Sea-Level Change and Vanishing Coastlines

Global mean sea level is not a fixed reference but a slowly moving target, and the Intergovernmental Panel on Climate Change's Sixth Assessment Report, published in 2021, found that it rose by approximately 0.20 meters between 1901 and 2018, with the rate of rise itself accelerating markedly over that period: from roughly 1.3 millimeters per year across 1901 to 1971, to about 1.9 millimeters per year across 1971 to 2006, to roughly 3.7 millimeters per year across 2006 to 2018, a near tripling of the rate within little more than a century. Two physical processes account for almost all of this change. Thermal expansion, the simple physical fact that water expands slightly as it warms, has historically contributed close to half of the observed rise, as the ocean has absorbed the great majority of the excess heat trapped by rising greenhouse gas concentrations. The remainder comes from the addition of previously land-locked water to the ocean, principally the melting of mountain glaciers worldwide and accelerating mass loss from the Greenland and Antarctic ice sheets, which satellite gravity missions have tracked losing several hundred billion tonnes of ice per year on average over the past two decades.