An unwelcome seachange

Sea-level rise reconstructed from tide gauges and satellite altimetry (credit J. Church)

Sea-level rise reconstructed from tide gauges and satellite altimetry (credit J. Church)

First published in the Adelaide Advertiser, 1 degree series, August 2007 – updated and hyperlinked.

One of the clearest and most worrying signs that something odd is happening to Earth’s climate is that sea levels are rising steadily. Records of long-term tidal gauges show that sea levels were 17-20 cm higher at the start of the twenty-first century than 100 years before, correlating with a globally averaged rise in temperature of 0.75 degrees. Recent and very precise satellite measurements taken over the past few decades have confirmed this trend. How much of a concern is this change?

Such a rate and magnitude of sea level rise is certainly not unprecedented in Earth’s past. For instance, at the end of the last ice age, around ten thousand years ago, the oceans rose by as much as 120 m over a few thousand years – and up to 1 m every 20 years for a sustained 400 year period – engulfing the previously dry land that connected mainland Australia to Tasmania and New Guinea. Looking deeper in time, global climate was an average of 2 to 3 degrees warmer than at present some 3 million years ago, and sea levels were 35 ±18 m above the shoreline of today.

The implications of these past changes in sea level for future rises are worrying, but uncertain. Our best models predict that the globe will heat up by anywhere between 1.8 to 6.4 degrees Celsius over the next century. The United Nations Intergovernmental Panel on Climate Change (IPCC), in their 2007 review, somewhat reassuringly indicated that the probable amount of sea level rise by the year 2100 will be 18 to 59 cm. Although a rise at the upper end of this range would be catastrophic for low-lying coastal and island communities and ecosystems, many societies will, at a cost, be able to adapt. However, the most recent science suggests that these IPCC projections could turn out to be gross underestimates.

The IPCC projections of sea level rise are based largely on the slow, steady and inexorable thermal expansion of the oceans (as water heats, its volume increases) with some additional contributions from the melting of mountain glaciers (almost all of which are expected to be gone by mid century). They do not include “rapid and dynamical changes in ice flow”, because the IPCC was too uncertain about how likely or influential these changes might be. This means that the 18 to 59 cm projection is really a best-case scenario, given that it assumes the great icecaps of Greenland and West Antarctica remain largely intact.

Yet the rate of ice loss from these two polar realms, as identified by satellite measurements of the change in gravity of the ice masses, has more than doubled over the last decade. It now represents a loss of up to 150 cubic kilometres of ice per year, from each region. Similarly, the rate of sea level rise has doubled over the last 10 years, and now exceeds the upper-end predictions made by the IPCC only a few years before. The suspicion of many scientists is that this acceleration is being driven by self-reinforcing feedbacks. This includes the loss of reflectivity (“albedo”) caused by sunlight striking dark exposed rock rather than white ice, the lubricating effect of melt water penetrating to the bottom of glaciers, and many other “dynamical changes”.

If both Greenland and West Antarctica shed the entirety of their ice burden, global sea levels would rise by 12 to 14 m. Although these icecaps would not disintegrate within a century, the loss of even a third of their mass – quite plausible if the rate of polar ice loss continues to double each decade – would force up the oceans by at least 4 m, with disastrous socioeconomic and environmental consequences. Further, the inertia accumulated by the slowly heating, yet physically vast oceans, means that should large-scale polar melting begin, it will almost certainly be impossible to halt.

The likelihood of such a frightening scenario unfolding is currently quite uncertain. Perhaps in 10 to 20 years, when science has accumulated considerably more information on ice flow dynamics, and measured another two decades of ice loss, we will be more confident about the true risk of catastrophic sea level rise.

The question is, can we afford to wait until we are sure?



  1. The question is, can we afford to wait until we are sure?

    Another way of phrasing this question is: “assuming uncertainty about sea level rise, and given a certain amount of risk aversion, what is the expected impact on our utility of waiting to be sure?”. In his paper On Modeling and Interpreting the Economics of Catastrophic Climate Change, the economist Martin Weitzman looked at this question for climate sensitivity, which is also uncertain and has a long tail. His conclusion was that the possibility of low probability catastrophic impacts, and the fact that climate change has “potentially unlimited downside exposure” implied that the expected cost would be infinite. Unfortunately the chance of Greenland and West Antarctica melting may not even be “low probability”.

    This suggests that the answer to the question “can we afford to wait until we are sure?” is no.


  2. Sea level rise rates rose from about 0.5mm/year early in the 19th century to about 3.5 mm/year at present (Rahmstorf, 2006).

    A fundamental contradiction emerges from the IPCC 4th Report, where its projected sea level rise of 18 – 59 cm by 2100 can in no way be reconciled with its temperature rise projection of 1.8 – 6.4 degrees C by 2100.

    No oceanographer or glaciologist would expect the Greenland and west Antarctic ice sheets to survive even +3 degrees C rise, let alone +6.4 degrees C rise!


    Nor can any such conclusion be reconciled with the relations between temperature and sea level rise during the recent history of Earth, which are in the very minimum +8 meters per +1 degree C (as in the mid-Pliocene (3 Ma), or about +20 meters per +1 degree C in the Youngest dryas (11 230 years ago).

    For the +0.8 degrees C global mean temprature rise since early 19th century, given delay factors, sea level rise would reach several meters.

    In a recent paper in Science Express (19 June, 2008) Stevensen et al. estimate from deuterium isotopes in Greenland ice cores a the rise in about 4 degrees C at the Youngest dryas took place over a few years! (the abstract is pasted below). There was much ice to melt at that stage (Laurentian and Scandinavian ice sheets) but global temperatures were also lower. What the paper demonstrates is the EXTREME INSTABILITY of the atmosphere to minor forcing, i.e. less than 0.4 Watt/m2 mean global orbital forcing at the last termination.

    How stable is the atmosphere at present under a total anthropogenic GHG forcing – since 1750AD – of 1.6 Watt/m2 (Figure SPM-2 in the IPCC “Climate Change 2007: The Physical Science Basis”)?

    High-Resolution Greenland Ice Core Data Show Abrupt Climate Change Happens in
    Few Years
    J. P. Steffensen et al., Science Express, 19 June, 2008


    The last two abrupt warmings at the onset of our present warm interglacial period, interrupted by the Younger Dryas cooling event, are investigated in high temporal resolution from the Greenland NGRIP ice core. The deuterium excess, a proxy of Greenland precipitation
    moisture source, switches mode within 1-3 years over these transitions and initiates a more gradual change (50 years) of the Greenland air temperature as recorded by water stable isotopes. The onsets of both abrupt Greenland warmings are slightly preceded by decreasing
    Greenland dust deposition, reflecting wetting of Asian deserts. A northern shift of the ITCZ could be the trigger of these abrupt shifts of northern hemisphere atmospheric circulation resulting in 2-4K changes in Greenland moisture source temperature from one year to the next.


  3. The Garnaut Review and sea level rise

    The Garnaut Review states (page 13): “If the Greenland ice sheet were to melt, it would add about seven metres to the world’s ocean, and the west Antarctic ice sheet up to six metres, over a long period”.

    However, the Garnaut Review does not specify the time scale meant by “over a long period”.

    Satellite data indicate sea level rise of 3.3 mm/yr during 1993-2006. Since 1990 the observed mean global sea levels have been rising faster than projected by IPCC models. Over the last 20 years the rate is 25 percent faster than the rate in any twenty years period in the preceding 115 years (Rahmstorf et al., Science Express, 2007, Potsdam Institute of Climate Impact Research, 2007).

    Leading climate scientists, including Professor James Hansen (NASA’s chief climate scientist) and Professor Steffen Rahmstorf (Potsdam Institute of Climate Impact Research), project sea level rise on the scale of metres through the 21st century. Paleo-climate studies by Glikson and Brook (in prep) indicate sea level rise rates of well over 5 metres per 1 degree C, consistent with these projections.

    Sea level rises reflect melting of the Greenland ice sheet, where melting since measurements began in 1979 increased by 30 percent (S. Konrad, University of Colorado, AGU, 2008), and of the west Antarctica ice sheet which is losing ice at rates 60 percent faster than 10 years ago (British Antarctic Survey, Nature Geoscience, 2008).

    Given the consequences of sea level rise around the world in terms of inundation of cultivated delta, coastal and lower river regions, where hundreds of millions of people live, and the flooding of port cities, including major Australian coastal cities, the warnings presented in the Garnaut Review would appear to underestimate of the global effects of sea level rise, should the levels of greenhouse gases in the atmosphere and associated carbon cycle feedbacks continue to rise.


  4. Greenland melting is probably a bit abstract for many people to comprehend, but I found a neat little website called which maps which areas may be submerged by a 7 meter rise in sea level, using a ‘google maps’ style interface. There are various sources of inaccuracy with this approach, most of which will lead to the actual consequences of sea level rise being worse. The most significant is probably coastal erosion.

    As well as significant flooding of inhabited areas, the most noticable impacts are on ports and airports. Transport infrastructure is also affected. The impacts on ports usually occur at levels signifantly less than 7 meters and there is not one port that I looked at that would not be severely affected by a 7 meter rise. One issue related to this is food security. If southeastern Australia dried out we may still be able to import food, but if both our ports and many other ports around the world are affected, then this will also be a serious problem. How much of a problem depends on the speed of sea level rise and on the degree of adaptation. Sydney and Brisbane airports and ports are particularly at risk, and most of the Gold Coast will be flooded.

    Here is a non-exhaustive list of suburbs affected:
    Adelaide: Adelaide Airport, some of Henley Beach, West Lakes, Ethelton, Glanville, Peterhead, Outer Harbour, Port Adelaide, Rosewater, Mansfield Park, Gillman, Wingfield, Dry Creek, and some of The Levels. Semaphore and Largs Bay will become an Island. Shipping infrastructure around Outer Harbour (2-6m). Severe disprutions to Grand Junction Road, Port River Expressway, Salisbury Highway, and Port Wakefield Road.

    Melbourne: Altona, Seaholme, the Docklands (2-4m), Port Melbourne, Albert Park, St Kilda, Elwood, Aspendale, Chelsea, Patterson Lakes. The Weribee Train line and West Gate Freeway will be disrupted.

    Sydney: Sylvania Waters, Sandringham, Ramsgate Beach, Monterey, Kyeemagh, Sydney Airport, Port Botany (1-3m), Banksmeadow, Sydenham and part of Marrickville, Wolli Creek, some of Homebush, Rosehill. Rail Infrastructure around Sydenham, Eastern Distributer tunnels around the airport.

    Newcastle: Wickham, Hamilton East, Maryvale, Mayfield, Carrington (including Carrington Coal Terminal), Stockton, Kooragang (including Kooragang Coal Terminal), parts of Maitland.

    Most of the Gold Coast will be flooded.

    Port of Brisbane (1-4m) and Brisbane Airport, some suburbs on the Brisbane River, which could probably be dammed with some sort of wall. Many suburbs along the coastline will be flooded.

    The port in Fremantle will be flooded (1-5m). Low lying areas around the Swan River can probably be protected with some sort of dam/sea wall. There will serious flooding of some urban areas south of Perth, including Rockingham and Mandurah.


  5. The issue of rising (sic) sea levels crystalises the whole issue of mitigation versus adaptation. The Stern-Garnaut approach prefers to wreck the whole economy now for the sake of avoiding putative and far-off local effects. The Dutch have continuously for hundreds of years adapted to changing sea-levels, most recently after the 1953 floods. Have they suffered since from rising sea-levels? Perhaps the denizens of Rose, Neutral, and Double Bays in Sydney are stoopid as well as rich, but when we see them raising sea walls then we might believe in the Brook-Gliksen thesis.


  6. The Dutch have continuously for hundreds of years adapted to changing sea-levels, most recently after the 1953 floods. Have they suffered since from rising sea-levels?

    You mean, like, other than the huge amount of money spent over the centuries to deal with it?

    Thank you for proving our point, that global warming is going to cost people REAL MONEY. Or, if they don’t have it, such as in Bangladesh, worse.


  7. Several points:

    One of my pet peeves is that many, if not most, government bodies refer to sea level rise by 2050, or by 2100, as if the levels will stop once these dates are reached. It would be much more appropriate to refer to the equilibrium amount of increase in sea level per unit of achieved equilibrium temperature increase. Whether it takes 40 or 90 or 500 years, the increase will still impact future generations and ecosystems. Incremental times to such equilibria, whilst nice on a calendar, are more distracting than useful in an overall sense.

    On another point, one of the factors that seems to be missed in some of the estimated of sea level increase is the amount that it is currently ‘decreased’ through impoundment of river flows by dams. I’ve seen two papers this year that refer to a figure of around 1mm/year increase being masked because of water collection by humans. Frustratingly I don’t have the papers with me at the moment, but a quick Google Scholar can probably take one to the information rapidly enough.

    And then there’s Tim Curtin…

    Tim, I am Dutch Australian, and very proud of my Dutch birth and heritage. Nevertheless, your promotion of Dutch reclamation and dyking technology ignores so many historic, economic, and technical factors as to be laughable as a more general strategy. In fact any promotion of an adaptation approach (whether in this or in any other area) as anything other than a last resort is not only misguided, it is irresponsible. We would always be chasing our tails with respect to cost and logistic requirements, let alone ignoring ‘side issues’, and having painted over the rust the original causes would only continue to increase in severity.

    With mitigation we stand a chance of avoiding the worst of unpredicatable synergies down the track: relying simply on adaptation would be no different to the Sorcerer’s Apprentice strategy of taking an axe to the broomsticks.

    But you just can’t let go of your idea that, according to your economic theory, it might cheaper and easier for someone else at some unspecified time in the future to scoop up your poop, can you? Not only is this a logically dubious notion, it is a morally repugnant one too.

    Oo, and I think that should be Maryville, not Maryvale.


  8. Most likely, reductions in sea level rises due to damming will be more than offset by lack of retention of stormwater due to urbanisation.

    Spangled drongo, I would really like to see the literature that you drew upon for this claim.

    Try “imperceptibly” instead of “more than”, and you will be closer to the mark.

    Or do you have some arcane twist on catchment areas, evapotranspiration, Manning’s equation and hydraulic resistance and other such hydrological fundamentals, that the rest of the world is not privy to?


  9. Learnard Bernard,

    “Or do you have some arcane twist on catchment areas, evapotranspiration, Manning’s equation and hydraulic resistance and other such hydrological fundamentals, that the rest of the world is not privy to?”

    While there has been endless calcs on stormwater runoff in cities, I have never seen any on the increase in runoff due to the world’s 6 billion people and their domestic animals.

    But the effect of our infrastructure, land clearing, farming and grazing would be an enormous increase in stormwater runoff.

    Almost always this is designed not to end up in dams.

    Undisturbed bushland often has no runoff even after very heavy rain.

    Conversely, dammed water would often never make to the sea even if the dams weren’t there.

    If you ever come across a paper that does this very involved calculation WRT sea levels I would like to read it.


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