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A wet warning from Australia’s Top End on rising sea levels

Rising sea levels are typically written about as a “threat to future generations” – something to worry about by 2050 or 2100, not now. But if you want to see why even relatively small increases in sea…

Riding underwater on Darwin’s most popular bike path, on 1 February 2014. Andrew Campbell

Rising sea levels are typically written about as a “threat to future generations” – something to worry about by 2050 or 2100, not now. But if you want to see why even relatively small increases in sea levels matter, come to Darwin.

The Arafura and Timor Seas off northern Australia are a global hotspot for warming oceans and rising sea levels. CSIRO, CC BY-NC-ND

Australia’s top end is a global hotspot for rising sea levels. In Darwin and the World Heritage-listed floodplains of Kakadu National Park, we’re seeing how the combination of gradual sea level rise and “normal” weather events - such as storms and king tides - can have surprisingly big impacts.

Small changes adding up to big damage

Storms and heavy rain are not unusual in the Darwin wet season. But recent weather has been spectacular, as monsoonal onshore winds coincided with king tides to batter the shoreline. Crowds gathered to see waves crashing over cliffs and jetties that usually overlook calm seas. Tragically, two people got into trouble in these rough seas, losing their lives, and a young boy drowned in a flooded stormwater drain.

Sea levels around Darwin, which abuts the warm, shallow Arafura Sea, have risen by about 17 centimetres over the past 20 years. As the CSIRO noted in its last State of the Climate report, the rates of sea-level rise to the north and northwest of Australia have been 7 to 11 millimetres per year, which is two to three times the global average. Along the eastern and southern coasts of Australia, rates of sea-level rise are around the global average.

Sea-level rise rates around Australia, as measured by coastal tide gauges (circles) and satellite observations (contours) from January 1993 to December 2011. CSIRO State of the Climate 2012, CC BY-NC-ND

Seventeen centimetres may not seem much, especially with a 7 to 8 metre daily tidal range. However, raising the underlying base makes a big difference, not just to the ultimate penetration of big tides and storm surges, but also in the everyday hydrodynamic fluxes on beaches, estuaries and floodplains.

The impact of recent Darwin weather on infrastructure — both built and natural — has profound implications for coastal planning, design, management and regulation. The recent confluence of 8-metre king tides with strong onshore winds after weeks of wet monsoonal weather was unusual, but well short of being even a Category 1 cyclone.

By Darwin standards, there has been nothing exceptional about this wet season’s wind or tides. There was heavier than average rain last month - but even that has been a long way short of the records, or even a 1-in-10 year event.

The chunk of bitumen with the white line used to be the bike path. Andrew Campbell

Yet the damage we are seeing in Darwin has been considerable. Near where we live, a significant stretch of the city’s most popular bike path (right) was washed away. Further north, a large casuarina tree, which 10 years ago stood atop the landward side of two dunes, toppled into the surf. A blowhole emerged where waves had undercut the cliffs.

As the City of Darwin has acknowledged for years, eroding coastlines are a growing problem for Darwin.

And as global maps in a recent article in the journal Nature showed, Darwin is just one of many cities - including heavily populated centres such as New York City, Kolkata and Shanghai - at growing risk of coastal flooding, in part due to accelerating sea-level rise.

How can we manage change better?

In Darwin, like other low-lying coastal settlements, we essentially have three options: start managing our retreat from the sea; try to engineer coastal defences; or get used to much more volatile and risky life on the edge, and modify our systems, policies and behaviour accordingly.

Of course, we could simply do nothing. But we contend that is the least credible and potentially most expensive option in the long run.

The other three options of managed retreat, investment in coastal defences, and accepting greater risk are not mutually exclusive. They can be blended within a well-conceived long-term strategy.

Managed retreat is the most confronting option, which some communities are already facing. Some low-lying coastal areas simply cannot be defended cost-effectively, and even the best adaptation strategies may be inadequate.

But there are also significant opportunities to reconfigure coastal settlement in ways that minimise social disruption.

In places with valuable assets, such as parts of some cities or Kakadu, we can improve coastal defences, natural and/or engineered.

On the Tommycut Creek: this used to be a freshwater melaleuca forest, like those seen in the film Ten Canoes, but saltwater intrusion has turned it into a hypersaline swamp. Eric Valentine

After our recent storms, Darwin’s coasts were more intact in sections where mangroves, trees and shrubs protected the soil. While the shoreline did retreat, damage was less than in cleared sections. We need to be replanting the dunes we want to keep, and retaining or restoring mangroves in estuarine and low-lying areas.

The North Australian Biodiversity Hub is working with Kakadu Traditional Owners to look at options for managing the impacts of weeds and sea level rise on the floodplains that are so important for food for local people, and more broadly for Top End fishing and tourism experiences.

A casuarina tree that used to be on the landward side of two dunes, now toppled on the beach. Andrew Campbell, CC BY

In Darwin, hard protection of foreshore made some difference. But even rock-walled sections were disassembled in places, with the rocks dragged back into the sea or thrown, with astonishing force, onto the tops of cliffs.

If expensive hard protection is going to be used, it needs to be done at a scale that is engineered to last for decades and withstand extreme weather events, taking into account projected future sea levels.

The latest climate science suggests that northern Australia may have less frequent cyclones in future, but a higher proportion of extremely intense (Category 5 or worse) tropical cyclones.

Darwin residents protest against a proposed residential island between Nightcliff and East Point. Andrew Campbell

Thirdly, the construction of new residential or tourism infrastructure in exposed zones of the coastal environment is inherently risky. At the very least, coastal planning must take into account the amplified risks from continuing sea-level rise.

Prepare now, or pay later

What we are seeing now in Darwin is a taste of things to come in many coastal areas of the world if we don’t take preventative and adaptive measures.

This has major implications for residents, investors, insurers, planners and policymakers. It also promises to create fertile grounds for litigation in the future, if people approving developments are not seen to be basing their decisions on the best available information.

Recent events in Darwin underline that sea level, especially in the monsoonal north, is rising fast, and old assumptions should no longer hold.

So we need to think long-term about which bits of coastal infrastructure we want to try to keep, and for how long, while steadily moving essential services to more secure places.

And we should remember that recent storms have been mild compared to the cyclone that will likely whack Darwin again sooner or later.