My Research: Flic Williams

Hello Hello!

I’m Felicity Williams and I study how sea level changes when the amount of ice on land either grows or melts.

It is very tempting to think of our earth as one large bath tub in which the water level goes up and down uniformly across the entire surface. The real world is far more interesting!

Every location around the world experiences a different sea level for the same amount of water being added to or taken away from the oceans.

Figure 1: Mountainside scoured by a retreating glacier –the Moiry Glacier – which can be seen to the left of the image.

Figure 2: A view down the valley scoured by the Moiry Glacier. Here you can see the edge moraines formed by the ice when it surged down the valley.

This is because when ice grows it squishes down the land underneath it. This causes sea level to effectively rise in that location even though ice is building up on land and so taking water out of the ocean. Conversely when the ice melts, the land springs back and sea level falls, even though more water is being added to the ocean. This process is called Glacial Isostatic Adjustment – and we feel it even today in the British Isles as a result of the great British and Irish Ice Sheet that was centered over Scotland around twenty thousand years ago.

Figure 3: Beautiful corals on the Great Barrier Reef in Australia. We can reconstruct past sea levels using fossil corals that were once alive.

We have some clues as to how sea level changed in the past gained from evidence like fossil corals, and we have some clues as to how much ground the ice sheets covered in past times as the ice sheets can push huge mounds of earth and rubble in front of them as they advance – these are known as moraines.

My job is to pull these clues together, so that we get a better idea of just how variable sea level was in past times – with the intention of applying it to the present day. If we know how fast sea level changed in the past, we have a better idea as to just how variable sea level could be in the future.

Flic.

roger fentem says:

October 27, 2014 at 9:59 pm

I have just come back from a short break on the isles of scilly. There, it is said, that the islands are still sinking as a result of the melting of the ice sheet you refered to … Is this right? Have you studied the drowning of scilly? Do you at what rate the isles are disappearing? When will the first of the currently inhabited islands notice the rising sea level? …
You don’t have to answer these questions, they gnaw at me ….

flic says:

November 2, 2014 at 7:53 pm

Hi Roger,

Apologies for taking a while to get back to you – the Isles of Scilly aren’t a particular focus of my research (though they are beautiful – I have enjoyed a holiday there myself!) so I wanted to hunt out a few references.

Your query covers two broad sections. The first is often referred to as “eustatic” sea level rise – by which we mean, something is happening that affects how much total water is in the ocean, and the absolute size of the ocean itself. So in general, land based ice melt or growth, or the widening of the ocean basin due to plate tectonics, are things which change this “eustatic” sea level. It may help to think of this as a global mean sea level. This global mean sea level has been changing on a timescale of about 100,000 years, for about the past 700,000 years. The second aspect of your question relates to Glacial Isostatic Adjustment (GIA), which tells us more about how sea level will change in a particular location. The glacial isostatic response doesn’t happen all at once; the earth continues to respond to a past loading by an ice sheet, for thousands of years.

So, in investigating sea level change in the Isles of Scilly we have to consider two things – how is it responding to past loading by the British and Irish Ice sheet, and how will it respond to changing sea levels in general. In terms of the GIA response, you don’t need to worry – Scilly isn’t about to drown! At present, Scilly is “sinking” at a rate of about 0.5 millimeters per year (Have a look at the second page of this document for a nice graphic of the UK response rates to the British and Irish ice sheet at the last glacial maximum: http://www.geosociety.org/gsatoday/archive/19/9/pdf/i1052-5173-19-9-52.pdf). We could make a linear projection and say that, given St Mary’s has a maximum elevation of approx. 50m it will take 100,000 years to disappear. But (and this is a very important one!) – this rate is absolutely not a constant or linear rate, so we must not make these calculations. What we are seeing and measuring with these rates is a system that is trying to return to an equilibrium position – think about memory foam – it deforms, but slowly and surely returns to it’s original position, and then stops deforming. I couldn’t say that Scilly has an “original” position, as our Earth is so dynamic, but I hope you agree that 0.5 mm per year is a very small change.

Global mean sea level change is another, huge, topic – so I hope you don’t mind me focusing in on the glacial isostatic adjustment portion in this reply.

I hope that helps with the gnawing questions!

My Research: Flic Williams

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