual global GDP losses each year by 2100 unless coastal regions prepare for rising seas.
Making available regional information on future coastal climate is one way communities can prepare for the impact of coastal change.
Dr McInnes’ team is in the process of finalising a new set of high-resolution sea level rise projections for Victoria to assist the government in developing policy to adapt to rising sea levels in the state. The team is also assessing the inundation, erosion and groundwater hazards for Melbourne’s Port Phillip Bay for a range of sea level increases to help inform council planning, risk assessments, community awareness and mitigation activities.
They’ve obtained the longest possible record of historic sea level rise in the region, and have isolated sources of climate variability, including those large-scale climate circulation patterns like ENSO.
“By teasing apart these different contributing factors, we can figure how much sea level rise has already occurred, and how much can be attributed to the background trend of sea level rise. As well as helping decision-makers to prepare for ongoing rise, it allows us to provide a much stronger line of evidence of a changing climate,” says Dr McInnes.
Managing sea level rise when disasters collide
Another aspect of Dr McInnes’ research is helping communities to understand the implications of compound extreme events in preparing for sea level rise. It’s not just about preparing for sea level rise on its own, but the many other potential climate events – bushfires, tropical cyclones and floods – that can come together to drive cascading impacts and place additional pressure on emergency services.
Dr McInnes says that this field is sometimes referred to as ‘wargaming’ – understanding the worst possible factors that could come together, and determining whether assets, infrastructure or management systems might be able to cope. This facilitates stress-testing for councils and planning agencies and a better understanding of risk.
Living shorelines for coastal resilience
Dr Becki Morris at the National Centre for Coasts and Climate, the University of Melbourne is on a mission to change the way we think about coastal management.
She’s heading up a team investigating how coastal ecosystems such as saltmarshes, mangroves and shellfish reefs can be restored or created to buffer against coastal hazards.
The so-called “living shoreline” has direct benefits over traditional structures like revetments and seawalls in terms of risk reduction – mostly because man-made structures need to be upgraded and rebuilt over time in order to continue to function, especially after storm surge events.
Coastal ecosystems, on the other hand, can adapt and self-repair as a living system by either growing higher or migrating inland.
Natural habitats protect shorelines in several ways. This includes by adding roughness and shallow areas that cause a reduction in wave height and wave breaking, as well as the accretion and stabilisation of sediments.
Their effectiveness depends, of course, on their location. Dr Morris describes the ideal location as one “not at immediate risk, but will be in the future – and has space and the environmental conditions required for restoring or creating a habitat”.
“You need to have space, both in the shallow intertidal zone to establish the habitat and between the foreshore and the nearest infrastructure,” explains Dr Morris.
“If the infrastructure is so close that the next storm will result in the structure being compromised, then you have a high-risk or ‘fix on failure’ situation, and this is no longer suitable (for nature-based defences), as they are living systems, which take time to develop.”
Hybrid solutions, where you use a combination of hard structures with natural habitats may be a novel way to reduce some of the time limitations of a ‘soft’ or entirely natural solution. For example, rock fillets have been used successfully for bank stabilisation and mangrove rehabilitation in estuaries in NSW for the last 20 years. The rock is placed parallel to the bank, with gaps that allows for mangrove recruitment behind and the movement of fish and other organisms at high tide.
Coastal protection with co-benefits
A major drawcard is that nature-based coastal solutions are significantly cheaper to construct. They have a number of co-benefits, too, like supporting marine life and improving water quality.
Although the research is only in early investigative stages, it is shaping up as an effective adaptive coastal protection alternative with the ability to provide more climate-resilient coastal protection outcomes for certain areas.
But for the technology to be adapted at scale, a shift in thinking – in scientific research, coastal management, and engineering design and practice – will be necessary.
“Most of our thinking about coastal management is reactive. But if we’re going to implement more diverse risk reduction tools, like nature-based methods, we’re going to need to change that.”
“The hope is that nature-based coastal defence will eventually be considered within a standard risk framework,” says Dr Morris.
Research conducted by Dr Kathy McInnes at CSIRO and Dr Rebecca Morris at the National Centre for Coasts and Climate, and their teams, was supported through the Earth Systems and Climate Change Hub of the Australia Government’s National Environmental Science Program.
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