RELATIVE SEA LEVEL

“Relative sea level” is a term used widely in coastal processes studies by geologists. The Glossary of Geology is the source to consult for its definition. The American Geological Institute has produced seven editions of the Glossary that separate into an initial series (1957, 1960) and a continuing series (1972, 1980, 1987, 1997, 2005). These Glossaries are valuable works. The seventh, and most recent, Glossary on paper is the 2005 Fifth Edition. “Relative sea level” first appears in that 2005 edition. It is absent from all prior editions.

Prefixing the adjective ‘relative’ to some nouns may make the term dimensionless. For examples, relative humidity, relative permeability, relative aperture, are all dimensionless terms after prefixing ‘relative’. ‘Relative’ also affects terms that can be ranked in order, such as ‘relative age’, ‘relative chronology’, ‘relative time’. A little more than half of relative terms become dimensionless when modified by ‘relative’, and the remainder is ranked entities. The number of dimensionless terms compared to the total ‘relative‘ terms in the seven editions are, in time order, 3 of 4, 3 of 4, 10 of 18, 11 of 18, 12 of 18, 13 of 23, and at least 13 of 26.

The 2005 Glossary defines relative sea level as follows: “The vertical position of global sea level with respect to an arbitrary scale, for example, extending from a low of 0 (position 30 million years ago) to a high of 1 (position 65 million years ago) (Vail et al., 1977, p.83).” This quote comes from a volume on Seismic Stratigraphy. Conversation with coastal specialists suggests that ‘relative sea level is not dimensionless, but is measured as a length in the vertical direction.

As used, ‘relative sea level’ often exaggerates sea level change. The term appears to represent sea level, but actually incorporates the much larger compaction. According to the most recent IPCC opinion on present sea level change, sea level is rising at not more than 2 mm per year, but local studies suggest that compaction on the Mississippi delta exceeds 10 mm per year. When combined, the two exaggerate sea level rise by a factor of five or six.

Nothing said above intends criticism of the AGI Glossaries. They are valuable aids to geological work. I purchased my copy of the first Glossary in July 1957, and I still have it.