Sea level rise: Meaning (information, definition, explanation, facts)

Sea level rise is a rise in sea level. Multiple complex factors may influence such changes.

Local and Eustatic Sea Level

Local “Mean sea level” (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time, such as a month or a year, long enough that fluctuations caused by waves and tides are largely removed. One must adjust perceived changes in LMSL to take into account vertical movements of the land, which can be of the same order (mm/y) as sea level changes. Some land movements occur due the isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age.

Atmospheric pressure (the inverse barometer effect), ocean currents and local ocean temperature changes can all affect LMSL.

“Eustatic” change (as opposed to local change) results in an alteration to the volume of water in the world ocean.

• Since the Last Glacial Maximum about 20,000 years ago, sea level has risen by over 120 m (averaging 6 mm/y) as a result of melting of major ice sheets. A rapid rise took place between 15,000 and 6,000 years ago at an average rate of 10 mm/yr which accounted for 90 m of the rise; thus in the period since 20 kyr BP (excluding the rapid rise from 15-6 kyr BP) the average rate was 3 mm/y.
• Based on geological data, global average sea level may have risen at an average rate of about 0.5 mm/yr over the last 6,000 years and at an average rate of 0.1 to 0.2 mm/yr over the last 3,000 years.
• Based on tide gauge data, the rate of global average sea level rise during the 20th century lies in the range 1.0 to 2.0 mm/yr, with a central value of 1.5 mm/yr
• Recent studies of Roman wells in Caesarea and of Roman piscinae in Italy indicate that sea level stayed fairly constant from a few hundred years AD to a few hundred years ago.
• Measurements have detected no significant acceleration in the rate of sea level rise during the 20th century. Sea-level rise estimates from satellite altimetry over the last 25 years (about 2-3 mm/y) exceed those from tide gauges. It is unclear whether this represents an increase over the last 25 years; variability; true differences between satellites and tide gauges; or problems with satellite calibration.

Various factors affect the volume or mass of the ocean, leading to changes in eustatic sea level.

• If temperature rises, the ocean expands, leading to an increase in ocean volume. Observational estimates are about 1 mm/yr over recent decades.
• The mass of the ocean, and thus sea level, changes as water cycles between oceans, glaciers and ice caps. Observational and modelling studies of glaciers and ice caps indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.
• Climate changes during the 20th century are estimated from modelling studies to have led to contributions of between –0.2 and 0.0 mm/yr from Antarctica (the results of increasing precipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation and runoff).
• Estimates suggest that Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr over the 20th century as a result of long-term adjustment to the end of the last ice age.
• In particular, scientists lack knowledge of changes in terrestrial storage of water. Between 1910 and 1990 such changes may have contributed from –1.1 to +0.4 mm/y.

Over much longer timescales, changes in the shape of the ocean basins and in land/sea distribution can affect sea level.

The current rise in sea level observed from tide gauges, of about 1.5 mm/y, is just about explicable by the combination of factors above [1] but active research continues in this field. The uncertainty in the terrestrial storage term is particularly large.

The fact that sea level has risen at a relatively rapid rate over the last century frequently encourages the conclusion that the activities of humans have changed the world environment. The Intergovernmental Panel on Climate Change (IPCC) projects that global warming will cause additional sea-level rise. This could lead to difficulties for shore-based communities: for example, many major cities such as London already need storm-surge defences, and would need more if sea level rose.

Most climatologists argue that rising atmospheric temperatures currently cause polar and alpine glaciers to melt and water in the oceans to expand.

A small minority (Fred Singer is the only known proponent) assert that effects of the increased temperatures causing water to evaporate and leading to increased snowfall (thereby storing water on ice sheets) have a larger effect than thermal expansion and ice sheet melting [2]. This view has gained few adherents and has not been published in peer reviewed literature.

Some assert that rising sea levels have started to force the evacuation of Tuvalu, an island nation of 11,000 people in the Pacific, North of Fiji. The Tuvaluan government announced the evacuation in 2001. Of two small islands in Oceania, Tebua Tarawa has already disappeared and Tepuka Savilivili no longer has coconut trees. However, the observed sea level rise during the 20th century is quite small - perhaps 20 cm - and it is more likely that recent storms have caused much of the problem.

• TAR chapter 11