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!