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...
