Tide gauge measurements show that the current global sea level rise began at the start of the 20th century. Between 1900 and 2017, the globally averaged sea level rose by 16–21 cm (6.3–8.3 in), or 1.4–1.8 mm (0.055–0.071 in) per year. More precise data gathered from satellite radar measurements reveal an accelerating rise of 7.5 cm (3.0 in) from 1993 to 2017, for an average rate of 31 mm (1.22 in) per decade. This acceleration is due mostly to climate change, which is driving thermal expansion of seawater and the melting of land-based ice sheets and glaciers. Between 1993 and 2018, thermal expansion of the oceans contributed 42% to sea level rise; the melting of temperate glaciers 21%, Greenland 15%, and Antarctica 8%. Climate scientists expect the rate to further accelerate during the 21st century, with the latest measurements saying the sea levels are currently rising by 3.6 mm per year
Projecting future sea level is challenging, due to the complexity of many aspects of the climate system and to time lags in sea level reactions to Earth temperature changes. As climate research into past and present sea levels leads to improved computer models, projections have consistently increased. In 2007, the Intergovernmental Panel on Climate Change projected a high end estimate of 60 cm (2 ft) through 2099, but their 2014 report raised the high-end estimate to about 90 cm (3 ft). A number of later studies have concluded that a global sea level rise of 200 to 270 cm (6.6 to 8.9 ft) this century is 'physically plausible'. A conservative estimate of the long-term projections is that each Celsius degree of temperature rise triggers a sea level rise of approximately 2.3 meters (4.2 ft/degree Fahrenheit) over a period of two millennia: an example of climate inertia. In February 2021, a paper published in Ocean Science suggested that past projections for global sea level rise by 2100 reported by the IPCC were likely conservative, and that sea levels will rise more than previously expected
The sea level will not rise uniformly everywhere on Earth, and it will even drop slightly in some locations, such as the Arctic. Local factors include tectonic effects and subsidence of the land, tides, currents and storms. Sea level rises can affect human populations considerably in coastal and island regions. Widespread coastal flooding is expected with several degrees of warming sustained for millennia. Further effects are higher storm-surges and more dangerous tsunamis, displacement of populations, loss and degradation of agricultural land and damage in cities. Natural environments like marine ecosystems are also affected, with fish, birds and plants losing parts of their habitat.
