Fundamental questions on Isostasy and Mean Sea-Level

Guest post by Andi Cockroft

Firstly, let me declare that I am not a current student of Geology or Geophysics. I studied Mathematics, Computer Science and Geology at University way back in the 60’s and I have focused my entire working life in IT. To this day, I still run a small IT company in New Zealand.

I follow the workings of the climate models with interest, since much of my University life was spent on Numerical Analysis and Stochastical Processing – the pre-cursors to today’s modelling. Although back then, Computer power was measured in kilo-flops, and the first mainframe I worked on was 20K. A 1Mb mainframe was but a pipe-dream – our biggest was 128K.

On the other hand, my groundings in Geology were at a time when my Professor was still agnostic about plate tectonics (although my tutor was an avid proponent thankfully). It was a time when magnetic striping of the mid-Atlantic was known but not totally understood. It was also a time when isostacy (although an old theory) was finding new support.

So, I offer this post not as any particular expert in the field, but rather to stimulate discussion from those far more familiarity with the concepts than I – please feel free to correct my impressions and limited understanding of the various topics

All I am proposing here are some of the fundamental questions that bother me regarding Isostacy, melting or re-freezing ice-caps and the effect on global mean sea-level (GMSL).

Firstly, just what is GMSL? I don’t believe it can ever be measured against one specific location on the globe – for just about everywhere on this planet is in motion to some degree or other – north/south, east-west, up/down – although our impression is of a stable terra-firma, geologically speaking it isn’t – it is more like the skin of a rice-pudding floating on the more fluid layers below.

If you ever played with your rice-pudding as a child, let the skin form and then pressed on it with your spoon? Press on one place and the skin will rise in another. So the Earth responds to increasing or decreasing pressures (or mass) on its crust.

The earth’s crust (or lithosphere) “floats” on the more fluid asthenosphere. The thickness and relative buoyancy of that part of the lithosphere will determine just how high or low it will “float”. When the buoyancy is in balance, it is said to be in isostatic equilibrium. There are many places that still today are not in isostatic equilibrium.

In terms of mountain building, where for instance the Indian plate is hurtling northwards into the Eurasian plate giving rise to the Himalaya Mountains – the gravity measurements tell us that just like an iceberg, there is just as much if not more “Everest” below the crust as there is above it – the buoyancy of that extra material below is needed to support the weight of the Mountain Range above.

Imagine ice floating in a bath – add more weight on top and the ice will sink such that the Archimedes principle of displacement is yet again in equilibrium. Indeed, add too much weight and the whole iceberg will sink regardless – but that’s a diversion (think 2Km thick glacier!)

So back to the question – what is GMSL? To my way of understanding, it can only be measured against a theoretical spot, and must in this age of satellite and gps be measured relative to a reference datum and/or the calculated centre of the earth. Since the earth is not a true sphere – it is I believe referred to as an “oblate spheroid” – the averaging out of these measurements must be translated back into local datum to be able to measure against tide-gauges etc.

Then again, I have always maintained that Sea-Level is a local phenomenon – for many reasons.

Tides, temperature, winds, air-pressure as well as such phenomena such as ENSO etc

But given that statements implying GMSL was 300’ lower than today during the last ice-age, I ask the question – “but what was the real or relative sea-level back then in say New York State?” – in places under Kilometres of ice apparently. Certainly what is now the Great Lakes was inundated by huge ice flows.

That sheer weight of ice would have had the effect of pushing the underlying land-mass downward, to some extent counteracting the drop in GMSL – in fact (just in my imagination) – I believe that these areas would have been significantly below GMSL even with its 300’ decline.

Sorry I do not have the knowledge to do the calculation, but I hope here that group-science can answer the question.

There have been statements that New York was about 150 – 200 Kilometres inland during the last Ice Age 20,000 years ago – but would it have been? Wouldn’t Isostacy have taken the land-mass down with it?

One of the problems faced by modern day measurements of GMSL, is that Isostacy is still at work.

Remove the massive ice-flows from ice-age North America, and the continental land mass does not rebound instantly – it rebounds (uplifts) slowly – it is still rebounding today. As things equal themselves out over Geologic time, the land masses continue to rise, and as a counter, the ocean basins are actually falling. In other words, the oceans themselves gain a greater capacity to store water.

This expansion needs some accounting for, and this Global Isostacy Adjustment (GIA) is estimated at about the equivalent of 0.3 mm per year in GMSL, this at least partially offsets the predicted 3mm per year expansion of the oceans due to warming (that seems to have paused just now). Indeed, if thermal expansion of the oceans has paused, GIA will continue unabated and actually serve to lower observed GMSL.

All of this is a very roundabout way of me arguing that Sea Level is a local phenomenon, and calculating a GMSL whilst academically interesting serves no actual use. Certainly not in terms of Government and Policy decision-making.

In England, long before Liverpool was ever heard of, the River Dee provided the main shipping for the North West. First Chester (used by the Romans), then further out to Burton and Neston and ultimately Parkgate were the main points of embarkation for Ireland. Visit Parkgate today, and the sea is many hundreds of metres away with a huge salt marsh in between.

In New Zealand, where I have lived this past quarter-century, there are many places where erosion is eating into the coast and houses are inexorably marching towards oblivion. In others, accretion of up to 1 metre per year is extending the land-mass ever seaward.

Yet Local Government in these areas is focused almost entirely on projected changes to GMSL rather than what is physically happening.

The local authority here is planning on Climate Change bringing inundation despite all the evidence that the sea is retreating !!!

However, I digress. Looking further at Geologic Changes, and back to Isostacy for a moment, where two tectonic plates collide, one usually will ride over whilst the other suffers subduction. This is where we will find seismically active areas (sadly Christchurch here and many more such as Japan, China etc). We will also likely find mountain-building – as mentioned earlier, the Indian plate is still moving northward (150mm or 6” pa), and the Himalayas are still rising (5mm or 0.2” pa) – in geological terms still very active.

At the same time, the subduction zone may well create significant volcanic activity – New Zealand has its fair share.

But just about anywhere plates collide will see some degree of uplift. Again using New Zealand as an example, the main mountain range in the South Island is the Southern Alps. Current measurements show an uplift rate of about 7mm per year.

How this translates into vertical movement at either east or west coast I have been unable to determine, but it has to be happening, and ongoing.

In Wellington, the biggest recorded earthquake was a magnitude 8.2 in 1855. This uplifted the east of the City between 2 and 3 metres. Today’s airport is built on this uplifted land, and the current business district sits on reclaimed marshland exposed at that time.

Wandering along Wellington’s Lambton Quay (where boats used to dock), reveals brass plaques set into the pavement showing the shoreline in 1840 – the current Quay is about 300 metres away.

Given the magnitude of that earthquake, I suspect Wellington’s rate of uplift over larger time-intervals will be outpacing GMSL, but I could be wrong. By thesis, Cole (2010) identifies a shift in vertical position around 1944 – this following two large earthquakes in the preceding years.

There is nothing to suggest that Wellington’s upward thrust (to the east of the Wellington Fault) has ceased, so the next “big one” will likely see further uplift and hence a relative drop in Sea Level.

Local effects of Plate collisions, Isostacy rebound (eg Great lakes) etc., all serve to modify the impact of GMSL .

If we are to believe recent reports, some of the small Island Nations such as the Maldives may in fact be growing rather than being swamped under an ever-rising tide. Explain that one – I understand coral growth may be at work here but I’ve not seen anything truly convincing yet.

In other places, we are seeing true inundation but usually accompanied by more rational explanation such as pumping ground water.

So is the observed increase in GMSL of 3mm pa actually causing any significantly observable effects?

I remember the Thames Barrier being built to protect London from inundation, but that was to protect against a storm surge – where a massive low pressure system in the North Sea causes a correspondingly massive increase in local Sea Level. This is of the order of metres, not 300mm per century.

And one final question. As Cole (2010) discovered, atmospheric pressure has been falling over time.

Ill-equipped as I am to perform the calculations, is this drop in barometric pressure yet one more factor affecting the calculation for GMSL?

As I said at the start, I do not have the skills necessary to answer these and many other questions posed above – just taking a leaf out of “justthefacts” methods – and asking from you the pundits to engage and answer via group-science. Be kind for I am not as used to this medium as Anthony and other major players.