Sea Level Rise

Abstract

Sea level is the height of the sea surface expressed either in a geocentric reference frame (absolute sea level) or with respect to the moving Earth’s crust (relative sea level). Absolute sea level variations result from changes in the volume of water filling ocean basins (due either to water density or mass changes), while relative sea level variations designate sea surface height changes with respect to the ground (thus accounting both for “absolute” sea level changes and ground motions). Sea level variations spread over a very broad spectrum. On geological time scales, roughly 5–100 million years ago, 330-foot-amplitude sea level changes depend primarily on tectonics processes, such as large-scale changes in the shape of ocean basins associated with seafloor spreading and midocean ridge expansion, as well as on the existence (or not) of polar ice sheets. On a 10,000–100,000-year time scale, glacial/interglacial cycles driven by changes of the Earth’s orbit and obliquity also cause about 330-foot-amplitude sea level variations. On shorter time scales, 3.3-foot-amplitude sea level changes occur in response to natural climate-forcing factors (change in solar irradiance and volcanic eruptions). Humans also influence climate, and associated sea level change is noticeable since about 1900. Sea level is a very sensitive index of climate change and variability. For example, as the ocean warms in response to global warming, seawaters expand and thus sea level rises. When mountain glaciers melt in response to increasing air temperature, sea level rises because of fresh water mass input to the oceans. Similarly, ice mass loss from the ice sheets causes sea level rise. A corresponding increase of fresh water into the oceans changes water salinity; hence, seawater density as well as ocean circulation affects sea level and its spatial variability. Finally, modification of water storage on land in response to climate variability and direct anthropogenic forcing also causes sea level to vary on interannual to multidecadal time scales. Because of the multidisciplinary character of the sea level topic, as well as enormous progress made since the late 20th century owing to new in situ and space-observing systems, the literature on the subject is vast and continually increasing. For that reason, most of the works listed in this article were published after 2000, although a few older works are also mentioned.