Sea Level: Earth’s Dominant Elevation—Implications for Duration and Magnitudes of Sea Level Variations

High-resolution digital elevation models of Earth’s solid-fluid interface reveal that Earth’s bimodal elevation distribution is characterized by a dominant mode at or very close to present-day sea level. It is hypothesized that sea level is Earth’s dominant modal elevation because it is the only elevation where two opposing first-order surface processes—erosion and deposition—act to increase the area of a single elevation. The observation that sea level is the dominant elevation implies that (1) the time needed for the modal elevation to adjust is very short and/or (2) long-term (∼106–107 yr) mean sea level has not varied significantly from its present height. The mass of material that defines the sea level modal elevation is quite small, whether integrated over 1 or 100 m of thickness, and could be transported in <104 yr. This would suggest that the shoreline, at no matter what height relative to the present 0-m elevation, was probably always Earth’s dominant elevation. However, Eurasia, North America, and Australia have dominant elevations near sea level, and their entire above-sea-level hypsometries appear to be graded to present-day sea level. In contrast, Africa and South America have dominant elevations significantly higher than sea level and appear to be graded to elevations significantly higher than sea level. The erosional adjustment time scale of continental hypsometries is >107 yr. This would suggest that Eurasia, North America, and Australia have been graded to a height of sea level close to present-day 0 m on a time scale of at least one to a few times 107 yr. Reconstruction of a new global sea level curve for the past 150 m.yr. using areas of continental flooding as a function of time suggest that long-term sea level has been at or close to present-day sea level on a time scale of 40 m.yr. and was not more than ∼150 m higher at any time in the past 150 m.yr. Given 106–107-yr time scales and modest amplitudes of sea level variability, it is perhaps not surprising that most continents are characterized by modern sea level as their dominant modal elevation.