Editing Sea level (section)

== {{anchor|Change}} Change ==
{{See also|Past sea level|sea level rise||}}

=== Local and eustatic ===
{{See also|Eustatic sea level}}

[[File:Mass balance atmospheric circulation.png|thumb|[[Water cycle]]s between ocean, [[Earth's atmosphere|atmosphere]] and [[glacier]]s]]

'''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 long enough that fluctuations caused by [[ocean surface wave|waves]] and [[tide]]s are smoothed out, typically a year or more. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can occur at rates similar to [[sea level rise|sea level changes]] (millimetres per year).

Some land movements occur because of [[isostasy|isostatic]] adjustment to the melting of [[ice sheet]]s at the end of the [[Last Glacial Period|last ice age]]. The weight of the ice sheet depresses the underlying land, and when the ice melts away the [[post-glacial rebound|land slowly rebounds]]. Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the [[geoid]] and [[true polar wander]]. [[Atmospheric pressure]], [[ocean current]]s and local ocean temperature changes can affect LMSL as well.

'''Eustatic sea level''' change (global as opposed to local change) is due to change in either the volume of water in the world's oceans or the volume of the [[oceanic basin]]s.<ref name="Oilfield Glossary">{{cite web|url=http://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20level|title=Eustatic sea level|work=Oilfield Glossary|publisher=Schlumberger Limited|access-date=10 June 2011|archive-date=2 November 2011|archive-url=https://web.archive.org/web/20111102190720/http://www.glossary.oilfield.slb.com/Display.cfm?Term=eustatic%20sea%20level|url-status=live}}</ref> Two major mechanisms are currently causing eustatic sea level rise. First, shrinking land ice, such as mountain glaciers and polar ice sheets, is releasing water into the oceans. Second, as ocean temperatures rise, the warmer water expands.<ref>{{Cite web|url=http://www.climatehotmap.org/global-warming-effects/sea-level.html|title=Global Warming Effects on Sea Level|website=www.climatehotmap.org|access-date=2016-12-02|archive-date=20 November 2016|archive-url=https://web.archive.org/web/20161120171530/http://www.climatehotmap.org/global-warming-effects/sea-level.html|url-status=live}}</ref>

=== {{anchor|Short term and periodic changes}}Short-term and periodic changes ===
[[File:Global sea levels during the last Ice Age.jpg|thumb|The [[Last Glacial Period]] caused a much lower global sea level.]]
[[File:Glaciers and Sea Level Rise (8742463970).jpg|thumb|Warming temperatures and melting glaciers are currently raising the sea level.]]

Many factors can produce short-term changes in sea level, typically within a few metres, in timeframes ranging from minutes to months:

{|
|-
! colspan="3" style="background:lightgrey;"| Periodic sea level changes
|-
| Diurnal and semidiurnal astronomical tides || 12–24 h P || 0.1–10+ m
|-
| Long-period tides || 2-week to 1-year P || <0.1 m
|-
| Pole tides ([[Chandler wobble]]) || 14-month P || 5 mm
|-
! colspan="3" style="background:lightgrey;"| Meteorological and oceanographic fluctuations
|-
| Atmospheric pressure || Hours to months || −0.7 to 1.3 m
|-
| Winds ([[storm surge]]s) || 1–5 days || Up to 5 m
|-
| [[Evaporation]] and [[Precipitation (meteorology)|precipitation]] (may also follow long-term pattern) || Days to weeks || <0.1m
|-
| [[Ocean surface topography]] (changes in water [[density]] and currents) || Days to weeks || Up to 1 m
|-
| [[El Niño]]/[[southern oscillation]] || 6 mo every 5–10 yr || Up to 0.6 m
|-
! colspan="3" style="background:lightgrey;"| Seasonal variations
|-
| Seasonal water balance among oceans (Atlantic, Pacific, Indian) || 6 months || &nbsp;
|-
| Seasonal variations in slope of water surface || 6 months || &nbsp;
|-
| River runoff/floods || 2 months || 1 m
|-
| Seasonal water density changes (temperature and [[salinity]]) || 6 months || 0.2 m
|-
! colspan="3" style="background:lightgrey;"| Seiches
|-
| [[Seiche]]s (standing waves) || Minutes to hours || Up to 2 m
|-
! colspan="3" style="background:lightgrey;"| Earthquakes
|-
| [[Tsunami]]s (catastrophic long-period waves) || Hours ||  0.1–10+ m
|-
| Abrupt change in land level || Minutes || Up to 10 m
|}

=== Recent changes ===
{{Further|Ocean heat content|Effects of climate change on oceans}}
{{excerpt|Sea level rise}}