Over the summer, North Carolina legislators ended up looking very stupid. They passed a law stating that sea level rise in their little corner of the world will only be linear, as extrapolated from historic 20th Century trends. You can read the actual wording here (PDF, Section 2, Part E). This is all the more crazy because there is published evidence that North Carolina is actually in a sea level rise hotspot. This example probably has more to do with the politics of costal development than actual science (one hopes), but it does highlight some very obvious flaws in the understanding of sea level change in general.
The ocean is dynamic, any surfer can tell you that. Tides come in, go out, currents stream, the wind can drive immense waves. Beneath these already complex day-to-day motions of the ocean is another world of complexity. In 2007 the IPCC summary suggested an average sea level rise of 3.5 mm/yr over the next century is likely. The figure is widely quoted in the popular press and most non-expert readers will have been left with the impression that the sea level change where they live would be 3.5 mm/yr. This is badly wrong. The misunderstanding comes from the fact that the 3.5 mm/yr is a global average value. Average is a useful statistical construct, not necessarily representing physical reality. Consider a room with 4 people in it: Emma is 1.74 m tall, Dave 1.80 m, Sam 1.67m and Sarah 1.79. The (mean) average height of people in the room is 1.75 m, but given this information no one would walk into the room and expect everyone to be 1.75 m tall. Indeed, in this toy example, no one is of ‘average’ height. This is so fundamental that it sounds like I am insulting your intelligence, but this is exactly what many supposedly intelligent people have done with sea level data. In practice, some places will get around 3.5 mm/yr rise but other places will get significantly more or less. This figure is also purely a rise in the ocean surface; no vertical motions of the land surface are included.
The sea level change that matters to us humans in a: “where do I build my house?” sense is known as relative sea level. This is what defines how a shoreline migrates over time. It is a function of ocean surface height and land surface height. If global sea level was static but you live in a region that is subsiding, you would experience (relative) sea level rise. If sea level was rising at 5 mm/yr but your region was uplifting at 6 mm/yr, you would experience 1mm/yr sea level fall. What sea level change you experience depends very much on where you are on Earth.
Some factors that affect local sea level are: sediment deposition, ongoing uplift of formerly glaciated regions and long term ocean surface dynamics. The Mississippi delta is a region of sediment deposition and is therefore subsiding, increasing the local rate of sea level rise above global averages. In Greenland, if the ice cap is reduced, the reduced mass of ice on the land causes local uplift. Ice caps are also large enough to have a significant gravitational effect, pulling ocean water towards them. Remove the ice cap and you remove this effect, resulting in further sea level falls. As a final example, long lived ocean currents tend to ‘pile’ water up where they meet the coast, shifts in these currents in response to a changing climate can change the location of these piles. Also consider that these processes don’t even include the issue of acceleration in the rate of sea level change. This is a very real possibility, given the apparent accelerating melting of the Greenland ice cap.
Clearly there are many aspects, which control experienced sea level change. Even using accurate global averages to make local policy is doomed to failure. It is said that all politics are local; the same is true for sea level change. Concerning North Carolina, many commentators have pointed out that a lesson from King Canute would be in order. I am inclined to agree.