Sea level
Author: C. Michael Hogan
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Sea level is defined on a global scale as the mean altitude of all seas of the world. Since actual sea height can vary appreciably with diurnal tides, lunar cycles, and due to local weather and storms, one must establish a method of spatial and temporal averaging to assign a meaningful value to global sea level. According to Bloom, "Changes in the volume of ocean basins occur over millions of years and are not directly responsive to climate." (Bloom, 2010)
Contents
Mechanisms of Sea Level Change
Several phenomena can produce a change in sea level. The most fundamental driver is alteration of ocean temperature. Since colder water is more dense, a rise in ocean temperature engenders a rise in sea levels. Melting of ice from temperate glaciers, the Greenland ice sheet or Eastern Antarctic ice sheet would also produce a sea level rise effect from meltwater addition to the oceans; however, this melted ice is subject to the negative feedback loop discussed in the following.
Feedback loops
There are a number of feedback loops that mitigate temperature variations and sea level change with natural consequences. A significant loop pertains to the melting of glaciers, ice sheets and polar ice. To the extent melting is occurring, colder meltwater serves to decrease surface ocean temperatures, reducing global temperatures overall. Calculations show that this effect may cool the Earth's oceans by about 0.4 degrees Celsius between 2021 and 2100, (Bronselaer et al, 2018) thereby obliterating most of any projected warming attributed to greenhouse gases and deforestation. Note that this ocean cooling effect is difficult to circumscribe, since many of the Earth's ice sheets and glaciers are advancing, not retreating; notably, the massive Eastern Antarctic Ice Sheet, most Himalayan glaciers, and many Argentine and Chilean glaciers are advancing.
Historic Sea Level Change
The greatest recent period of sea level rise occurred between 15,000 and 6000 years before present, immediately prior to the Holocene and enduring to the mid-Holocene. Since the late nineteenth century global sea levels have risen approximately one millimetre per year. (Royal Society, 2020)
The last time the Earth was 2 °Celsius warmer than pre-Industrial Revolution temperatures, sea levels were about five metres higher than the year 2000; this situation existed in the prior interglacial, about 120,000 years before present, and clearly arose from totally natural causes.
Scenarios of Future Sea Level Change
The most widely discussed forecast of of sea level change projected to the year 2100 indicates that sea levels will rise about three to six millimetres per decade from present time to the year 2100, assuming little is done to mitigate deforestation or production of greenhouse gases (e.g. methane, nitrous oxide, carbon dioxide, nitrogen trifluoride).References
- Bindoff, N.L., Willebrand, J., Artale, V., Cazenave, A., Gregory, J., Gulev, S., Hanawa, K., Le Quéré, C., Levitus, S., Nojiri, Y., Shum, C.K.; Talley L.D., Unnikrishnan, A. (2007), "Section 5.5.1: Introductory Remarks", in IPCC AR4 WG1 (ed.), Chapter 5: Observations: Ocean Climate Change and Sea Level
- Arnold J. Bloom (2010) Global Climate Change: Convergence of Disciplines. UCVerse of the University of California. ©2010 Sinauer Associates and UC Regents
- B. Bronselaer et al. (2018) Change in future climate due to Antarctic meltwater. Nature, doi:s41586-018-0712-z
- The Guardian (12 September 2018) The strange science of melting ice sheets: three things you didn't know
- The Royal Society (2020) Climate Change: Evidence and Causes. The Royal Society, London, UK
