What makes sea level rise uneven

By Bryan Walker 23/03/2010

An illuminating article by Michael Lemonick just published in Yale Environment, which I summarise here, communicates some of the developing understanding of just how uneven sea level rise is likely to prove.  It will vary greatly by region. There are a number of reasons for this.  One is that the land is actually rising in some places, including northern Canada and Scandinavia, which are still recovering from the crushing weight of the Ice Age glaciers, albeit from 10,000 years ago. Their sea-level increases are less than the global average would suggest, since their land areas are rising a few millimeters a year.   On the other hand land around the periphery of where the glaciers sat, such as Chesapeake Bay and the south of England, was squeezed upwards by the downward pressure nearby and has been sinking back by a few millimetres a year ever since, so sea level rise is greater than average in these regions. Land is also subsiding in coastal places where massive oil and gas extraction has occurred such as Louisiana.

A larger effect is from changes in prevailing winds, which can push water consistently toward the land or keep it at bay. The trade winds that blow west across the tropical Pacific, for example, boost average sea levels by as much as 24 inches on the western side of the ocean – in places such as the Philippines – compared with those in northern South America. If those winds shift with climate change, so would local sea levels.

Ocean currents also affect sea level rise. If the Gulf Stream were to slow, for example, that would force water to pile up behind what amounts to a partial blockage of the overturning current. That could force sea level along the U.S. coast to rise another 8 or so inches over the next century beyond the global average, given a medium-emissions scenario.

But the ’gorilla in the room’ according to Ronald Stouffer, of the U.S. Geophysical Fluid Dynamics Laboratory in Princeton NJ, is gravitation. The extra gravitational attraction of an undersea mountain range pulls water toward it, creating a literal, permanent bump on the surface of the sea, while the deficit of gravity near an undersea valley creates a depression in the water up above. A coastal mountain range pulls the water in its direction, raising sea level nearby. So do the massive icecaps that smother Greenland and Antarctica. They keep sea level higher than it would otherwise be for thousands of kilometers around both land masses, and correspondingly lower elsewhere.

If the polar ice sheets shrink, though — as they’re currently doing, especially in Greenland and West Antarctica —  their gravitational pull weakens and so does their hold on the surrounding water. Their loss of mass not only contributes to overall sea level rise through meltwater but also allows some of the water held by their diminishing gravitational pull to go elsewhere — including the threatened east coast of the US.  And it’s not a small effect. In Hawaii, for example, Stouffer estimates that a seven metre sea level rise caused by the disappearance of the Greenland ice sheet would have an extra two or three metres added to it. Whereas a beachfront property in Iceland would end up with more beach.

Jerry Mitrovica, a Harvard geophysicist who is working with Stouffer, comments that when he gives talks about this people don’t believe him. He doesn’t blame them. ’It’s just wacky when you think about it, completely counterintuitive,’ he says. ’But it’s true.’

Mitrovica recalls that when he started looking at regional effects, some climate change deniers were noting that sea-level rise was happening at different rates in different regions, arguing that this proved there was no global trend, and thus no global warming. That was already a bogus argument, but now that he and others have begun investigating the gorilla in the living room, it’s even more absurd. The science is so straightforward, he says, that ’if you saw that sea level was rising uniformly around the world, it would be proof that the big ice sheets are not melting.’

One wonders what the Christchurch City Council might make of all this. They’ve settled for planning for a 50 cm rise. ’We’re following the Government’s advice and we’re not going out on a limb,” their spokesperson said primly.  Apart from the fact that 50 cms is now inadequate advice for the century, the dynamics of regional variation suggest that an already  complex set of considerations when planning for  future sea level rise may have to be open to even more complication. Nick Smith will surely have to descend from his high horse: “The Government is not going to consider adjusting its policy every week.”  Not that I’ve heard anyone asking for weekly adjustment — but annual reconsideration might be sensible.