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]