Sea Level

Sea levels around the world are rising. Current sea-level rise potentially impacts human populations (e.g., those living in coastal regions and on islands) and the natural environment (e.g., marine ecosystems). Global average sea level rose at an average rate of around 1.7 ± 0.3 mm per year from 1950 to 2009 and at a satellite-measured average rate of about 3.3 ± 0.4 mm per year from 1993 to 2009, an increase on earlier estimates. It is unclear whether the increased rate reflects an increase in the underlying long-term trend.

Two main factors contributed to observed sea level rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the contribution of land-based ice due to increased melting. The major store of water on land is found in glaciers and ice sheets.
Sea level rise is one of several lines of evidence that support the view that the climate has recently warmed. It is highly likely that human-induced (anthropogenic) warming contributed to the sea level rise observed in the latter half of the 20th century.

Sea level rise is expected to continue for centuries. In 2007, the Intergovernmental Panel on Climate Change (IPCC) projected that during the 21st century, sea level will rise another 18 to 59 cm (7.1 to 23 in), but these numbers do not include "uncertainties in climate-carbon cycle feedbacks nor do they include the full effects of changes in ice sheet flow". Although IPCC explicitly refrained from projecting an upper limit of total sea level rise in the 21st century, one meter of sea level rise is well within the range of more recent projections.

On the timescale of centuries to millennia, the melting of ice sheets could result in even higher sea level rise. Partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, could contribute 4 to 6 m (13 to 20 ft) or more to sea level rise.

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