The encroaching sea: new NZ sea level rise maps

This guest post is by Jonathan Musther, who has just published an amazing series of highly detailed maps projecting future sea level rise scenarios onto the New Zealand coastline. If you live within cooee of the sea, you need to explore his maps. Below he explains why he embarked on the project.


The effect of 10m sea level rise on Christchurch: say goodbye to St Albans, prepare to paddle in the CBD. Full map here.

For humans, sea-level rise will almost certainly be the most directly observable effect of climate change, and specifically of global warming. As the climate changes, many of the effects will be subtle, or if not subtle, they will at least be very complex. Summers may be warmer, or cooler; we may experience more rain at some times of year, and less at others; tropical storms may increase and they may be sustained further from the equator, but all of these changes are complex, and not necessarily obvious against the background complexity of any climate system. In contrast, there is something obvious and unstoppable about sea-level rise, there is no question that it will send anyone in its path running for the hills.

For some time I have been involved in searching for land appropriate for specific uses such as arable farming, water catchment, and off-grid living. When searching for land in this way, there are many, many criteria to consider, and of course one of these is potential future sea-level. Using GIS (Geographical Information System) software, and elevation models of the New Zealand landscape, it is possible to visualise sea-level rise, and select sites accordingly. Naturally, the next question is what sea-level rise to consider. It is possible to place an upper-limit on sea-level rise – after all, there’s only a finite amount of ice that could melt – but beyond that, we’re limited to informed guesswork.


25m sea level rise: a sunken city and Banks Island. Full map here.

What is the maximum possible sea-level rise? It depends who you ask. Many sources place the maximum potential sea-level rise at around 60-64 metres, but these figures are rarely referenced, and don’t concur with the latest research. Other sources place the figure at around 80-81.5 metres, and while this appears to be well referenced and researched, it is based on work that is somewhat out of date. The best estimates I’ve been able to locate, based on recent measurements (and lots of them) are around 70 metres, but quite what the margin of error is remains uncertain. Of course, when considering future sea-level, we must remember that here in the South Pacific, we will likely experience increased numbers of more powerful tropical storms, with associated storm surges.


At 80m, West Melton is a seaside township. Full map here.

The maps I created showing sea-level rise for the whole of New Zealand depict rises of 10, 25 and 80 metres. I have certainly received criticism for not focussing on more modest sea-level rises (e.g. 1 or 2 metres), but there are some good reasons for this: firstly, the resolution of the elevation models of New Zealand do not allow accurate predictions of such small rises. Secondly, larger sea-level rises pose a huge threat, and are therefore worth considering. I made a point of avoiding time frame predictions when producing the sea-level rise maps, partly because the time frame is largely irrelevant (if 80% of our homes are flooded, it’s bad news, no matter when) and partly because the range of expert estimates is huge. Study after study shows that we have underestimated ice-sheet instability, and it is almost universally accepted that large sea-level rise will be a consequence. Unfortunately, most studies place this sea-level rise at some unspecified time in the future – when, we’re not sure, but it’s far enough away that we needn’t worry…

So is a 10 or 25 metre sea-level rise likely? Unfortunately, the broad answer is yes. The Greenland, West and East Antarctic ice sheets are showing growing instability, and many researchers agree that they may have past a ‘point of no return’. Remember, the Greenland ice sheet alone, if completely melted, would lead to approximately a 7 m rise in global sea-level. Of course, we return to the issue of when this is likely to happen, and on that, the jury is out.

I firmly believe that to be good scientists, we must investigate the possibility of large sea-level rise, and its consequences. The time frame is unclear, the absolute rise is also unclear, but there really is something unstoppable about rising oceans. We are now well outside the sphere of collective human experience and expertise, and we should be very careful to prepare, as best we can, for a range of scenarios.

15 thoughts on “The encroaching sea: new NZ sea level rise maps”

  1. Sea level rise has moved up the lists as far as threats are concerned but not in a way that makes a twenty meter sea rise flood the farm and ruin your carpets way. The ice in Greenland and West Antarctica is melting faster than anyone predicted but even so there is a limit to the speed at which such a vast amount of ice can melt. In our lifetime we could see one meter or, if you are very young, two metres. The danger form one metre is mostly economic in that so much infrastructure will be lost that it will cripple the economy and also that of most countries around the world. My blog points to some of the city losses in NZ but a lot of Florida disappears, a huge amount of farmland in the UK and of course Holland.
    The biggest worry is the displacement of hundreds of millions of people which will cause civil strife as families are displaced and will cross borders looking for a safe place to live. Worrying about the occasional flood is the least of your worries when you are sharing your property with twenty refugees and the economy is bankrupt.

  2. The 2m rise is probably a very important one because it will do just enough damage with storm surges to scare the bejesus out of a lot of folk who currently have their thumbs jammed in their ears singing “La la la la…”
    Can’t come too soon….
    (OK, sad for the unfortunates that get caught in the firing line, but humanity is going to have a LOT worse if we don’t pull finger!”

  3. I wrote a blog post on this subject recently and came to the same conclusions as you:

    “…From recent satellite data and scientific studies, SLR appears to be in the beginning phase of an exponential growth pattern that will decimate thousands of coastal cities by 2070. Last year we learned that the ice mass loss rate from both Greenland and Antarctica has more than doubled in the past 5 years. Ominously, the West Antarctic ice sheet has been found to be less stable than originally thought. Warming ocean waters are infiltrating beneath the ice shelves and irreversibly melting West Antarctica from below. And more recently we learned that the stability of East Antarctica is being undermined in the same insidious way. In fact, Antarctic ice shelves have been thinning up to 70% faster than average in some spots. These ice shelves extend out over the polar waters and are what hold back and support all the land-based glacial ice. Once the ice shelves are eroded, land ice will have an open path to slide down into the ocean and melt, greatly accelerating SLR. Congruent with these disturbing trends is the revelation that SLR has been increasing much faster than we thought in the last couple decades. The rate of change per year has been 3.2mm since 1990 versus 1 to 1.4mm for the previous nine decades. That is a 100% to 200% increase in just the last couple decades…

    …Hansen had posited a doubling time of ten years for land ice melt rates, but satellite data has revealed a doubling time that is occurring twice as fast. This would put those measurements more in line with the projections of physicist/climatologist Paul Beckwith who calculates we may be on track for a 7 meter(23 feet) SLR by 2070 if the doubling period of ice cap melt from both Greenland and Antarctica hold up over this century…”

    1. I would not want to contradict Hansen as he is a hero of mine but AProff Andrew Macintosh of the Antarctic Research Centre said that the rate of ice melt has a physical limit that will not allow huge increases in such short periods. Two metres is easily enough to bankrupt the economies of every country in the world and start massive wars.

      1. “Unfortunately, over the past several years, support has crumbled for the tidy belief that ice sheets require millennia to disintegrate.
        First, as I argued in my 2005 ‘Slippery Slope’ paper and discussed in Chapter 5, it became clear that the ice sheet models fail to incorporate physics components that are critical during ice sheet collapse. This deficiency has been confirmed by the models’ inability to simulate the rapid changes observed on Greenland and Antarctica during the past few years.
        Second, the belief that ice sheets are inherently lethargic is based mainly on the average rate at which they grew and decayed during Earth’s history. The overall size of ice sheets grew and decayed over tens of thousands of years. But the ice sheets responded slowly because that was the time scale for changes of Earth’s orbit—the time scale for the forcings that caused ice sheets to grow or melt. Those slow orbital changes imply nothing about how fast the ice sheets would respond to a rapid forcing. On the contrary, as I and five coauthors showed in a paper published in the Philosophical Transactions of the Royal Society in 2007, during the deglaciation there was no discernible lag between the time of maximum solar forcing of the ice sheet and the maximum rate of melt (maximum rate of sea level rise). In other words, Paleoclimate data indicate that ice sheets are able to respond rapidly, with large changes within a century. Sea level 13,000 to 14,000 years ago rose at a rate of 3 to 5 meters (10 to 17 feet) per century for several centuries.
        Third, evidence has mounted during the past several years that it is not unusual for sea level to fluctuate by several meters within an interglacial period. The most comprehensive study for the immediately prior interglacial period was published by geologist Paul Hearty and several colleagues in 2007. They showed, from sedimentary and fossil evidence on the shorelines of Australia, Bermuda, the Bahamas, and other locations, that sea level was about 2 meters above the present level for most of the interglacial period, but near the end of it, about 120,000 years ago, sea level increased to a maximum between 6 and 9 meters higher than today. The additional water must have come from Antarctica, Greenland, or some combination of the two.
        The Hearty study and others show that the sea level stability of the late Holocene cannot be taken for granted. The Holocene’s stable sea level, so far, may be related to the fact that temperature peaked early in the Holocene and at a level slightly cooler than in most interglacial periods—and this peak warming was followed by a slight cooling trend.
        Whatever the reason for sea level stability, it helped spur the development of civilization, as mentioned earlier. The stable sea level not only provided early humans with a high-protein marine food supply, but it also made possible grain production in estuary and floodplain ecosystems. With these conditions, food for the human population could be produced by a fraction of the people, thus allowing a transition from the Neolithic way of life to urban social life and the development of complex state-governed societies.
        The period of stable sea level is almost surely over…”
        ~ James Hansen, Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe…

        “…Humans, by burning fossil fuels, are now increasing atmospheric carbon dioxide by 2 ppm per year. In other words, the human climate forcing is four orders of magnitude—ten thousand times—more powerful than the natural forcing.”
        ~ James Hansen, Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe…

        “The combined radiative forcing due to increases in carbon dioxide, methane, and nitrous oxide is +2.30 [+2.07 to +2.53] W m–2, and its rate of increase during the industrial era is very likely to have been unprecedented in more than 10,000 years (see Figures SPM.1 and SPM.2). The carbon dioxide radiative forcing increased by 20% from 1995 to 2005, the largest change for any decade in at least the last 200 years.”
        IPCC Group I Summary Report, “The Physical Science Basis”

        1. My understanding of the early stability of the Holocene period is that it was early slash and burn farming techniques that added just enough CO2 into the atmosphere to keep the temperature stable.
          The big jump in sea levels where it increased at 4 metres every hundred years may have been the mass of ice over Canada and Russia melting quickly.
          I dont think that the detail is that important as just one metre will bankrupt most economies and including the USA, China and Europe and there will be so many millions of people on the move that civil wars will be all over the world. This is just a sample of our losses.
          Could our economy survive? I doubt it.

        2. Xraymike79

          Thank you for the very useful posts, I have often wondered whether a rapidly increasing forcing might also lead to a rapid ice melt. You haven’t commented on the effects of albedo loss, I assume that would be another tipping point with disastrous consequences.

  4. Much infrastructure will have to be rebuilt further inland, or land reclaimed. But if you build new settlements and infrastructure, how far inland do you go? Do you assume a 60 metre sea level rise and build far inland, or design on the basis of a one or two meter rise and assume settlements will relocate and rebuild many times over? This appears to be the approach NZ is taking.

    Clearly whatever scenario applies a lot of roading infrastructure will be rebuilt and that is where the costs will be. Modern buildings are designed to only last about 80 years (although they often have longer lives in practice). If sea level rise is gradual adaptation at contained costs may be possible, but if there is acceleration it will be difficult. If there is a sudden period of rapid sea level rise as there has been in the distant past, costs will be horrendous.

    However there are other problems, like loss of farmland and possible refugee problems and tensions. We have enough challenges with refugees right now, due to war and political situations.

  5. And the North Island becomes two islands. The channel between the two will be named Groser Strait, after the “straight” talking Minister responsible for Climate Change who helped to allow this to happen.

    Auckland will be inundated causing the peaceful residents of Remuera to flee their homes. Chased away by the hordes from Pakuranga and Mangere Bridge whose homes are now underwater – thanks to the lifestyles of the rich and famous.

  6. Not 100% sure about the maps. All the areas where I have a reasonable idea of the topography, the suggested 10m line is off-course. There should be a big kink west of Papanui in ChCh corresponding to the well-defined ‘swamps’ between Wairakei Rd and Memorial Ave that probably extend right up to Burnside and I find it hard to believe that the Otagon/York St in Dunedin are below 10m and South Dunedin is above.
    Anyone else spot discrepancies?

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