As a sort of follow up to this post, more from the Coastal Engineering Manual on this subject. I was unaware of the “Committee on Engineering Implications of Changes in Relative Mean Sea Level (CCMSL),” but they published a report and you can find it here.
There are a couple of things that “lay people” should keep in mind when looking at reports of sea level rise:
- Sea level is relative to the land it meets with. When people say that “sea level is rising,” is the sea level rising? Or the land subsiding? Or both?
- “Sea level” is not a fixed datum, although it’s frequently presented with that kind of authority. As the report notes, “…mean sea level has been fluctuating through a range of not more than 40-150 cm (in long-term fluctuations) for at least 300 years,” and “…there has been a general, longterm rise (with short-term fluctuations) probably not exceeding 200 cm during the last 1,500 years.” It also notes that sea level can vary at different rates in different parts of the world.
(1) The apparent rise in worldwide sea level has been of great concern to the United States, as well as other countries, for several years. Much of this concern stems from the claims of some climatologists and oceanographers that the rise will accelerate in the future due to warming of the atmosphere associated with the “greenhouse effect,” a global warming produced by increased levels of carbon dioxide and other gasses in the atmosphere. Because of the potential consequences associated with sea level rise, a Committee onEngineering Implications of Changes in Relative Mean Sea Level (CCMSL) was formed to examine existing knowledge concerning sea level change, to document existing relative rise rates, and to provide recommendations concerning their conclusions.
(2) Relative mean sea level change can be defined as the difference between local changes in land elevation and global sea level changes. These changes result from a variety of processes, several of which can occur simultaneously. The following six processes can contribute to long-term relative mean sea level change; however, all processes do not necessarily apply to all geographic locations:
(a) Eustatic rise. Refers to a global change in the oceanic water level. Examples of eustatic rise include melting of land-based glaciers and the expansion of near-surface ocean water due to global ocean warming.
(b) Crustal subsidence or uplift from tectonic uplifting or downwarping of the earth’s crust. These changes can result from uplifting or cooling of coastal belts, sediment loading and consolidation, or subsidence due to volcanic eruption loading.
(c) Seismic subsidence. Caused by sudden and irregular incidence of earthquakes.
(d) Auto-subsidence. Due to compaction or consolidation of soft underlying sediments such as mud or peat.
(e) Climatic fluctuations. May also create changes in sea level; for example, surface changes produced by El Niño due to changes in the size and location of high pressure cells.
(3) The above processes have been evaluated with respect to their historical and potential contribution to sea level change on U.S. coasts. The Committee report assesses changes in sea level as well as the affected hydrodynamic processes and the effect on the coastal zone. The report also investigates feasible response strategies that could be used to mitigate the effects of sea level change. Although it is beyond the scope of this chapter to reproduce the contents of the report, conclusions relevant to this chapter are reproduced below.
(a) Relative mean sea level, on statistical average, is rising at the majority of tide gauge stations situated on continental coasts around the world. Relative mean sea level is generally falling near geological plate boundaries and in formerly glaciated areas such as Alaska, Canada, Scandinavia, and Scotland. Relative mean sea level is not rising in limited areas of the continental United States, including portions of the Pacific Coast.
(b) The contrasting signals concerning relative mean sea level behavior in different parts of the United States (and the world in general) are interpreted as due to differing rates of vertical motion of the land surfaces. Subsidence and glacial rebound are significant contributors to vertical land displacements.
(c) Large, short-term (2- to 7-year) fluctuations worldwide are related to meteorological phenomena, notably shifts in the mean jet-stream path and the El Niño-Southern Oscillation mechanisms, which lead to atmospheric pressure anomalies and temperature changes that may cause rise or fall of mean sea level by 15-30 cm over a few years.
(d) Studies of a very small number of tide gauge records dating more than 100 years (the oldest being Amsterdam, started in 1682) show that after removal of the subsidence factor where known, mean sea level has been fluctuating through a range of not more than 40-150 cm (in long-term fluctuations) for at least 300 years.
(e) The geological record over the last 6,000 years or so indicates that there has been a general, longterm rise (with short-term fluctuations) probably not exceeding 200 cm during the last 1,500 years.
(f) Monitoring of relative mean sea level behavior is at present inadequate for measuring the possible global result of future climate warming due to rising greenhouse gases.
(g) The risk of accelerated mean sea level rise is sufficiently established to warrant consideration in the planning and design of coastal facilities. Although there is substantial local variability and statistical uncertainty, average relative sea level over the past century appears to have risen about 30 cm relative to the east coast of the United States and 11 cm along the west coast, excluding Alaska, where glacial rebound has resulted in a lowering of relative sea level. Rates of relative sea level rise along the Gulf Coast are highly variable, ranging from a high of more than 100 cm/century in parts of the Mississippi delta plain to a low of less than 20 cm/century along Florida’s west coast.
(h) Accelerated sea level rise would clearly contribute toward a tendency for exacerbated beach erosion. However, in some areas, poor sand management practices or navigational modification at channel entrances has resulted in augmented erosion rates that are clearly much greater than would naturally occur. Thus, for some years into the future, sea level rise may play a secondary role in these areas.
(i) As noted previously, the two response options to sea level rise are stabilization and retreat. Retreat is most appropriate in areas with a low degree of development. Given that a “proper” choice exists for each location, selecting an incorrect response alternative could be unduly expensive.
(j) There does not now appear to be reason for emergency action regarding engineering structures to mitigate the effects of anticipated increases in future eustatic sea level rise. Sea level change during the design service life should be considered along with other factors, but it does not present such essentially new problems as to require new techniques of analysis. The effects of sea level rise can be accommodated during maintenance periods or upon redesign and replacement of most existing structures and facilities. There are very limited geographic areas where current subsidence rates may require near-term action as has been the case in Japan and Terminal Island, California.
(4) The above conclusions represent the state of knowledge on the subject of relative sea level change. For additional information, the reader is referred to the Committee report. It presents a complete and comprehensive investigation of the subject based on known facts and engineering and scientific principles.
(5) For the purposes of this report, the primary conclusion is that, with some regional exceptions, sea level is not rising at a rate to cause undue concern. Results of the report indicate an average sea level riseover the past century of approximately 30 cm/century on the east coast, and 11 cm/century on the west coast, and a range along the Gulf of Mexico coast of less than 20 cm/century along the west coast of Florida to more than 100 cm/century in parts of the Mississippi delta plain. The above summary remarks lead to the conclusion that normal design criteria should be followed in which the design life of a project should consider the possible local relative sea level rise rates shown above.