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Sea level rise


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

Sea level has risen more than 120 m since the peak of the last ice age 20 kyr ago. The bulk of that occurred before 6 kyr ago. From 3000 years ago to the start of the 19th century sea level was almost constant; since 1900 the level has risen at 1-2 mm/y; since 1992 at about 3 mm/y.

Contents

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.

Past changes in sea level

  • 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 piscina e 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 since 1992 (about 3 mm/y) exceed those from tide gauges. It is unclear whether this represents an increase over the last decades; variability; true differences between satellites and tide gauges; or problems with satellite calibration.

Factors affecting present-day sea-level change

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.
  • If all glaciers and ice caps melt, the projected rise in sea-level will be around 0.5 m. If the melting includes the Greenland and Antarctic ice sheets (both of which contain ice above sea level), then the rise is a more drastic 68.8 m. [1] http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113
  • Ice Shelves float on the surface of the sea and, if they melt, to first order they do not change sea level. Because they are fresh, however, their melting would cause a very small increase in sea levels, so small that it is generally neglected. It can also be argued that if ice shelves melt it is a precursor to the melting of ice sheets on Greenland and Antarctica.

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 1.8 mm/y, is just about explicable by the combination of factors above [2] http://www.grida.no/climate/ipcc_tar/wg1/428.htm but active research continues in this field. The uncertainty in the terrestrial storage term is particularly large.

Since 1992 we have a series of data from the TOPEX and JASON satellite programs. The current data are available at [3] http://sealevel.colorado.edu . The data shows a mean sea level increase of 2.8(+/-0.4) mm/yr. Another analysis at [4] http://membrane.com/sidd/sealevel.html indicates that the rate of sea level rise over the period 1999 through 2004 increased to 3.7(+/-0.2) mm/yr from 2.1(+/-0.2)mm/yr for the period 1992-1999.

Future sea level rise

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.

The IPCC say that projections of components contributing to sea level change from 1990 to 2100 using a range of AOGCMs following the IS92a scenario (including the direct effect of sulphate aerosol emissions) give:

  • thermal expansion of 0.11 to 0.43 m, accelerating through the 21st century;
  • a glacier contribution of 0.01 to 0.23 m;

The results from Dyurgerov show a sharp increase in the contribution of mountain and subpolar glaciers to sealevel rise since 1996 (0.5 mm/yr) to 1998(2mm/yr) with an average of approx. 0.35 mm/yr since 1960. (Dyurgerov, Mark. 2002. Glacier Mass Balance and Regime: Data of Measurements and Analysis. INSTAAR Occasional Paper No. 55, ed. M. Meier and R. Armstrong. Boulder, CO: Institute of Arctic and Alpine Research, University of Colorado. Distributed by National Snow and Ice Data Center, Boulder, CO. A shorter discussion is at [5] http://nsidc.org/sotc/sea_level.html ) Of interest is also Arendt et al, (Science,297,p382, July 2002) who estimate the contribution of Alaskan glaciers of 0.14 (+/-0.04)mm/yr between the mid 1950s to the mid 1990s increasing to 0.27 mm/yr in the middle and late 1990s.

  • a Greenland contribution of 0.02 to 0.09 m;

Krabill et al(Science, Vol 289, Issue 5478, 428-430 , 21 July 2000) estimate a net contribution from Greenland to be at least 0.13 mm/yr in the 1990s. Joughin et al have measured a doubling of the speed of Jacobshavn Isbrae between 1997 and 2003(Nature,432,p608, Dec. 2004). This is Greenland's largest outlet glacier and drains 6.5% of the icesheet, and is thought to be responsible for increasing the rate of sea level rise by about .06 millimeters per year, or roughly 4 percent of the 20th century rate of sea level increase. A description of the results is at [6] http://www.spaceref.com/news/viewpr.html?pid=15611

  • an Antarctic contribution of 0.17 to 0.02 m.

Including thawing of permafrost, deposition of sediment, and the ongoing contributions from ice sheets as a result of climate change since the Last Glacial Maximum, they obtain a range of global-average sea level rise from 0.11 to 0.77 m. This range reflects systematic uncertainties in modelling [7] http://www.grida.no/climate/ipcc_tar/wg1/409.htm .

The Effects of Current Sea Level Rise

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.


Links

  • TAR chapter 11 http://www.grida.no/climate/ipcc_tar/wg1/408.htm
  • changes in the Earth's shorelines during the past 20 kyr caused by the deglaciation of the Late Pleistocene ice sheets http://www.pol.ac.uk/psmsl/palaeoshoreline_webpage/HTML/HOME.htm , from the Permanent Service for Mean Sea Level
  • Includes picture of sea level for past 20 kyr based on barbados coral record http://www.pol.ac.uk/psmsl/palaeoshoreline_webpage/HTML/Science.htm
  • Global sea level change: Determination and interpretation http://www.agu.org/revgeophys/dougla01/dougla01.html
  • Sea level rise FAQ http://www.radix.net/~bobg/faqs/sea.level.faq.html
  • REVIEWS OF GEOPHYSICS, VOL. 42, RG3001, doi:10.1029/2003RG000139, 2004; Present-day sea level change: Observations and causes; A. Cazenave and R. S. Nerem http://www.agu.org/pubs/crossref/2004/2003RG000139.shtml


Last updated: 02-10-2005 10:23:15
Last updated: 02-21-2005 12:20:05