Imagine for a moment that Joe Romm and his Warmy Screamers are right. Sea levels in 2111 will be two metres higher than they are now, and he can prove it. I'm standing on the beach in 2011, looking out to sea, pondering. What to do, what to do?
Consider the evolution of a coastal city over a century. The average useful life of a building or structure is seldom more than a century. A commercial structure may have a design life of 30 to 50 years. Even a reinforced concrete bridge or a steel-piled harbour wharf will be pretty dilapidated after a hundred years. Roads and railways are upgraded, rerouted, changed, even abandoned. Airports and harbours grow and shrink as time goes by and user patterns change. Technologies emerge, change, die. In 1911 there were no container ports or airports, and all railways ran on steam. What now-undreamed-off ways of living and moving will have arisen by 2111? A bank or a mansion or a tenement may seem solid and unchanging to the casual observer, but at the century timescale it's merely an anthill.
Now, we understand pretty well, since we create and direct them, the social and economic forces and processes that affect the development of the urban space. Planning and zoning; property taxes; the provision of infrastructure and services; economic forces of the market ; these are the influences that determine whether a particular piece of land supports a hotel or a prison, a bank or a parking lot, a funfair or a supermarket; or lies empty. A century from now, almost the entirety of the urban landscape will have been rebuilt and replenished, perhaps several times. Huge amounts of capital are continually invested, recovered through profits and taxation, and re-invested in the process of constant organic renewal.
All we need to do to stay ahead of even a two-metre-per-century rate of sea-level rise is to use these tools to direct new developments uphill beyond the flood level. Uncontroversial rules exist in many countries to prevent new development in flood-plains where the risk of catastrophe is high. Planning blight is a well-known phenomenon; urban corridors for infrastructure projects are commonly kept clear of development for many years before construction begins. A little targeted planning blight is all that's needed to mitigate the effects of sea level rise. The trick, simply, is to allow existing structures in the danger zone to age past their design lives, and to interdict their replacement in situ.
So let's pencil in the rough lines of a land-use and planning policy. Legislate the rules, so that everybody knows, long-term, how they will apply. The idea is to influence, with as light a touch as possible, investment and government-infrastructure decisions in order to move new development up the hill and out of danger. Simple.
Start by designating zones a metre deep (50 years' worth of sea-level rise at our assumed pessimistic-case rate) by height above current high tide level. Let's label them OneMinus, TwoMinus, ThreeMinus, ThreePlus. In zone OneMinus (less than one metre above current high tide level) the rules will be:
Y+10 (that is, ten years from now): No new major capital work will be approved. That allows currently-planned schemes to go ahead without disruption, and they should have between 10 and 35 years of useful life in front of them.
Y+35: Municipal services no longer guaranteed - water, sewerage, gas, elec, street-lighting, telecom will be maintained year-on-year only as long as physically possible, and as long as property taxes from the affected areas pay for them. Private utilities to be released from obligation to supply. Restrictions and obligations on building insurance abolished. In essence, you can continue to occupy the land, but entirely at your own risk and expense. Coastal defences will no longer be maintained or approved.
Y+50: By this time, the tide will be coming right in, so: no new buildings, structures or infrastructure permitted within the zone, with the exception of those solely to the benefit of ThreePlus districts, say a new harbour bridge or a heightened quay. Prohibit or super-tax insurance to help drive out occupiers. Make it clear that post Y+60, no public aid or compensation of any sort will be available to property holders.
Now, offset the same rules by 40 years (to accommodate a metre or so's sea level rise, and apply them to zone TwoMinus, so that anybody buying property there is aware that no new building projects will be permitted past Y+50 (2061), and that they'll be off the grid in Y+75. 75 years is plenty of time for a property investment to mature and pay for itself, so nobody who puts up money now in zone TwoMinus should have any cause to complain. We're getting a long way out on the horizon now; but do the same, with another 40-year offset for zone ThreeMinus. 90 years on, you won't be allowed any new buildings. Invest accordingly.
Taking this approach is, I think, pretty flexible, as the zones and the "Y+" cutoffs can be adjusted depending on how sea level actually pans out. Property owners are given certainty about where they stand, and can take decisions accordingly. No hugely expensive coastal defence and relocation work needs to be covered by taxpayers. The city naturally and organicly draws back from the rising sea, with minimum fuss and expense. Life goes on.