Wednesday, 10 October 2012

Fingerprinting the ocean

Which is more likely to cause flooding in Europe; the Greenland ice cap or the west Antarctic ice sheet? They are both currently melting. And we can measure how fast, but it’s only been a recent development we have satellites that can resolve this, so it’s hard to draw conclusions on future melt rates from that. We might want to look at the past. And glaciologists have ways to find clues on how big ice sheets have been in times gone by (like they show here), but that information is often patchy. Sea level itself provides clues too. There are ways of telling where water that runs into the oceans has come from.

If you have an ice sheet, and a part of that ice sheet melts, several processes take place. The ice sheet becomes lighter and smaller, causing the Earth’s crust to bounce back up like a lilo, and the ice also lessens its gravitational pull on the sea water around it. The whole sea would in effect flow away from the shrinking ice sheet. So strangely enough, the most sea level rise you would find would be on the other side of the globe. Near the ice sheet, relative sea level would only fall.

Modelled results of what happens if 1mm sea level equivalent melts from the Greenland Ice Sheet: the resultant sea level change ranges from <0mm (blue) to>1.2mm (dark orange). From: Mitrovica, Tamisiea, Davis and Milne, Nature 409, 2001

So what if the Greenland ice sheet melts? That would be ~6m overall sea level rise, so that would be felt everywhere, but the southern hemisphere would be hit hardest (apart from the northern hemisphere having many more big cities in low-lying coastal areas). For Europeans, it’s the west Antarctic ice sheet that’s the main threat.

So how can that feature be used? If you want to know where past sea level rises originate from, you need to make reconstructions at a wide range of latitudes. The spatial pattern of where the rise is highest to where it even may be negative will tell you where the water involved came from. Simply speaking, the hemisphere where you find the smallest rise is the culprit. This process is called "fingerprinting"; this term has a rather chemical ring to it, but sea level scientists use it in a more spatial way. And if you can then find out under what circumstances it is which ice sheet that reacts, you may get an idea of what will happen in our future. And that is information of which it is quite imaginable it will be ignored by the relevant authorities, but at least everyone with access to scientific literature will have an idea of where not to buy a house...

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