Earth's hypsometry and what it tells us about global sea level

Over geological time scales, the combination of solid-Earth deformation and climate-dependent surface processes have resulted in a distinct hypsometry (distribution of surface area with elevation) on Earth, with the highest concentration of surface area focused near the present-day sea surface. However, in addition to a single, well-defined maximum at the present-day sea surface, Earth's hypsometry is also characterized by a prominent maximum ∼2–5 m above this level, with the range accounting for uncertainties in recent digital elevation models. Here we explore the nature of this enigmatic maximum and examine, using a gravitationally self-consistent model of ice-age sea-level change, how it evolved over the last glacial cycle and may evolve moving towards a near-ice-free future. We argue that the hypsometric maximum captures topographic conditions at the end of the last deglaciation phase and subsequent glacial isostatic adjustment (GIA) raised it from the sea surface to its present-day elevation. Moreover, ongoing GIA will raise the maximum a further ∼2 m in the absence of future ice mass loss. If a portion of the hypsometric maximum has persisted for longer than Holocene time scales, the resulting GIA-converged elevation of the hypsometric maximum at +4–7 m above the sea surface implies a longer-term mean state of the Earth that may reflect lower ice volumes, trends in erosion, dynamic topography, or a combination of these. The signature of these various contributions on present-day hypsometry is intimately connected to the time scale of erosional and depositional processes near shorelines.