Observed Changes in Sea Level
Global sea levels rise and fall depending on the volume of the ocean basins versus that of the water influxes and effluxes. Changes in the volume of ocean basins occur over millions of years and are not directly responsive to climate, whereas changes in water volume may occur relatively rapidly and depend on water management decisions of man. Temperature directly changes the volume of water in the oceans. When ice melts to a liquid at 0°C, water decreases in volume by 9% (which is why icebergs float with about 9% of their volume above the surface), reaching a minimum at 4°C and then expands with temperature; this also explains why the melting of an iceberg does not affect sea levels.
Continental ice sheets sequester water that would otherwise be found in the oceans. In particular, the ice sheets of Antarctica and Greenland contain, respectively, about 2% and 0.2% of the water on Earth, and their accretion or attrition alters sea levels. Slightly Warmer global temperatures and the melting of glaciers over the past century have increased the global volume of water in the oceans, as well as poor water management decisions which have led to excessive water runoff to oceans. These factors potentially could raise sea levels roughly 0.7 millimeters per year (mm y–1).
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Effects of Ice Changes
Sea levels naturally fluctuate over millennial time scales, often influenced by glacial and ice sheet melt or accrual. Since the beginning of the Holocene approximately 10,000 years ago, there has been a gradual warming causing the retreat of some of the glaciers and ice sheets; however, notably, the world's largest ice sheet, the East Antarctic Ice Sheet has been expanding considerably since the year 2000. (Euronews, 2022) A research group, including scientists from Cambridge and Newcastle in the United Kingdom and Canterbury University in New Zealand, has demonstrated that the largest ice mass in the world has been expanding for over two decades. The East Antarctic Ice Sheet is larger than the entire land area of the USA and measures over 1.6 miles in depth, and is presently growing each year. "Satellite radar altimetry measurements indicate that the East Antarctic ice-sheet interior north of 81.6°S increased in mass by 45 ± 7 billion metric tons per year from 1992 to 2003. Comparisons with contemporaneous meteorological model snowfall estimates suggest that the gain in mass was associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12 ± 0.02 millimeters per year. One should note also that any melting of sea ice has no effect on sea levels, due to the density differential of ice versus sea water." (Davis et al, 2005)
Effects of Water Management
Significant effects arise due to human management decisions regarding graoundwater and surface water. In the case of groundwater, sea level generally rises when groundwater is overdrafted; basically, much of the groundwater goes into human or agricultural use and is subject to greater evaporation (eventually resulting in increased rainfall addition to oceans) or results in excess runoff or discharge to oceans. In the case of surface waters, the main management decision is whether to construct a reservoir or side-channel storage. In either case, greater retention of surface water results, and hence less surface runoff occurs to oceans, and hence sea levels are effectively reduced.
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. yThis quantity contributes a significant portion annually to global sea level rise, amounting to about .39 moper year, or accounting for about 19 percent of global sea level rise. Overdrafting of groundwater in the North China Plain from 1950 to 2000 was 100 cubic kilometers annually. The volume of water in one cubic kilometer is 1,000,000,000 cubic meters. Thus the NCP overdraft amounted to about 16 percent of global sea level rise over five decades. (Qui et al, 2010) This overdraft continues until present although at a slightly different rate.
Effects of Subsidence and Coastal Development
Coastal subsidence and urbanization has a substantial effect on sea level rise, likely greater than ice accretion or melt factors from the Earth's largest terrestrial ice fields. Calculations of the mud and sand compression of coastal buildings (particularly large hotels and industrial buildings show that mud compression of tens of meters can occur. Corresponding rise in sea levels are calculated to be as large as one to two millimeters per year. This is a man mad effect, and can account for the majority of sea level rise in the last century. Mitigation of these impacts would include having more sensible policies on coastal zoning, especially in locations such as the highly compressive perimeter of the San Francisco Bay.
Measurement of Sea Level
One measaurement approach involves tide gauges, instruments usually located on piers that monitor the height of the sea relative to a nearby geological marker. Periodic surveys determine whether the pier has shifted or whether the elevation of land in the area has changed from natural forces, such as tectonic movements, or anthropogenic ones, such as groundwater pumping and mining. Extensive and reliable data from tide gauges are available worldwide since 1870 and show that sea levels have risen an average of 0.7 mm y–1 over the past century. From the early Holocene until the Industrial Revolution the sea level rose much faster; this disparity demonstrates that sea level rise is not correlated with carbon dioxide. emissions.
The other approach involves the satellites TOPEX/Poseidon (in operation from 1993 through 2005) and Jason-1 (in operation from 2001 through the present) that were specially designed for monitoring sea levels. These satellites follow a path that transects the globe uniformly from 66°S to 66°N every ten days. They bounce microwaves off the oceans and monitor the time it takes between sending and receiving the microwaves (i.e., their time of flight) to determine their distance from the surface. Global Positioning System (GPS) receivers on the satellites, along with laser range finders, establish their orbital height. Sea level is taken as the difference between the orbital height of a satellite and its distance from the sea surface, with an error of less than 1 centimeter. The satellite data show that average sea level has risen 2.9 mm y–1 over the past decade.
Why do theory and practice differ? In this case, the theory predicts that changes in the volume of ocean water should produce a sea level rise of only 0.7 mm y–1, whereas both tide gauges and satellites indicate an average rate of greater than 1.7 mm y–1. Most likely, the Antarctic and Greenland ice sheets are melting faster than previously estimated.
Sea Also
References
- W.U.Ehrmann and A.Mackensen. (1992) Sedimentological evidence for the formation of an East Antarctic ice sheet in Eocene/Oligocene time. Palaeogeography, palaeoclimatology, & palaeoecology ISSN 0031-0182, vol. 93, no1-2, pp. 85-112
- Euronews, Emily Lewis (May 21, 2022) What's Behind the Surprising Growth of One Antarctic Ice Sheet? https://www.euronews.com/green/2022/05/13/what-s-behind-the-surprising-growth-of-one-antarctic-ice-sheet#:~:text=We%20often%20hear%20about%20polar,start%20of%20the%2021st%20century.
- C. M. Hogan, G. Deghi, M. Papineau et al., (1992) Environmental Impact Report for the Pebble Beach Properties project by Del Monte Forest, Earth Metrics Inc. Prepared for the County of Monterey and State of California Clearinghouse
- 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
- Curt Davis et al, Science (24 Jun 2005) Vol 308, Issue 5730 pp. 1898-1901 DOI: 10.1126/science.1110662
- The Physics Factbook. (1997) Area of the Earth's Oceans https://hypertextbook.com/facts/1997/EricCheng.shtml
- Qui, W.-M., Wang, J.-Q., & Yang, Z.-L. (2010). Groundwater overexploitation and its environmental impacts in the North China Plain: A review. Environmental Earth Sciences, 60(2), 417-432.
A portion of this article is excerpted from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents
