Articles have appeared in the media recently quoting different studies on projected sea level rise. Some have said a 20 ft. (6m) rise will take place. Another predicts a 250 ft. (76m) increase. Those projections are for sea level rise hundreds of years from now. Either one would put all coastal cities under water.
In the near term, the UN Intergovernmental Panel on Climate Change (IPCC) predicts a sea level rise of up to 1 meter (3.3 ft.) by 2100, based on the current rate of global warming increase, thermal seawater expansion, and the melt rate of the Greenland and Antarctica ice sheets.
However, a new paper published the week of July 20, 2015, in the European Geosciences Union journal projects a sea level rise of up to 10 ft. (3m) by 2100. The study authored by James Hansen, former head of NASA’s Goddard Climate Center and now a professor at Columbia University’s Earth Institute, and 16 co-authors, bases its projections on evidence that the Antarctic ice sheet is melting 10 times faster than thought, due to warming ocean waters and increases in carbon emissions. Not all climate scientists agree with the new study, but most are taking it seriously.
But even the more modest UN projection puts coastal communities at risk. A 1m (3.3 ft.) sea level rise would flood much of Miami and a large part of lower Manhattan, driving millions of people from their homes and causing trillions of dollars in property loss. The UN panel also predicts that tropical storms such as Hurricane Katrina and Superstorm Sandy will be more common and more intense. If you add the threat of storm surge to the projected sea level rise, many coastal communities will be in dire flood danger by the end of the century.
That is, unless steps are taken to protect those communities against the rising sea by building seawalls or dike systems. The question is would building such mitigation systems cost more than projected property loss if nothing were done?
The answer may well be found in a paper published in the February 3, 2014, edition of Proceedings of the National Academy of Sciences. According to this study conducted by the Global Climate Forum, the cost of property loss if nothing is done could be as high as $100 trillion worldwide. On the other hand, if dike systems are built in exposed locations, the cost of flooding could drop to around $80 billion.
If the decision is made to protect coastal communities with dikes or seawalls, several questions arise. First, where does the money come from? The 2015 budget for the US Corps of Engineers allocates only $28 million for flood control and coastal emergencies, which won’t go far when construction will run into the tens of billions. The funds required would have to be either voted by Congress or provided by local jurisdictions by raising taxes or issuing bonds. It will probably take several disasters before either is likely to happen.
If funding is provided, the next decision is whether to build hard seawalls of reinforced concrete, or use the so-called soft system preferred by the Dutch. Hard seawalls are usually erected at the water’s edge to protect existing structures that have been built close to the high tide line. Drawbacks are high cost, degradation of beaches, and a landscape eyesore. Also, even the highest seawalls can be overtopped by a strong storm surge or major tsunami.
The Dutch “soft system” utilizes a wide beach with dunes as the first barrier, then a belt of woodland, and finally a wall of low dikes built of sand, clay, and a straw binder. Wetlands and drainage canals are also used to handle excess water. The eroding beach sand is constantly replenished with a device called a sand engine. The Netherlands system requires more land and mandates that structures be built much farther back from the tide line than in the US.
Whether we see a 1 meter or a 10 meter sea level rise, and whether concrete seawalls or earthen dikes are used, it appears that some form of protection against rising and stormy seas is in our future.