Rising Sea Level and the Potential Impact on Canterbury


We have all heard about global warming and the potential for sea level to rise. But what does it actually mean for people living in Canterbury? How much is sea level predicted to rise and just what will that mean in terms of the topology in Christchurch? These are a few of the questions I will attempt to answer here.

Image Cusack, D


I am not the first to ask this question and there have been some excellent technical work already done on this issue by Tonkin & Taylor and the Council but I wanted to make this blog more reader friendly and hopefully more accessible to non technical interested parties. 

Before we get into the mechanics of sea level rise let us first quickly review what climate change is and some of the major drivers of climate change.


Climate change is not something new, the climate on the planet is always changing whether it's hourly changes caused by the sun’s azimuth, seasonal changes due to our elliptical orbit around the sun or the thousands to million year timescale of the Milankovitch cycles.

- Milankovitch Cycles

There are three cycles that exist in our solar system that influence our weather on Earth. The first of these is the Eccentricity cycle, the variation in our orbital ellipse. This cycle has a period of about 100,000 years. At its peak we are further from the sun so the amount of warming radiation reaching us is less. This difference can amount to as much as 30%. (Indiana University Bloomington, No Date)

Eccentricity zero
Eccentricity half
The second cycle is Obliquity (Axial Tilt). We may remember that the earth's rotation is not congruent to our poles but at an angle of 23.4 Degrees. This is how we get our Summer and Winter differences in temperature. As the axial tilt changes the differences in temperature increase or decrease accordingly. This cycle has a period of 41,000 years and ranges from 21 to 24.5 degrees.

Earth Obliquity Range 
The final cycle is the precession of the equinoxes. This is best envisioned as the wobble you see in a spinning top. The top spins about its axis, but as it slows it starts to wobble as it spins. The same thing is happening to the Earth as it rotates about the sun. This means that in some years our summer will be at the closest part of our orbit around the sun. So we would expect hotter summers and 11,500 years later it will be at the furthest part. So we would expect colder summers  This cycle has the shortest period of about 23,000 years.

Earth Precession
If you are still finding this hard to visualize, then you can watch a short youtube video explaining it here:

Here is an image of the joint American French satellite.  I wonder where they buy that cool gold tin foil?

Jason 2

 - The Rock Cycle and its Effect on Climate

Rock Cycle (Image Cusack, D)

Aside from the celestial-mechanical influences that are outside the earth system there are internal causes as well. You may have already heard about Carbon Dioxide, CO2. This gas causes warming because it absorbs heat from the sun in the atmosphere and re-emits it in all directions. The rock cycle is a long term carbon sink. CO2 is locked away in rock where it has little effect on temperature. But being a cycle that is only half the story, as rocks weather they release that COback into the atmosphere or oceans. This is the idealised and simplified equilibrium formula for the rock cycle.

CO2 + CaSiO3  CaCO3 + SiO2

CO2 is also locked away in the ocean.

CO2 + H2O -> H2CO3

H2CO3 is an acid; this acid weathers out other ions like calcium and sodium from its surroundings. These ions then react to make carbonates and CO2.

H2CO3 + H2O -> HCO3- + Ca2+ + Clay

Ca2+ + 2HCO3- -> CaCo3 + CO2 + H2O

CO2 is released back into the ocean.

We see that CO2 is locked away in limestone on the sea floor until it’s brought to the surface and starts to weather again thus releasing COback into the atmosphere. We can see that things that increase the speed of weathering will increase the speed at which this sequestered CO2 will be released back into the atmosphere. But fortunately, the dissolved CO2 in the oceans is at equilibrium with the CO2 in the atmosphere. So by removing CO2 from the ocean by limestone formation, it causes more atmospheric CO2 to be dissolved to maintain equilibrium. As shown by this simplified equation.

CO2 + H2O + Ca2+     CaCO3 + 2H+

Volcanoes and subduction zones release a lot of COand SOGas. SOis usually considered to have a cooling effect by increasing the Earth’s albedo. Although some scientists have put forward the idea that SOforms Sulphuric acid crystals that trap heat and thus lead to warming. (Notholt, J., et al., 2005) 

 - Antropogenic Climate Change

This refers to the human contribution to climate change. Some human activities contribute to global warming. Activities that cause the release of the gasses that are strongly associated with warming like CO2 (Carbon Dioxide), CH4 (Methane), and N2O (Nitrous Oxide). CO
is largely released through combustion and global concentrations of CO2 increased dramatically during the industrial revolution (1750's) and the invention of coal fired steam engines (circa 1712) and continues today with coal fired electricity generation.(Huntly Power Station is the only remaining example in New Zealand).  CH4  is a waste product of agricultural practices. Methane is a product of the breakdown of organic material.

Cow Farts (Image Cusack, D)

Nitrous Oxide (laughing Gas) is most abundantly created by fertiliser. Either through the breakdown of the fertiliser in the soils or through manure.

Antropogenic Sources of Nitrous Oxide (Image Cusack, D)

Now we have an idea of what causes the heating, let’s look at how the heat causes sea level to increase.

Steric Rise and Glacial Ablation, two drivers of sea level rise.

The two leading contributors to global sea level rise are the thermal expansion of the ocean  and melting of the water that’s trapped as ice on land. As, global atmospheric temperatures continue to increase in the long term, the ocean starts to warm as a result. Like all things that are heated the oceans expand. This thermal expansion (Steric rise) is one of the causes of seal level rise.  The sea level in the South Tasman Sea has been calculated to have a 0.3mm/year steric rise. Data around Australia was showing a warming trend, however, New Zealand’s water temperatures are influenced by a southern artic flow and a northern tropical flow. This is very different to what influences ocean temperatures around Australia. So no extrapolation is made. Meehl et al., 2007 suggests New Zealand’s sea level rise will be in accord with global averages.

How much will it rise?

The American space agency NASA and CNES, the French space agency launched a satellite in 1992 (TOPEX/Poseidon) to map ocean surface topography. This enabled very accurate sea level measurements. Several follow-up satellites have since replaced this ageing satellite. (Jason1, Jason2 and the future Jason3) Using the trends established over the twenty three years of data collection the sea level research group at the University of Colorado have shown a current rate of rise of 3.3 millimetres per year. However, some argue that it is much less (SteveF, 2011)

Image credit CU Sea Level Research Group University of Colorado

Notice the change in rate around 2003-2004. What caused that, is it a lower steric contribution? (SteveF)  And note the linear trend; there is no acceleration to doomsday in this graph. The IPCC, Intergovernmental Panel on Climate Change, has made some predictions that show how much sea level rise is increasing. They suggest that in the one hundred years from 1990 to 2090 sea levels will increase by 4 mm a year. Thermal expansion will drive this, but as temperatures increase more and more melt water will enter the ocean.

What would happen if all that water that’s currently locked away as ice melted? The U.S. National Snow and Ice Data Center predicts that if both Antarctica and Greenland, the world's largest ice sheets, both melted completely, the sea level would rise more than 70 meters. Let’s do some calculations using the IPCC numbers and leave out the Antarctic and Greenland Ice sheets for now.

The rate given is four millimeters per year:

Equation 1

Therefore for 10 years we get:

Equation 2

And in one hundred years we expect:

Equation 3

We can sea level currently is running up just under half a meter in a human lifetime based upon our current observations and historical trends. What does a half meter increase in sea level look like for Christchurch city? Using the university's LIDAR data that was collected after the Earthquake and shading it we can see the changes in height above sea level as a gradual shift in colour from the sea back towards the plains. We can also see the low gradient around the rivers (Avon and Heathcoat)

Christchurch Lidar Map Contour Shading

Here is the same map with the major streets and suburbs marked in.

Christchurch Basemap

Height Above Sea Level

In this picture I have highlighted three topographic contours. The black and grey areas are above 18 meters. The red shaded area is from 15.5 to 18 meters. The yellow shaded area is sea level to 15.5 meters Seas level is at 12.2 meters. The yellow area is at most 3.3 meters above sea level and at least sea level. At the current rate of sea level rise it would take about 825 years to flood all the yellow zone and 1450 years to flood the red zone. But if all that landlocked ice melts and sea level does rise by seventy meters then Canterbury is well and truly flooded.

One meter Contours

Now as homage to those macrame blankets my Grandma used to make here is a colour contoured image, where each colour represents an approximate one meter change in height. The roads, and rivers are highlighted so you can probably find your house on this map. Notice how the lower areas align with the rivers as you would expect. The large gunship grey area in St Albans / Richmond is 18 meters above the sea floor which is no higher than four meters above sea level.

400mm sea level rise

This image shows the new coast line for those of us lucky enough to still be alive in 2115. Assuming that the half meter rise established in Equation 3 is correct. You can see that the ephemeral Brighton Spit is an archipelago of small islands. In reality the river mouth would retreat and carve out large parts of Aranui and Burwood as it created new channels. This might allow the creation of a new bar as the long shore currents draw the river sediments southward.

One meter sea level rise coastal impact

In this image I have shown the new east coast shoreline at one meter. If sea level rise reaches a meter we probably lose Linwood, and Avonside and Woolston. Some of the basin areas might now be lower than sea level and start flooding. Halswell is already experiencing groundwater issues.  And here is the Armageddon scenario I know you all want to see of the full ice cap melt down

Ice sheets melt -70m Rise (Image D, Cusack)

The thin black line separates the seventy meter mark inland back towards the Southern Alps and the Lyttelton volcanic's sit high enough in many parts. Good news if you live in Oxford but we will need a new airport after this happens.

The West Antartic Shelf sits on the continent of Antarctica, it is sitting on land not in the water. This shelf averages 2 km thick and is over 5 km thick in some parts. Antarctica is 14 million square kilometers in size. (Nearly twice the size of Australia.) And ice covers approximately 97.5% of the continent.

What about the other effects of sea level rise? The fresh water aquifers’ base levels are determined by sea level so what would happen to them? If sea level does rise then base level will change. Septic systems could be flooded, and building foundations could be flooded as the fresh water ground table rises. Another major problem would be saline contamination of our fresh water aquifers as the base level rise brings more salty water inland. Since sea water is salty and more dense than fresh water it might be some time before we taste salty water from our drinking taps.

Other impacts might include an increased risk of liquefaction and a change in drainage patterns during storms and floods. There would be morphological changes to the river mouths of the Waimakariri, Ashley, Avon and Heathcoat rivers as their en-trained sediment is deposited. Beach dunes and bars will migrate as tidal patterns shift. King tides and major storms will cause damage further inland than their current reach. The 18th - 20th of July 2001 storm that caused extensive flooding along the Canterbury coast recorded a 1.41 meter swell above mean sea level at the Sumner beach tidal gauge (Cope 2001) . These damaging weather events will continue to occur reaching further inland with the rising shoreline. Compound these storms with the ongoing erosion of Canterbury's coast a rising sea level and the storm surges reach further inland.

"Excluding Banks Peninsula, 75% of the coastline of Canterbury is in a long-term erosional state. Rates of retreat have ranged from the extremely high 3.0 metres experienced in one year for the low beach ridge at Washdyke, to the moderately low rate of 0.2 metres per year for mudstone cliff locations in Hurunui District." Canterbury Regional Council (2005)

There is little doubt that sea level is changing in Canterbury. Records from  Port Lyttelton's tidal gauges, that began collecting data in the 1920's, show we are now 1.091 meters above the 1918 gauge zero.  A change in sea level will bring about many changes to the Christchurch landscape. Tonkin and Taylor prepared a report for the Christchurch city council to help the city plan for this. The report can be read here.

I also wondered what other things could accelerate the inundation. Specifically, I asked one of the Geologists at the university if Christchurch was sinking. Not as silly as it sounds, cities can sink for a variety of reasons. The remnants of the large volcano would have pushed down on the crust. Possibly causing an isostatic depression as it sunk. Subsidence also can cause sinking as the Quaternary sediments compact down and reduce the inter grain spaces. In his opinion Canterbury is not sinking and is relatively stable (Pettinga, 2015). So one less factor to consider in sea level rise.

As you can see in this map from Brown & Webber (1992), Canterbury's coastline has always been moving as the Southern Alps have eroded, now, maybe the coastline will erode faster as sea level rises.

(Image Brown & Webber 1992, adapted by Marney Brosnan)


Brown & Webber (1992) Geology of the Christchurch Urban Area 1:25 000. p104, GNS Science

Canterbury Regional Council (2005) Regional Coastal Environment Plan for the Canterbury Region

[Online] Available from: http://ecan.govt.nz/publications/Plans/RegionalCoastalEnvPlanNovember05.pdf [Accessed 7th September 2015].

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[Accessed 7th September 2015].

"Eccentricity zero" by NASA, Mysid - Vectorized by Mysid in Inkscape from NASA image at http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/Images/ecc_zero.gif.. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Eccentricity_zero.svg#/media/File:Eccentricity_zero.svg

"Eccentricity half" by NASA, Mysid - Vectorized by Mysid in Inkscape from NASA image at http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/Images/ecc_half.gif.. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Eccentricity_half.svg#/media/File:Eccentricity_half.svg

"Earth obliquity range" by NASA, Mysid - Vectorized by Mysid in Inkscape after NASA image http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/Images/obliquity.jpg.. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Earth_obliquity_range.svg#/media/File:Earth_obliquity_range.svg

"Earth precession" by NASA, Mysid - Vectorized by Mysid in Inkscape after a NASA Earth Observatory image in Milutin Milankovitch Precession.. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Earth_precession.svg#/media/File:Earth_precession.svg

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1 comment:

  1. Wicked awesome, Dale! So well communicated in an engaging and scientifically robust way that is accessible to a wide audience. Perfect!

    Love the original analysis balanced with light touches of humour. Also very well done.