Rising temperatures cause sea levels to rise in two ways. A warmer sea is less dense, so its volume increases and the level rises as it expands. A warmer climate can also cause glaciers to melt into the sea, raising its level.
The term sea-level rise generally designates the average long-term global rise of the ocean surface measured from the centre of the earth (or more precisely, from the earth reference ellipsoid), as derived from satellite observations. Relative sea-level rise refers to long-term average sea-level rise relative to the local land level, as derived from coastal tide gauges.
Sea levels are highly variable over periods ranging from seconds to decades. Sea-level rise is the rising trend averaged over longer periods, which is observed at many coastal stations since a few centuries. It is almost certain that global warming due to human emissions of greenhouse gases is responsible for steepening this trend since at least a few decades. The most recent projections for future sea-level rise are presented in the Special IPCC Report on the Ocean and Cryosphere in a Changing Climate (2019). This report is an update of the previous IPCC AR5 report (2013), and includes newer insights in the response of the Greenland and Antarctic ice sheets to global warming. It also provides an estimation of the possible sea-level rise up to the year 2030, see Fig. 1. Two scenarios for greenhouse gas emissions are considered in this figure: (1) a “low” scenario, called RCP2.6, with strong reduction of global greenhouse gas emission, such that global warming will probably not exceed 2 oC; (2) a “high” scenario, called RCP8.5, in which no measures are taken to limit greenhouse gas emissions (‘business as usual’). The high scenario can lead to a rise of up to 5 m of the global average sea level in 2300, but with great uncertainty.
Several phenomena contribute to sea-level rise. On a global scale, sea-level rise is mainly due to an increase of the water mass and water volume of the oceans. This global sea-level rise (often termed Eustatic sea-level rise) has three components:
(1) thermal expansion of ocean waters related to decrease of the density (also referred to as thermo-steric component of sea-level rise, related to increasing temperature),
(2) water mass increase, which is mainly due to melting of mountain glaciers and decrease of the Greenland and Antarctic ice sheets, and
(3) decreasing storage of surface water and groundwater on land.
Other phenomena can substantially influence sea levels at regional scale, inducing either sea-level rise or sea-level fall. Most important are:
(4) vertical earth crust motions – in particular earth crust adjustment to melting of polar ice caps, the so-called isostatic rebound,
(5) land surface subsidence, related in particular to extraction of groundwater and oil/gas mining and compaction of soft deltaic soils,
(6) changes in the earth gravitational field, related in particular to decrease of the Greenland and Antarctic ice sheets,
(7) regional atmospheric pressure anomalies and changes in the strength and distribution of ocean currents, related in particular to ocean-atmosphere interaction, and
(8) regional sea-level change related to changes in seawater salinity.
Due to these phenomena, sea-level rise is not uniform around the globe, but differs from place to place. Relative sea-level rise is the locally observed rise of the average sea level with respect to the land level. It is the sum of the components (1-8).
Picture Credit : Google