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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.

Join the conversation

50 Comments sorted by

  1. JB Rawson

    Writer

    Really interesting article: thanks, Andrew and Stephen.

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  2. Ron Steele

    Retired Geophysicist

    The high rate of sea level rise across northern Australia compared to the rest of Australia coincides with the area of collision between the Australian tectonic plate and the Eurasian and Pacific plates. I wonder how much of the "sea level rise" in that area is of epirogenic / tectonic origin (ie the sinking of the northern area of Australian crust relative to the geiod).

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    1. mike flanagan

      retired

      In reply to Mark Duffett

      Thanks Mark for that clarification and supporting links. I find the 17cm figure indicated by the authors astounding although believable with the growing evidence from both Arctic and Antarctic data.
      As an interested layman I am searching for some data about the influence on water levels at what maybe described our choke between the Pacific and Indian oceans (Darwin, Indonesia, New Guinea, Nth Aus. Malay peninsula etc..) of both the planetary Coriolis Effect, and the Pacific Walker Circulation. Perhaps you may have some suggested links?
      From a recent internet encounter I suspect the denier lobby are about to confuse the GHG and Climate Change debate with the injection of an avalanche of specious arguments in order to further confuse the public debate with and for spurious reasons
      .

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    2. Mark Duffett

      logged in via Twitter

      In reply to mike flanagan

      Sorry Mike, unfortunately I'm a bit beyond the limit of my expertise here already. As Andy S said, 'the CSIRO pages are interesting, but still don't get to the cause'; that site would be even better if it made it easier to get to the source literature.

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  3. Andy Saunders

    Consultant

    Question from ignorance: how can there be such marked regional differences in sea-level? Why don't they equalise (more) over time (especially given the large tidal volumes sloshing around)?

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    1. Mike Swinbourne

      logged in via Facebook

      In reply to Andy Saunders

      The reason that sea levels rise at different rates in different parts of the world has to do with - among other things - gravity. The gravitational attraction of the Earth varies from place to place, resulting in different sea levels. For example, you might be surprised to learn that the sea level at different ends of the Panama Canal varies by about a foot, which created problems for the people building it.

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    2. Andy Saunders

      Consultant

      In reply to Mark Duffett

      Thanks to all. I understand the overall sea-level drivers of thermal expansion and ice-cap melt.

      And I understand local (or even regional) subsidence (although I guess technically this isn't sea-level rise, just looks like it). And even rebound from Greenland/Antartica ice-cap melts (second-order effect).

      And I guess current/wind pile-up effects from changing weather and ocean-current drivers are understandable (maybe that is the cause of the Panama Canal effect?). I don't understand the gravity…

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    3. Nick Kermode

      logged in via email @hotmail.com

      In reply to Mike Swinbourne

      Also ocean currents can have a large effect. The South Equatorial Current piles up against Indonesia so much sometimes I think there is a metre or so difference either side of an island. When it gets compressed between the islands it makes for some hair raising drift diving though!

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    4. John Nicol

      logged in via email @bigpond.com

      In reply to Mike Swinbourne

      Mike Swinburne

      I think the differences in sea levels have little if anything to do with gravitational variations and relative heights of sea levels vary with time because of small changes in winds and ocean currents. I believe that the difference between the levels at either end of the Panama Canal has to do with the rotation of the earth and the general drag of the tides, a phenomenon known as the "Diurnal Lag".

      Certainly various parts of our coastline and else where have suffered varying…

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    5. Andy Saunders

      Consultant

      In reply to Nick Kermode

      Thanks, Nick.

      The Harvard/Nature work shows two other effects: gravitational pull of the glacier mass, and a shift of the axis of rotation of the earth.

      But still leaves a conundrum (or likely just my ignorance). The gravitational effect of the glacier mass is relatively localised to near Greenland and Antartica, and the rotational effect is (roughly) hemispheric. Doesn't explain the higher-frequency changes around northern Australia in particular seen in the map in the TC article - are these simply from piling-up effects? Or subsidence (which seems unlikely, given the magnitude is orders less than measured/forecast)?

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    6. Mike Swinbourne

      logged in via Facebook

      In reply to John Nicol

      John

      I'm afraid that what you think about the issue of seal level rise is immaterial - and wrong - and is more a reflection of your ideology on the issue of climate change than anything rational and evidence based. Did you even read the link I provided?

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    7. Nick Kermode

      logged in via email @hotmail.com

      In reply to Andy Saunders

      Hopefully someone with some expertise can tell us both the answer :)

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    8. Craig Read

      logged in via Twitter

      In reply to Andy Saunders

      Put simply: the moon and sun. Tides are typically bigger closer to the equator for the same reason(s).

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    9. John Nicol

      logged in via email @bigpond.com

      In reply to Mike Swinbourne

      Mike Swinburne

      My comments had nothing top do with global warming or my own ideology, which I admit has little respect for the lobby on CAGW.

      The glacial melt in the article you refer to - if that is the article you are suggesting here obviously refers to very much longer time scales than the relatively few years being considered here. Indeed there are no useful numbers in the article to demonstrate the magnitude of the effect.

      BTW, another reference given somewhere on this page is to an article by the CSIRO projecting sea level changes relative to 2030 and 2070. Those countries along the tropics such as Darwin are shown as having practically zero change in 100 years time while nearer the poles the differences are significant. They show rises at the poles where presumably the "glaziers" will melt but the gravitational forces will apparently be negligible. I have no idea upon what "theory" their predictions are based but their diagrams are clearly presented.
      John Nicol

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    10. Felix MacNeill

      Environmental Manager

      In reply to John Nicol

      John, I was just enjoying a calm, rational exchange of ideas, questions and information without any pointless distractions with irrelevant fictions and fetishes - just what one values about this site. Thanks for once again spoiling that.

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    11. Robert Tony Brklje
      Robert Tony Brklje is a Friend of The Conversation.

      retired

      In reply to Andy Saunders

      It is to do with the tides. A rise is sea level also results in a proportional rise in height of the high tide in addition to the sea level rise. Where tidal movements are particularly large that proportional rise is much higher, basically because gravitational affect of the sun and moon now has more water to work with. Should they measure low tide rise in sea level they will find much more uniformity.

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    12. Rob Painting

      logged in via Facebook

      In reply to Andy Saunders

      Andy - the greater-than-average sea level rise around Australia in recent decades is largely a result of stronger tropical trade winds. Satellite altimetry became available in 1993 and it just so happens that this coincided with the strengthening of the trade winds. When the winds strengthen there is a strong flow of water pushed through the Indonesian archipelago by the spin-up of the subtropical gyres in the Pacific Ocean. Since the year 2000 the trade winds have been in an intensfied state, otherwise…

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    13. Jim Inglis

      retired

      In reply to Mike Swinbourne

      Well, well, Mike, Another alarmist, history-and-logic-defying projection.

      So believable. Which model did he use?

      "Alarming though these projections may be, they are not immediate. “We’re talking a scale of hundreds of years before this could ever happen,” says Mitrovica. Predictions of what will happen are not his bailiwick; he leaves that to climate scientists and focuses instead on what he compares to “detective work.” "

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    14. David Arthur

      resistance gnome

      In reply to Jim Inglis

      Perhaps you could move to Darwin and conduct your own observations, Mr Inglis?

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  4. Brian Keyte

    Potter

    Thank you very much Andrew and stephen, all very much news to me. Part of the reason I'm here, to find out new stuff. Great!
    Both Ron Steele and Andy Saunders raise points I would be very interested in hearing more on. Maybe material there for another article or two? Please!

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  5. Mike Pope

    logged in via email @hotmail.com

    Erosion of the coastline will occur where there is a rise in sea level such as that experienced in northern Australia. Bruuns Rule states that on average each 1cm of sea level rise results in about 1m of coastal recession. In other words, for each meter of sea level rise, the coastline is eroded, over time, by 100 meters.

    The authorities can attempt to slow erosion with coastal defences but eventually their efforts will prove fruitless, particularly as both global and regional sea level rise is an ongoing and accelerating phenomenon.

    Ultimately, the only alternative to retreat to higher ground, is to curb greenhouse gas emissions and their effect on global warming – and that is not going to occur as long as commercial interests see on-going or new opportunities for higher profits from using fossil fuels.

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  6. david leitch

    research analyst

    Great article. Exactly on my reading interests of adapation and impacts

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  7. Craig Myatt

    Industrial Designer / R&D

    EVERYONE should be reading this article.

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  8. Brian Westlake

    Common Sage

    Funny that the sea level around Narrabeen 4,000 years ago was 1.5M higher than today http://www.abc.net.au/catalyst/stories/2278381.htm what was the driver for that change ?

    In the 70's we were taught that Australia was sinking at 3mm/yr now it is the sea level that is rising by that amount and the land mass is holding steady ! I guess when the sea level gets back to where it was 4,000 years ago we all know what process to blame, don't we.

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    1. Robert Tony Brklje
      Robert Tony Brklje is a Friend of The Conversation.

      retired

      In reply to Brian Westlake

      So what you are saying is as for Narrabeen so the whole planet, hmm, I'm quite sure you can see the fallacy in that.
      It is becoming pretty obvious that the current government and it's PR crew are quite disgusting and no amount of News Corporation gloss can cover it over.
      Basically with regard their intention is quite clear, they fully intend to privatise the profits of developing at risk land and then in an act of total shamelessness turn those privatised profits into ten or even a hundred times that in socialised losses.
      Same old, same old, except now that pattern is being recognised.

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  9. John Holmes

    Agronomist - semi retired consultant

    So Casuarina beach is eroding.

    This article brings back memories of being in the Office of the Administrator along with 5 other agricultural scientists from the Department being threatened with the sack by the Administrator. Our crime was to talk to the local paper re sand mining on the main swinging beach, and its effect on the dune system and the destruction of a fresh water swamp system behind the beach. How dare we draw peoples attention to landscape degradation. A few leaks by our boss…

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    1. John Holmes

      Agronomist - semi retired consultant

      In reply to John Holmes

      Note - Why only 6 of us in the office. The rest of the group were out in the field at the time. Young and idealistic, but not as savvy politically as I hope most of us are by now.

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  10. Henry Verberne

    Former IT Professional

    A good article which focuses on the potential strategies to ameliorate the worst projected impacts of sea level rise. It also reminds me that though climate change is real we also need to adapt- and the authors have outlined various adaptations.

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  11. Warwick Rowell

    Permaculturist

    A good article on the current dilemma and its implications, and some good links to explain the detail.

    BUT.....

    Climate sensitivity, sea level and atmospheric carbon dioxide

    James Hansen, Makiko Sato, Gary Russell and Pushker Kharecha

    Phil. Trans. R. Soc. A 2013 371, 20120294, published 16 September 2013

    Differences between our inferred sea-level chronology and that from the ice sheet model [46] are relevant to the assessment of the potential danger to humanity from future sea-level…

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  12. Simon Campbell

    Postdoctoral Researcher in Astrophysics at Monash University

    Hi Andrew and Stephen,

    I don't doubt the statement that:
    "[Insert any number] centimetres may not seem much, especially with a 7 to 8 metre daily tidal range. However, raising the underlying base makes a big difference..."

    However a a quick read of the source (CSIRO, BOM?) of the very large value of 17cm (=8.5 cm/yr) that underpins this article shows that this value is most likely *not* the long term rise in sea level at all - 20 years is a short time in the oceans it seems.

    The original…

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    1. Mark Pollock

      Analyst

      In reply to Simon Campbell

      Simon,
      You have come to the home of poor/ misleading and sensational science. And yes, it does provide ample fodder to the climate denialists. You capitalised the phrase is if it was some sort of proper noun but I don't think any one really denies the climate. You seem an exact sort sort of chap so may I suggest "poor/misleading and sensation climate science denialists" for people like me?

      I lived inDarwin in the early eighties and and the place wasn't as developed as it is now. The whole place is fluid. There is no real coastline. Old timers would talk about the madness of building in surge areas, of how this was once sea and now is land, this was now land and now is sea.

      One casuarina falling into the sea, or one poorly planned bike path does not a catastrophoby make.

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    2. Simon Campbell

      Postdoctoral Researcher in Astrophysics at Monash University

      In reply to Mark Pollock

      Hi Mark,

      1) I did NOT say the science was poor/misleading and sensational - I said the _reporting_ of the science was. Poor science reporting is a major problem in Australia. The BOM and CSIRO scientists were clear and precise in their reports (which I linked to).

      2) I did make a mistake leaving out 'change' in the term Climate Change Denialists. However that is probably just as apt given the mentailty of these people - they may as well be denying the existence of climate. Earth is (mostly…

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    3. Simon Campbell

      Postdoctoral Researcher in Astrophysics at Monash University

      In reply to Simon Campbell

      Also a typo in my post: 8.5 cm/year sea level rise rate should of course be 8.5 mm/yr (compared to about 3mm/year globally).

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  13. Stephen Garnett

    Professor of Biodiversity and Sustainability at Charles Darwin University

    First thank you to all of you who have contributed so constructively to this discussion.

    Secondly, in response to the comments by a number of you on sea level cycles, what was striking about the recent storms was their exposure of a compressed but fragile bed of shells beneath a dune that, until recently, was the landward of a pair of such dunes over 5 m high. I struggle to think of circumstances in which such an extensive bed of shells would be laid down then have a dune on top except at a time…

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  14. Jim Inglis

    retired

    Generally the ocean equilibrates about the centre of gravity of this flat spotted geoid, give or take some change in levels due to constant wind direction and earth rotation but then this tends to even out with currents that vary in strength to bring it back to level.

    The sea doesn't just keep mounding up at that higher rate in various spots year after year as some of those maps would have us believe.

    Where people are claiming SLR in places where there have been recent storm surges, history…

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    1. Jim Inglis

      retired

      In reply to Jim Inglis

      " now know that errors in satellite SL measurement are considerably overstated ".....should read:... now know that satellite SLR measurements are considerably overstated...

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