Schematic of sea level (black) and rate of change (blue) over the last 25 kyr

There are multiple complex factors may influence sea level change.

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.

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 piscinae 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 until the last decade of 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 change

The late 20th century sea level rise rate lacks any sign of acceleration. Satellite altimetry indicates virtually no changes in the last decade. Therefore, observationally based predictions of future sea level in the year 2100 will give a value of +10±10 cm (or +5±15 cm), by this discarding model outputs by IPCC as well as global loading models. This implies that there is no fear of any massive future flooding as claimed in most global warming scenarios. (Mörner, 2004)

IPCC results

The fact that sea level has risen at a relatively rapid rate since 1840 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. TAR chapter 11 (http://www.grida.no/climate/ipcc_tar/wg1/408.htm)

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 sea level could rise above its current level if more polar ice melts. However, compared to the heights of the ice ages, today there are very few continental ice sheets remaining to be melted. It is estimated that Antarctica, if fully melted, would contribute more than 60 metres of sea level rise and Greenland would contribute more than 7 metres. Small glaciers and ice caps might contribute about 0.5 metres; this number is in the uncertainty of the estimates from Antarctica or Greenland but could be expected to be fast (within the coming century) whereas Greenland would be slow (perhaps 1500 years to fully deglaciate at the fastest likely rate) and Antarctica even slower [8] (http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113).

Critique of IPCC results

Nils-Axel Mörner, at the time president of the INQUA Commission on Sea Level Changes and Coastal Evolution reviewed the IPCC TAR Synthesis Report in 2000. [9] (http://www.pog.su.se/sea/HP-14.%20IPCC-3.pdf)

"0.09 to 0.88 metres between 1990 and 2100” – from SRES scenarios. These values are completely misleading and false. Our INQUA Commission on “Sea Level Changes and Coastal Evolution” (which hosts the leading world experts on just the topic) has a totally different view (expressed in my previous reviews). The expert/observational-based figure is 10 cm +10 cm."

"It has been popular the threaten “small islands and low-lying coasts” with scenarios of disastrous future flooding. The Maldives has been the most utilised target. We have undertaken a careful analysis of actual sea level changes in the Maldives. No rise has been recorded either in the present or the past centuries. Instead we have documented a significant sea level fall in the last 20-30 years. Take this as Reality contra Models."

"All handling by IPCC of the Sea Level questions have been done in a way that cannot be accepted and that certainly not concur with modern knowledge of the mode and mechanism of sea level changes."

On TAR Chapter 11:

"Chapter 11 on "Sea Level Changes" was written by 33 persons; none of which represents actual sea level research. I have now finished a 7 pages review report. It is a most shocking reading; lots of modeler wishes but very little hard facts based on real observational data by true sea level specialists. It seems that the authors involved in this chapter were chosen not because of their deep knowledge in the subject, but rather because they should say what the climate model had predicted. This chapter has a low and unacceptable standard. It should be completely rewritten by a totally new group of authors chosen among the group of true sea level specialists." [10] (http://www.pog.su.se/sea/14_news.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.

Very long term changes

At times during Earth's long history, continental drift has arranged the land masses into very different configurations from those of today. When there were large amounts of continental crust near the poles, the rock record shows unusually low sea levels during ice ages, because there was lots of polar land mass upon which snow and ice could accumulate. During times when the land masses clustered around the equator, ice ages had much less effect on sea level.

During the glacial/interglacial cycles over the past few million years, the sea level has varied by somewhat more than a hundred metres. This is primarily due to the growth and decay of ice sheets (mostly in the northern hemisphere) with water evaporated from the sea.

The sedimentary record

For generations, geologists have been trying to explain the obvious cyclicity of sedimentary deposits observed everywhere we look. The prevailing theories hold that this cyclicity primarily represents the response of depositional processes to the rise and fall of sea level. In the rock record, geologists see times when sea level was astoundingly low alternating with times when sea level was much higher than today, and these anomalies often appear worldwide. For instance, during the depths of the last ice age 20,000 years ago when hundreds of thousands of cubic miles of ice was stacked up on the continents as glaciers, sea level was 390 feet lower (~120 meters), locations that today support coral reefs were left high and dry, and coastlines were miles farther basinward from the present-day coastline. It was during this time of very low sea level that there was a dry land connection between Asia and Alaska over which humans are believed to have migrated to North America (see Bering Land Bridge).

However, for the past 6000 years (long before mankind started keeping written records) the world's sea level has been gradually approaching the level we see today. During the previous interglacial about 120,000 years ago, sea level was for a short time about 6 m higher than today, as evidenced by wave-cut notches along cliffs in the Bahamas. There are also Pleistocene coral reefs left stranded about 3 meters above today's sea level along the southwestern coastline of West Caicos Island in the British West Indies . These once-submerged reefs and nearby paleo-beach deposits are silent testament that sea level spent enough time at that higher level to allow the reefs to grow (exactly where this extra sea level came from - Antarctica or Greenland - is open to question). Abundant similar evidence of geologically recent sea level positions can be found around the world.

References

• Cazenave, A.; Nerem, R. S.. (2004). "Present-day sea level change: Observations and causes". Rev. Geophys 42, RG3001. doi:10.1029/2003RG000139 (http://dx.doi.org/10.1029/2003RG000139)
• Mörner, Nils-Axel. (2004). Estimating future sea level changes from past records. Global and Planetary Change 40 (1-2), 49-54. doi:10.1016/S0921-8181(03)00097-3 (http://dx.doi.org/10.1016/S0921-8181(03)00097-3)
• Mörner, Nils-Axel; Tooley, Michael; Possnert, Göran. (2004). New perspectives for the future of the Maldives. Global and Planetary Change 40 (1-2), 177-182. doi:10.1016/S0921-8181(03)00108-5 (http://dx.doi.org/10.1016/S0921-8181(03)00108-5)

Links

• "The expected sea level changes in the next century. (http://www.pog.su.se/sea/07_research_topics/rt5.htm)." Sea Level Changes and Coastal Evolution, Research Topics (RT) 5; Mörner (2000). Accessed on February 9, 2005.
• "Sea Level Changes in the Past, Present and in the Near-Future Global Aspects Observations versus Modells (http://www.dsm.unile.it/Bacheca/IGCP437FinalConference/AbstractBook/Morner5_10.pdf)." IGCP Project No. 437 Puglia 2003 - Final Conference. Accessed on February 10, 2005.
• "Sea Level Changes: The Maldives Project Freed From Condemnation to become Flooded (http://www.dsm.unile.it/Bacheca/IGCP437FinalConference/AbstractBook/Morner175_176.pdf)." IGCP Project No. 437 Puglia 2003 - Final Conference. Accessed on February 10, 2005.
• 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) (1997)