Sea level
Published: October 11, 2011 Updated: October 30, 2022
Author: C. Michael Hogan
Editor: David Hassenzahl
| Topics: |
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. A 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. Another driver of sea level rise is the mismanagement of freshwater resources, best exemplified by the US state of California, (Los Angeles Times, 2019) which wastes about 30 trillion gallons of water each year by failure to provide adequate surface water storage and groundwater recharge. This phenomenon of mismanagement is often termed Water Wastage
Two other important ongoing drivers of sea level rise are land subsidence and soil compression from urban development near coastlines. Land subsidence may be from natural crustal changes, but can also be driven by human factors such as groundwater overdrafting or certain types of mining. In those human caused cases, the land surface simply subsides, when large quantities of groundwater or earthen material are extracted in near coastal areas.
Another important cause of sea level change is Isostatic Rebound, a circumstance where sea levels effectively fall, due to an ongoing Holocene interglacial action, where land surfaces are generally rising after glacial melt has alleviated pressure from the prior glaciers. This effect in certain parts of the world actually exceeds any sea level rise from tha addition of water from glacial melt. This effect can also be viewed as a natural negative feedback loop to mitigate any sea level rise in the event of global warming of the atmosphere.
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) Sea level change as of current times is much smaller than in any time in the past 20,000 years. (Fleming, 1998)
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). It is notable that Nitrogen Trifluoride has now superceded the other greenhouse gases in terms of potency, persistence in the atmosphere and rate of increase; Nitrogen Trifluoride is a by-product in the manufacture of solar panels and a few other electronic products. The greenhouse potency is one of the reasons that the Brookings Institute found that combined cycle natural gas plants are less environmentally harmful than solar photovoltaic.One of the most dramatic examples of sea level change has occurred in California, where groundwater overdrafting and lack of creation of water storage (in spite of a massive increase in human population since 1980) has led to an enormous loss of freshwater due to lack of water retention and corresponding huge discharge (and loss) of freshwater to the ocean. State of California drought official Max Gomberg has called out the Newsom administration for failure to act on key water storage projects, and resigned his post in 2022. (Los Angeles Times, 2022) Basically most of the measured global sea level rise in the last four decades can be attributed to the mistakes of a single state, California. This fact can be deduced by starting with the world ocean surface area of 360 million square kilometres (The Physics Factbook, 2017) As of 2022, about seventy wix percent of California rainwater is wasted, or runs off into the Pacific Ocean without human usage. Statewide rainfall in California is approximately 39 trillion gallions, of which about percent is wasted by flowing directly to the ocean without human or agricultural utilization. This quantity contributes a significant portion annually to global sea level rise. If these mistakes in water resource management propagate to other jurisdictions, threat of sea level rise is amplified on a global basis.
See Also
- Little Ice Age
- Nitrogen Trifluoride
- Observed Changes in Sea Level
- Ice Sheet
- Isostatic Rebound
- Subsidence
- Water Wastage
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
- Brookings Institute (2016) NATURAL GAS DRIVING DECOUPLING OF ECONOMIC GROWTH AND GHG EMISSIONS. https://www.energyindepth.org/brookings-institute-natural-gas-driving-decoupling-economic-growth-ghg-emissions
- 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
- Fleming, Kevin, Paul Johnston, Dan Zwartz, Yusuke Yokoyama, Kurt Lambeck, and John Chappell (1998). "Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites". Earth and Planetary Science Letters 163 (1-4): 327-342. doi:10.1016/S0012-821X(98)00198-8
- The Guardian (12 September 2018) The strange science of melting ice sheets: three things you didn't know
- Forbes, Michael Zakaras (April 15, 2015) Why does California let Billions of Gallons of Fresh Water Flow Straight into the Ocean? https://www.forbes.com/sites/ashoka/2015/04/15/why-does-california-let-billions-of-gallons-of-fresh-water-flow-straight-into-the-ocean/?sh=798a44e8517c
- Los Angeles Times, Mark Gold (Feb 20, 2019) California Wastes most of its Rainwater. https://www.latimes.com/local/lanow/la-me-rainwater-lost-wet-winter-california-20190220-story.html
- Los Angeles Times (Jul 28, 2022) California drought official blasts Newsom administrationhttps://www.latimes.com › california › story › 2022-07-28
- The Royal Society (2020) Climate Change: Evidence and Causes. The Royal Society, London, UK
