Past Sea Levels: Part 2, The Findings

Tide Gauges

Tide gauge data provides us with average sea levels, for the past 100-200 years, in relation to the height of the land that the measuring device is positioned on [1]. 

Despite the efforts of tide gauge survey groups to produce universal readings for sea level rise, published predictions of change are numerous and widely varying (table 1).

Table 1: Various predictions of sea level change. Region refers to the area of measurement, VLM the method used to correct the readings for vertical land movement and the final column shows the different rates of sea level change. Source: [1]

Table 1 illustrates the differences of opinion concerned with sea level change, a trend that has not subdued in the 21st century:

Miller & Douglas (2004): 1.5 - 2.00 mm/yr 

Holgate & Woodworth (2004): 1.3 - 2.1 mm/yr

Church et al (2004): 1.5 - 2.1 mm/yr

These are just a few of the many predictions made, for more detail, please see Observations: Oceanic Climate Change & Sea Level [2]. 

The variance in predictions can be attributed to the weakness in tide gauge measurement systems; flaws that must be appreciated, if accurate measurements can be made.

Tide Gauge Weaknesses

Local Land Movement: one of the major weaknesses of tide gauge measuring systems is the movement of the ground that the gauge is attached to being mistaken for changes in sea level [3]. Such movement may occur due to tectonic action, glacial rebound, sedimentation or subsidence [2&3] and must corrected before the data can be used. 

To perform such correction, geological data of adjacent ground can be used to find movement levels, or geophysical modelling can indicate the level of rebound, after glacial melt [1]. However, such movement can only be estimated, a point that increases the uncertainty in results and brings about the variance in predictions [1&2].

Different Ages of Tide Gauges: predictions are more reliable with a larger amount of data to inform them, a point that warrants the use of older tide gauges. However,there are less than 100 gauge records that span more than 50 years, and a lot of these contain unusable information [3]. 

Such a lack of long term data forces the use of younger records, which may show large variation, due to a focus on short term changes [3]. The records are averaged to avoid this; a technique that whilst avoiding anomalies, may have dulled the data, causing the rapid rise in estimates seen with the advent of satellite measurement. 

Northern Hemisphere Domination: Most of the usable tide gauges are placed along the coast of norther hemisphere countries [2]. The domination of the northern hemisphere creates large unknowns when predicting sea level change, as it is known that the long term climates of the northern and southern hemisphere work in opposition. Until southern hemisphere readings increase, a truly global sea level measurement can not be made.

Sea Level Reconstruction Using Paleotemperatures

The study of oxygen isotope rations allows the reconstruction of long term climatic variation, as long records are stored in ice sheets and sediment layers. The long term records also help to avoid the noise of short term changes, allowing the creation of a smooth representation of past temperatures [4]. 

The findings in ice cores and sediment records are compared with the known changes of 1980 and 1999, as this period was very well documented for temperature changes and sea level response. Such efforts enable the prediction the past 2,000 years of sea level change [4]. 

Highest sea level: Medieval Warm Period (1120-1200 AD) - 12 &21 cm higher than 1988-1999 [5&6]

Lowest sea level: Little Ice Age (1730 AD) [4] (figure 1)

Figure 1: Two separate predictions [5&6] of sea level rise over the past 2,000 years. each inset box shows the final 20 years of measurements in detail and the IPCC predictions of sea level rise until 2100. Source: [4]

As can be seen from the grey bands of uncertainty surrounding each of the above predictions, this technique also carries many uncertainties.

Sea Level Reconstruction Using Paleotemperatures Weaknesses

Understanding of ice sheet dynamics: The reactions of large ice sheets to warming is still not fully understood, as the processes occurring withing an ice sheet are near impossible to observe, without causing disturbance [4]. Until such processes are quantified, the predictions of past sea levels will carry a large band of uncertainty.

Understanding of influencing factors to sea level: All of the processes that contribute to sea level rise are not yet known, such a point was emphasised by the IPCC's prediction of the known sea level rise of 1988-1999 being nearly 40% out [4].

Seismic Stratigraphy

Seismic Stratigraphy studies sea level through the study of sediment deposition, the direction of which indicates times of coastal on lap, still water and retreat, the modelling of which can be seen below.

This technique permits the study of vast and inaccessible terrains, allowing a comprehensive sea level study to be performed [7]. The findings of this technique are however,  very large and qualitative (figure 2), a point that may reduce the audience level and use of any information. 

Figure 2: Seismic Stratigraphy reconstructions of Phanerozoic sea levels (past 542 million years) A and B are the findings of two different projects [7&8 respectively]

 Weaknesses of Seismic Stratigraphy 

Quantifying information: As can be seen from figure 2, the data provided by this technique is highly qualitative [8], a point that makes the production of a definite set of facts a hard task. Such a weakness could lower the impact of any findings, as the results may not be widely published or quoted. 

Accuracy of time scales: Recent advances in research have begun to produce finer detailed and more accurately timed records of past sea levels [9], raising questions of the use of this information.

Plotting imbalance: During the research period, it has been found that it is easier to plot the rises in sea level than the falls [7]. This occurrence will lower the reliability of the results by a sever amount, as the study of sea level can not rely on half accurate results. 

Percentage Flooding of Continents

This technique also allows the study of long sea level change, with mapping of Phanerozoic sea levels possible. Examples of the work include the mapping of the last glacial maximum ice extent (figure 3) and the percentage flooding of continental regions, to gauge past sea levels (figure 4).

Figure 3: Map of Northern Hemisphere glacial extent, during the last glacial maximum [10]

Figure 4: (a) percentage flooding of North America and the Soviet Union. (b) Relative paleo-sea level [11]

 Weaknesses of Paleo-Mapping

Not widely used: This technique is not as widely used or documented as others, a point that may reduce reliability, due to the lack of available points of reference and comparison. 

Assumes sea surface area does not change: The mapping of previous flooding relies on the fact that during the time of study, the area holding the water did not change [10]. This assumption may work on a short time scale, but if efforts were made to combine observations for the entire Phanerozoic, the tectonic movement of continents and resultant change in ocean surface areas would have to be considered. 

Raised Marine Terraces

The study of raised terraces combines the observation of the terrace extent and the oxygen content of cores taken, with the deep sea core oxygen ratios. Such a combination allows the findings of past sea extent from  marine terrace length to be confirmed by  the modelling of temperatures and ocean response [12 &13].

The position of the uplifted terrace is important, as the area must be highly reactive to sea level changes. When such an area is found, the observations made there can be likened to global change, as the conformation of observations with those from deep sea cores reduces uncertainties [12] (figure 5).

Figure 5: Sea level reconstruction from the start of the younger dryas, showing the findings from deep sea core KL11 and various uplifted coral reefs. Source [12] (please excuse the poor formatting, the results are illegible at a smaller scale)

Weaknesses of Raised Marine Terraces

Isostatic change: Isostatic sea level change due to land movement could easily ruin any  attempts to make global estimates from the study of uplifted marine terraces. To avoid this, rigorous checking should be continued, highlighting any results that have occurred due to land movement, rather than sea level change.

What does it all tell us?

The various findings documented in this post could easily become confusing, as different time scales, techniques and levels of uncertainty could easily mess up any efforts to produce a long record of change.

Luckily, it has been done for us (figures 6&7):

Figure 6: Phanerozoic sea level [14]

Figure 7: Global mean sea levels, showing estimates, tide and satellite measurement and future predictions [2]

As figure 6 shows, sea level has been estimated as being much higher in the past, a point that some may find comforting. However, never before have humans faced the risk of sea level rise and with levels on the increase again and the apparent acceleration in rise, shown in figure 7, the potential implications to the human population are devastating.

It must now be asked if sea level rise is actually accelerating and if so, what should the human race do about it? These questions will be addressed in the following posts. 

References

[1]  P. Huybrechts, M. Kuhn, K. Lambeck, M.T. Nhuan, D. Qin, P.L. Woodworth (2001) Changes in Sea Level. IPCC Third Assessment Report - Climate Change 2001

[2] Bindoff, N. L., J. Willebrand, A. Artale, A. Cazenave, J. Gregory, S. Gulev, K. Hanawa, C. Le Quere, S. Levitus, Y. Nojiri, C. K. Shum, L. D. Talley and A. Unnikrishnan , (2007) Observations: Oceanic Climate Change and Sea Level. Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change

[3] Douglas B.C. (1991) Global Sea Level Rise. Journal of Geophysical Research, 96, 6981-6992

[4] A. Grinsted, J. C. Moore, S. Jevrejeva (2010) Reconstructing Sea Level From Paleo and Projected Temperatures 200 to 2100 AD. Clim. Dyn

[5] P.D. Jones, Mann M.E. (2004) Climate over past millennia. Rev Geophys 42:RG2002

[6] A. Moberg, Sonechkin D.M., Holmgren K., Datsenko N.M.,Karle W. (2005) Highly variable northern hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433:613–617

[7] A. Hallam (1984) Pre-Quaternary Sea-level Changes. Annual Review of Earth and Planetary Sciences Vol. 12: 205-243

[8] Vail P. R., R.M. Mitchum, R.G. Todd, J.M. Widmier, et al. (1977) Seismic stratigraphy and global changes of sea level. Stratigraphic Interpretation of Seismic Data, ed. C. E. Payton.

[9] Haq B.U., J. Hardenbol, P.R. Vail (1987) Chronology of Fluctuating Sea Levels Since the Triassic. Science 235, 1156-1166

[10] Tushingham A.M. and W.R. Peltier (1991) Ice-3G' A New Global Model Of Late Pleistocene Deglaciation Based Upon Geophysical Predictions Of Post-Glacial Relative Sea Level Change. Journal of Geophysical Research, 96, 4497-4523

[11] Turcotte, D.L. and K. Burke (1978) Global Sea Level Changes and the Thermal Structure of the Earth. Earth and Planetary Science Letters, 41, 341-346

[12] Dodge, R.E., R.G. Fairbanks, L.K. Benninger, F. Maurrasse (1983) Pleistocene Sea Levels from Raised Coral Reefs of Haiti. Science, 219, 4591,1423-1425

[13] Siddall, E.J. Rohling, A. Almogi-Labin, Ch. Hemleben, D. Meischner, I. Schmelzer & D.A. Smeed (2003) Sea-level fluctuations during the last glacial cycle. Nature, 423, 853 - 858

[14] Fjeldskaar, W. (n/d) The Challenge of Predicting Sea Level Changes [online]. Available at: http://www.geoportalen.no/planetenjorden/klima/sealevel/ [13.03.2010]

Past Sea Levels: Part 1, Measurement

The causes of sea level change vary considerably:

• Tectonic movement, changing the shape of ocean basins
• Climatic variations (changes in temperature, precipitation rates etc)
• Isostatic land movements
• Back ground noise in records (volcanic eruptions, anthropogenic influences etc.) [1]

The various causes of change have caused much debate over the best way to modelling past sea level rise, a debate that has not yet and may never be satisfied. 

This post will document the various techniques of past sea level modelling and illustrate the weaknesses with each, an important point as it has been noted that no single technique should be relied upon to produce a history of sea level change [2].

Tide Gauges

Whilst the first tide gauge was set up some 300 years ago [1], reliable sea level records are only available for the past 150 years [3]. Such records began after the use of tide gauges in ports, harbours, rivers and estuaries became the most popular way of recording monthly and annual means of sea level change.

Tide gauges record the sea level, in relation to a set norm. water enters the gauge through the bottom and the height reached is recorded by electronic sensors, allowing the production of raw tide data to an accuracy level of 1mm (figure 1). The data is then used to produce month and yearly averages of sea levels.

Figure 1: An example of raw tide gauge data, taken from Ponta da Armação, Brazill [4]

Tide gauge networks have now been set up across the globe, greatly increasing the reliability of the measurements. Examples of such tide gauge organisations include the Global Sea Level Observing System and the Permanent Service for Mean Sea Level, both of which aim to observe sea level changes and use the collected data to warn of dangerous events, such as storm surges and tsunamis.

For more information, please see the websites of the aforementioned organisations:

Permanent Service for Mean Sea Level

Global Sea Level Observing System

Whilst tide gauges allow the monitoring of current and recently past sea level changes, the study of paleo-sea levels requires the use of other techniques.

Sea Level Reconstruction Using Paleotemperatures

Temperature has a leading roll in the story of sea level rise, a point illustrated in modern day sea level rise, as rising temperatures cause thermal expansion of the oceans and ice sheet melting. It is this modern day occurrence that allows past temperatures to be related to sea level rise: the study of glacier and ocean expansion reaction to modern temperature changes can be used to predict past changes, once temperature is know [3].

The study of oxygen isotopes in proxy records, such as ice cores, coral reefs, foraminfera and fossilised diatoms allows the reconstruction of past temperatures, as the different weights of oxygen isotopes cause varying levels of evaporation [3].

To find the temperature of past climates, the ratio between Oxygen-16 and the 12% heavier Oxygen-18 [5] isotope is considered:

Colder Climates

Ice Cores: Higher content of Oxygen-16 in the ice core, as these are lifted by weak evaporative forces, to form precipitation and extend glaciers.

Corals, Foraminfera and Diatoms: Higher content of Oxygen-18, as this heavier isotope is left in the ocean waters by the weak forces of evaporation. (figure 2)

Figure 2: Variations in the content of Oxygen-18 isotope of fossilised oceanic calcitic and phosphatic shells, illustrating the increase in Oxygen-18, as temperatures fall towards glacial level [6]

Such a technique allows the reconstruction of past temperatures and the relation of such temperatures to sea levels, as modern day reactions to change are able to be used as a point of reference. This technique has however been accused of being too simplistic, with little appreciation of other possible differences in the past, such as the temperature of ocean waters and varying isotopic content of paleo-ice sheets [2&6]. It is for this reason that other methods must be researched.

Seismic Stratigraphy

The study of seismic stratigraphy allows the reconstruction of sea levels from some 250 million years ago [1]. The technique uses coastal sediments to identify periods of maritime erosion and deposition [7], a variation that indicates the rise and fall of sea levels over the coast. 

The maritime sediment is studied for duration and magnitude of the change, a perk that allows the identification of the speed and length of sea level change and can allow the construction of charts that demonstrate the observations [7]. Whilst such an approach can supply a good representation of sea level change in an area, it can be easily affected by isostatic sea level change and the tectonic alteration of ocean basins, it must therefore be cross referenced against other techniques. 

Whilst the tectonic effects on sea level may seem whimsical, the following video illustrates the evolution of the earth, aptly demonstrating tectonic movement's ability to alter ocean basin size.

Percentage Flooding of Continents

This less documented technique uses paleogeographic maps to calculate areas of the land previously covered by ocean (figure 3). Such maps are created through the study of sediment cores, enabling the identification of areas that were once connected land masses. The extent of sea water coverage is also assessed through sediment observation, as maritime sediment in terrestrial cores indicates that the land was once covered. 

Figure 3: Paleogeographic map illustrating the hypothesised positions of the continents 245 to 228 willion years ago [8]

Such a method falls to the same shortcomings as Seismic Stratigraphy; local changes could easily affect the sediment core and cause errors in any conclusions made. The techniques should not however be dismissed, as the study of other techniques should help to avoid mistakes.

Raised Marine Terraces

The study of uplifted marine terraces and coral reefs is one that can prove very useful, as long as all of the necessary information is available; the researcher must be aware of:

• Date of uplift
• Rate of uplift
• Extent of uplift
• Height of original position, with relation to the seafloor [1]

With such information, the research is in a good position to estimate the previous sea levels, as this is simply as far as the terrace extends. The use of uplifted coral reefs (figure 5) makes the study particularly easy, as the extent of the previous sea levels is demonstrated by the extent of the reef.

Figure 5: An example of an uplifted coral reef in Moria [9]

The technique does however, fall when isostatic sea levels are considered, as uplift in one area may have no influence on sea levels across the globe. Again, the study of past sea levels must combine the findings of different techniques, in an effort to identify suggestions of sea level rise, that are in fact caused by local changes.

The techniques documented above have all provided sea levels estimations for varying periods of time, the next post will document the different findings, illustrating the need to combine the techniques, as the strengths and weaknesses of each become clear. 

To help you relax after reading this information-heavy post, please watch the following video of my main man David Attenborough talking about fish and shrimps. 

References

[1] Pirazzoli P.A. (1992) Global Sea-level changes and their meaurement. Global and Planetary Change, 8, 135

[2] Rabineau, M. S. Berné, J-L. Olivet, D. Aslanian, F. Guillocheau, P. Joseph (2006) Paleo sea levels reconsidered from direct observation of paleoshoreline position during Glacial Maxima (for the last 500,000 yr). Earth and Planetary Science letters, 252, 119

[3] Grinsted, A., J.C. Moore, S. Jevrejeva (2009) Reconstructing sea level from paleo and projected temperatures 200 to 2100AD. Clim. Dyn.

[4] Lieutenant Commander M.F. Cavalcante (2003) Brazilian Navy Hydrographical Centre of the Navy the Digilevel [online]. Available at http://www.mares.io.usp.br/aagn/7/dhn/presentation-brasil-digilevel.htm [4.3.2011]

[5] Emsley, John (2001). "Oxygen". Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press.

[6]  Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O. and Strauss, H. (1999) 87Sr/86Sr, d13C and d18O evolution of Phanerozoic seawater. Chemical Geology 161, 59-88.

[7] Vail, P.R., R.M. Mitchum Jr., Thompson, S. (1977) Seismis Stratigraphy and Global Changes of Sea level

[8] Rosli, A. (2009) Malaysian Triassic Blog [online]. Available at: http://malaysiantriassic.blogspot.com/2009_03_01_archive.html [4.3.2011]

[9] cruising newcaledonia (2010) Coastal Uplift [online]. Available at: http://www.cruising-newcaledonia.com/ncal80/3UPLIFT.HTM

The 2009 Updates and Advances in Knowledge

The 2009 Copenhagen Diagnosis [1] used satellite data, dating back to advent of its use in 1993, to update predictions of sea level rise made by the International Panel on Climate Change (IPCC) in the 2001 Third Assessment Report (TAR) [2] and the 2007 Fourth Assessment Report (AR4) [3].

The findings indicated an 80% increase of the most sever IPCC prediction. A point made even more sobering when it is considered that the prediction was based on the following assumptions:

• Top level of pollutant emissions
• Top level of climate sensitivity
• Ad hock measurement of sea level rise, due to the uncertain input from ice sheet melt [4].

The predictions of increase have been supported by studies of ocean warming, which identified thermal expansion of the oceans as some 50% greater than IPCC projections [5]. Further support came with observations of the actual rise in sea level between 1987 and 2007 indicating a rise 25% higher than any predictions made in the past 115 years [6].

There were two main causes identified for the of sea level rise between 1961 and 2003 [1]: 

• Thermal expansion of the oceans, caused by an increase in energy supplied to particles within the water column: 40%
• Continental ice sheet melt, particularly the ice sheets of Greenland and Antarctica: 60%

It was a single advancement in technology bought about fears of an acceleration in sea level rise and forced the increases in estimations: satellite observation of ice sheet dynamics [7].

The Ice Sheets

In both the 2001 and 2007 IPCC sea level rise predictions, ice sheet dynamics were not properly understood or acknowledged. For example in the AR4 predictions [3], Antarctica was identified as a land form responsible for countering a rise in sea levels during the 21st century. Such a point that was reversed during the creation of the 2009 predictions [1] and resulted in the doubling of the observed contribution of ice sheet melt to sea level rise.

Concerns were raised again when it was realised that doomsday scenario of the complete disappearance of the Greenland and Antarctic ice sheets would result in nearly a 60 metre rise in sea levels [1], the effects of which can be seen in the video below.

The two major continental ice sheets of Greenland and Antarctica are those under the most scrutiny, information of which follows:

Greenland: The IPCC AR4 identified the Greenland ice sheet as contributing between -0.07 and 0.17 millimetres a year to sea level rise between 1961 and 2003, a point that aptly illustrates the lack of knowledge of the IPCC in 2007 [3]. 

However, after satellite monitoring allowed further study, it was identified that the loss of ice sheet mass from Greenland had doubled between 2002 and 2009 [8]. It was also found that surface temperatures had increased by more than 1.5 times between 2002 and 2006, two points that could greatly increase the effect of Greenland ice melt on sea levels (figure 1)

Figure 1: The general increase in summer melt area of the Greenland ice sheet. two diagrams illustrate the difference between 1992 and 2007 total melt area (coloured in red) [1]

Antarctica: Being one of the most hospitable places on earth, little was known of Antarctica ice shelf dynamics before the use of satellite observation. However, a change in the view of ice sheets came with the observation; the  reliable and slowly changing masses were discovered to be rapidly changeable and easily affected landforms [7].

The research revealed that the solid land supported, east of Antarctica was far more stable than the west and the Antarctic peninsula, both of which were susceptible to ice shelf collapse. Ice sheet collapse occurs when an already floating ice shelf is warmed by ocean waters from beneath and breaks away from the ice sheet [7]. After such an event occurs, glacier flow increases in speed, unimpeded by and previous resistance, [ 7].

The video below documents the collapse of the northern point of the Larsen b ice shelf of the Antarctic Peninsular, a loss of 3,250 square kilometres of ice in just over a month. This collapse allowed a tenfold increase in glacier flow [7].

Such an occurrence could severely increase the inflow of melt water from the Antarctic ice sheets, as increase glacier flow delivers ice to the coasts and brings it into contact with a warmer ocean.

It was the realisation of these facts that bought polar warming to the forefront of sea level study. 

However, the knowledge of the earth's reactions to global warming is still thin in areas, an example of which can be seen in the observations of Stefan Rahmstorf [4]. Modern modelling systems are failing to model the changes in sea level that have been observed over the past decades, indicating that something is still missing from the calculations.

This point illustrates the need to study past sea level changes, in an effort to aid predictions of future changes; it is this subject that my next post will cover.

References

[1] Allison, I. , N.L. Bindoff, R.A. Bindschadler, P.M. Cox, N. de Noblet, M.H. England, J.E. Francis, N., Gruber, A.M. Haywood, D.J. Karoly, G. Kaser, C. Le Quéré, T.M. Lenton, M.E. Mann, B.I. McNeil, A.J. Pitman, S. Rahmstorf, E. Rignot, H.J. Schellnhuber, S.H. Schneider, S.C. Sherwood, R.C.J., Somerville, K. Steffen, E.J. Steig, M. Visbeck, A.J. Weaver, (2009) The Copenhagen Diagnosis, Updating the World on the Latest Climate Science. The University of New South Wales Climate Change Research Centre (CCRC), Sydney, Australia.

[2] Inter Governmental Pannle on Climate Change (2001) IPCC Third Assessment Report: Climate Change 2001 (TAR) [online]. Available at: http://www.grida.no/publications/other/ipcc_tar/ [02.03.2010] 

[3] IPCC, (2007) Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC, Geneva, Switzerland.

[4] Rhamstorf, S (2007) A Semi-Empirical Approach to Projecting Future Sea-Level Rise. Science 315, 368

[5] Domingues, C.M., J. A. Church, N. J. White, P. J. Gleckler, S.E. Wijffels, P.M. Barker & J.R. Dunn, (2008) Improved Estimates of Upper-ocean Warming and Multi-decadal Sea-level Rise. Nature, 543, 19

[6] Rahmstorf, S. A. Cazenave, J.A. Church, J.E. Hansen, R.F. Keeling, D.E. Parker, R.C.J. Somerville (2007) Recent Climate Observations Compared to Projections. Science, 316, p.709

[7] Rignot, E. (2006) Changes in ice dynamics and Mass Ballance of the Antartic Ice Sheet. Physolophycal Transitions of the Royal Society, A. 364, 1637-1655

[8] Velicogna, I., (2009) Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters 36, L19503.