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2018:  JAN | FEB

Sea Level Rise and Its Challenges
Dr. Peter Folger
Sea level change is a global, regional, and even local phenomenon. Globally, sea levels rose during the 20th century at an average rate of 1.7 mm per year, based on tide gauge records, with the rate increasing to 3.2 mm per year since 1992, according to satellite data. Warming oceans and melting glaciers were the dominant contributors to the global increase in the 20th century. Sea levels could continue to rise at roughly the same rates for another 100 years, but the rate of change may also increase dramatically. The wild card appears to be the Greenland and Antarctic ice sheets in a warming world. Some assessments project a global average sea level rise of at least 0.2 m, or possibly 2.0 m, by 2100. This order-of-magnitude uncertainty largely depends on how much ice melts from Greenland and Antarctica. The additional possibility of a sudden collapse of a large Antarctic ice sheet, though unlikely during the next century, adds even more uncertainty to the challenge of anticipating and mitigating the effects of sea level rise.

Regional sea levels can differ from these global rates due to phenomena such as winds, ocean currents, water temperature, salinity and climate patterns, such as the Pacific Decadal Oscillation. For example, the West Coast of the U.S. did not experience significant sea level rise between 1993 and 2012 because sea surface heights in the eastern Pacific Ocean decreased over that time period. At the same time, sea surface heights in portions of the western Pacific rose more than 10 mm per year.

Coastal communities dealing with global sea level rise also have to deal with shifts in the elevation of the ground beneath their feet. That global sea levels will continue to rise seems a pretty sure bet, but the amount of relative sea level (RSL) rise—the change in sea level relative to land surface elevation—often is the more immediate local issue. The trend and direction of RSL rise varies dramatically in the United States, although most coastal communities are experiencing rising relative sea levels. RSL is rising between 9 to 12 mm per year along Louisiana’s Mississippi Delta region around New Orleans, for example, but is dropping in parts of southern Alaska and the Pacific Northwest coastline. Why the difference? Natural forces are responsible in part. For example, land pressed down by the weight of continental glaciers during the last ice age is rebounding up (e.g., Alaska), whereas land that was not buried under ice (e.g., the Mid-Atlantic coastline), is tilting down, like a continent-sized see-saw. At Sewell’s Point, Virginia, RSL rise has averaged 4.4 mm per year since 1927, most of that due to the land sinking.

In many places, natural forces are not driving RSL as much as what humans are doing, sometimes at the local level: pumping groundwater, extracting oil and gas, and draining organic-rich agricultural lands. Sorting out the various causes that contribute to RSL rise is of first-order importance for coastal communities, as is dealing with their consequences: inundation of low-lying lands; increased erosion; shoreline change; increased coastal storm damage; impaired drainage; and saltwater intrusion of coastal freshwaters. How will those consequences affect economic development, safety and quality of life for coastal residents, as well as the health of coastal ecosystems and their societal benefits, such as commercial fisheries and opportunities for tourism and recreation?

Decision makers must choose when to protect, accommodate or retreat in the face of rising sea levels. In the United States, these decisions are not easily sorted out, in part because of an array of policy challenges at the federal, state and local level. Land-use decisions are largely the domain of local and state governments, as are requirements for coastal development (zoning, setbacks, building codes). The federal government, however, bears part of the financial burden associated with the consequences of coastal flooding through its disaster assistance programs and the National Flood Insurance Program. There is a natural tension between the federal government’s interest in managing disaster costs and the local, state and private roles in shaping coastal development, as well as the shared challenge of preserving ecosystem health.

Policy options might include: maintaining the status quo (no change to current programs and policies); addressing global sea level rise (e.g., greenhouse gas mitigation); reducing relative sea level rise (slowing/stopping land subsidence); improving coastal resilience (shore protection measures, building dunes, storm surge gates, protecting coastal habitats); and/or forestalling the consequences (restricting development, flood-proofing, buyouts of vulnerable properties, improved evacuation routes). Some of these options are not mutually exclusive; they may be taken in concert. For all policy options, assessing costs and benefits (and who bears the costs and who receives the benefits) are underlying questions.

Sea levels are rising, but broad questions challenge scientists, private decision makers, and local, state and federal governments. These include: how well understood are current and projected rates of sea level rise; what contributing factors represent natural variability, and which are influenced by human activities; how may these factors change; and what are the local, state and federal responsibilities for managing and mitigating activities that may exacerbate rates of relative sea level rise?

Future growth in coastal areas will likely be shaped by answers to these questions, and by private and public responses to mitigate risks of sea level rise.

Dr. Peter Folger specializes in energy and natural resource policy at the Congressional Research Service (CRS). He and his colleague Nicole T. Carter work on sea level rise and coastal policy considerations, among other issues. The views expressed here are those of the author and are not presented as those of CRS or the Library of Congress.

2018:  JAN | FEB

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