SEA LEVEL RISE.

A sea level rise is an increase in global mean sea level as a result of an increase in the volume of water in the world’s oceans. Sea level rise is usually attributed to global climate change by thermal expansion of the water in the oceans and by melting of ice sheets and glaciers on land.The melting of floating ice shelves and icebergs at sea would raise sea levels only by about 4 cm (1.6 in).

Sea level rise at specific locations may be more or less than the global average. Local factors might include tectonic effects, subsidence of the land, tides, currents, storms, etc.Sea level rise is expected to continue for centuries. Because of long response times for parts of the climate system, it has been estimated that we are already committed to a sea-level rise within the next 2,000 years of approximately 2.3 metres (7.5 ft) for each degree Celsius of temperature rise. IPCC Summary for Policymakers, AR5, 2014, indicated that the global mean sea level rise will continue during the 21st century, very likely at a faster rate than observed from 1971 to 2010.Projected rates and amounts vary. A January 2017 NOAA report suggests a range of GMSL rise of 0.3 – 2.5 m possible during the 21st century.

Sea level rises can considerably influence human populations in coastal and island regions and natural environments like marine ecosystems

Two main mechanisms contribute to observed sea level rise: (1) thermal expansion: because of the increase in ocean heat content (ocean water expands as it warms);and (2) the melting of major stores of land ice like ice sheets and glaciers. Based on figures from between 1993–2008 two thirds (68%) of recent sea level rise has been attributed by melting ice, and roughly one third has come from thermal expansion.

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.

Each year about 8 mm of precipitation (liquid equivalent) falls on the ice sheets in Antarctica and Greenland, mostly as snow, which accumulates and over time forms glacial ice. Much of this precipitation began as water vapor evaporated from the ocean surface. To a first approximation, the same amount of water appeared to return to the ocean in icebergs and from ice melting at the edges. Scientists previously had estimated which is greater, ice going in or coming out, called the mass balance, important because a nonzero balance causes changes in global sea level. High-precision gravimetry from satellitesdetermined that Greenland was losing more than 200 billion tons of ice per year, in accord with loss estimates from ground measurement. The rate of ice loss was accelerating, having grown from 137 billion tons in 2002–2003.

• The total global ice mass lost from Greenland, Antarctica and Earth's glaciers and ice caps during 2003–2010 was about 4300 billion tons (1,000 cubic miles), adding about 12 mm (0.5 in) to global sea level, enough ice to cover an area comparable to the United States 50 cm (1.5 ft) deep.
• The melting of small glaciers on the margins of Greenland and the Antarctic Peninsula would increase sea level around 0.5 meter. At the extreme potential, according to the Third Assessment Report of the International Panel on Climate Change, the ice contained within the Greenland ice sheet entirely melted increases sea level by 7.2 meters (24 feet). The ice contained within the Antarctic ice sheet entirely melted would produce 61.1 meters (200 feet) of sea-level change, both totaling a sea-level rise of 68.3 meters (224 feet).

It is estimated that fully melting Antarctica would contribute over 60 metres of sea level rise, and Greenland would contribute more than 7 metres. Small glaciers and ice caps on the margins of Greenland and the Antarctic Peninsula might contribute about 0.5 metres. The latter figure is much smaller than for Antarctica or Greenland, but it could occur relatively quickly (within the coming century), whereas full melting of Greenland would be slow (perhaps 1,500 years to fully deglaciate at the fastest likely rate) and Antarctica even slower. However, this calculation does not account for the possibility of accelerate melting as meltwater flows under and lubricates the larger ice sheets, which would begin to move much more rapidly towards the sea.

In 2002, Eric Rignot and R.H. Thomas found that the West Antarctic and Greenland ice sheets were losing mass, while the East Antarctic ice sheet was close to in balance (they could not determine the sign of the mass balance for The East Antarctic ice sheet).Kwok and Comiso (J. Climate, v15, 487–501, 2002) also discovered that temperature and pressure anomalies around West Antarctica and on the other side of the Antarctic Peninsula correlate with recent Southern Oscillation events.

In 2005 it was reported that during 1992–2003, East Antarctica thickened at an average rate of about 18 mm/yr while West Antarctica showed an overall thinning of 9 mm/yr. associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12 ± 0.02 mm/yr.