Event

Sea level is the relation between the elevations of land surface and the sea surface. However, both change independently so an appropriate frame of reference is necessary. Relative sea level is simply the elevation of the sea surface with reference to any particular spot on the shoreline. It changes differently depending on where you are because epeirogenic (vertical) motions of the land are spatially and temporally variable. A more universal concept is eustatic sea level, which reflects the relative volumes of the entire planet’s ocean water and the volume of the ocean basins. A reference frame for this is more problematic, but now feasible. At long time scales during non-glacial times, tectonics (primarily age of ocean floor) controls eustasy. At the short time scales of greatest interest to coastal infrastructure and ecosystems, ocean water volume controls eustasy. During recent glacial times (Pleistocene) it was driven by glacial cycles at the 100 Kyr time scale. However, during the industrial era (post-1850) it has been driven by anthropogenic climate change. Modern drivers of sea level rise include the melting of land-based glaciers (primarily Alpine and Greenland) as well as the thermal expansion of ocean water itself due to warming. During the 20th century, however, we “hid” the true rate of sea level rise by continuously building dams and filling new reservoirs. Now that we have stopped building major dams (and in fact are removing some), the rate of sea level rise has sharply increased from 1.5-1.8 mm/yr to over 3 mm/yr, or doubled over the span of a decade or two. The rate of sea level rise is expected to continue increasing, even if greenhouse gas emissions are sharply reduced, because the earth system has not yet equilibrated with the perturbations imposed during the 20th century.