Climate Change in the Arctic - The Changing Arctic Environment: The Arctic Messenger - page 277

10 times faster than the sea level rise we actually observe today. At present, the long-term
commitment is growing at about 30.5 cm (12 inches) per decade. According to Levermann,
even the much touted but elusive target of restricting warming to 2°C still implies a long-
term commitment for 4.8 m of global mean sea level rise.
If the concept of commitment to future sea level rise is making you feel uneasy about
the future, there is more unsettling news to come. I want to return to my earlier prom-
ise concerning global variability in sea level. Imagine you are a city engineer or govern-
ment policymaker entrusted with ensuring the long-term safety of people, infrastructure
and property in low-lying coastal areas. You will need to know more than just projections
for global mean sea level. You will be asking if your geographical region is one that is ex-
pectedtoconformwiththeglobalmeanorisaregionthatcanexpectgreatersealevelsthan
the mean. In the case of the latter, you will need to know the size of the expected deviation
over a certain time frame associated with a given ice sheet melting scenario.
It is a tall order, but estimations of geographic variability in global sea level rise (and
of its causes) have been carried out and they are quite the eye-opener.
The following will be a very brief summary of one of the causes of variability. For
further reading, I recommend the two references by Tamisiea and colleagues ( 2011 ) and
Mitrovica and colleagues ( 2009 ). To make these estimates, scientists cannot just quantify
the familiar elements responsible for the global mean (such as the thermal expansion of
water and the output from identified glacial melt sources). Now they must also estimate
the causes of geographic variability. The easiest source of variability to understand and to
measure concerns vertical movement of Earth's crust along the coasts caused by plate tec-
tonics or isostatic adjustment following glaciation or de-glaciation. The big surprise (for
me at least) was to learn about the possible influence of the gravitational pull of the Green-
landandWestAntarctic icesheetsonregionalsealevel.Theycauseagravitational tidethat
pulls water towards them, thus creating a regional elevation of sea level. I say “regional”
because the hinge line, which separates sea level elevation to sea level depression is be-
lieved to be about 2,000 kilometres from Greenland. Therefore, Greenland's gravitational
influence is estimated by Mitrovica and colleagues to reach as far as Scotland. In a lecture