Poster Created for Presentation

The following academic poster was created for a mid way presentation of my blog ideas and progression. The poster covers the topics of satellites, alternative forms of measurement, the findings and hints at the direction the blog will take. Please click on the image to open it to zoom for closer inspection.

What to do with it all?

Sea level rise and the resultant effects on human life developed into a widely discussed and hotly debated issue, during the late 20th and throughout the 21st century. 

The need for such debates to reach a consensus of action is clear, with events such as hurricane Katrina creating 28 foot high storm surges [1] and causing 3,560 miles of Levee failure, along the Mississippi River and canal systems of New Orleans [2]. Katrina death toll estimates range between 1,600 and 2,000 and the area suffered huge biodiversity loss and industrial decline as a result of the flood. Such impacts have all been identified as avoidable, if flood protection had been built correctly and areas of land were allowed to return to their natural salt marsh environments [3]. 

As sea level rise and flooding have the potential to accelerate to the highest levels of prediction made to date [4], decisions concerning flood defence must be held sooner rather than later. This post will document some of the main choices available to policy makers and hopefully shed some light on the options of future flood defence. 

The Options

Figure 1: 5 Policy Options for Dealing with Sea Level Rise [5]

As figure 1 shows, there are five generic options for sea defences, all of which will be explained henceforth. 

Option 1: Do Nothing

As the name suggests, this technique allows the sea to engulf the land as it rises, with no resistance from man made constructions. This option is taken when the economic value of the land does not justify spending money to save it [6]. This option is considered environmental friendly, but does involve the handing over of national land to the elements, which could prove to be a very hard pill for some governments to swallow. 

Option 2: Managed Realignment 

As figure one shows, this form of management allows a certain area of the land to become an intertidal mudflat or salt-marsh, through the removal of a sea wall. The extent of the land loss is controlled by a new, spatially depressed wall. 

An example managed realignment at work can be seen on Wallasea island, in the Thames Estuary, where six breaches in the current sea wall allowed the creation of 115 hectares of wetland, at a cost of £8 million [7]. As well as providing a habitat for a vast selection of fauna and flora, the land can act as a natural flood defence, if a storm surge was to occur along the Thames Estuary again, making the realignment economically, socially and environmentally viable. Please see the following video for more information on the benefits of the move [7]. 

Of course, there are issues of land ownership, which will inevitably get in the way and increase the cost of any realignment that is proposed. The simple solution is to pay the land owners an equal amount to what they would expect to get from the land within a determined time frame, however such issues require delicate negotiation and should not be approached in a brash manner.

Option 3: Hold the Line

Such an approach is adopted when the consequences of letting the sea inundate the land are too economically damaging, socially unpopular, or environmentally harmful. The defences, such as sea walls and groynes, are maintained and improved, in an effort to keep the sea at bay and protect the land behind from flooding. This option is the traditional choice for European flood defence [8]; as it has been mentioned before, the option of handing over land to the sea has never been overly popular with politicians and landowners alike. 

The process of maintaining flood defences does however have one major draw back; the loss of intertidal mudflats, salt-marshes, beaches and other shoreline habitats. Landward migration of such ecosystems is prevented by the coastal defence (figure 2), removing natural habitats and raising issues of ethics and environmental sustainability. 

Figure 2: What a Conundrum - Coastal Squeeze [9]

This potentially ecologically catastrophic process is the subject of much debate. The final line falls between the economic drive to maintain anthropogenically populated land and the ecological desire to maintain natural habitats, a debate that seems to run throughout modern development policies.

Option 4: Move Seaward

The choice to tackle erosion through the deposition of sediment in the sea and the artificial creation of land is a largely anthropocentric practice. The decision to move seaward is usually is driven by the want to provide nice beaches for tourism or protect economically valuable land with an extended, humanly constructed area. 

Two examples of such an approach can be seen in Koge Bay, Denmark and the Slovenian Coast, Slovenia (figures 3 and 4 respectively) 

Figure 3: Koge Bay, Denmark. Red line indicates areas of reclaimed land. [10]

Figure 4: Slovenian Coast, Slovenia Reclaimed land around edge of headland [11]

Such an approach to sea level rise seems counter intuitive and stands a high change of resulting in failure, high economic cost and rapid land loss, is sea levels were to rise above the vertical height of the humanly created land. 

For more information on both of the projects mentioned, please head to the websites below:

Koge Bay: http://www.eurosion.org/shoreline/6kogebay.html

Slovenian Coast: http://www.eurosion.org/shoreline/46sloveniancoast.html

Option 5: Limited Intervention

Again adopted in areas of low economic importance, this option solves the problem of sea level rise, to an extent that prevents the loss of more important land further inland. The near coast is usually allowed to develop into salt marshes and intertidal mud flats, in the hope that these will provide protection for more affluent areas, inland. 

Finding Direction in a World of Possibilities


Whilst it is undeniable that some areas of coast line, such as the aforementioned Netherlands, need hard, hold the line defences, to protect areas from rapid and devastating inundation, other areas of the world are currently in a more flexible position. Acknowledgement of such a fact could allow management to find an equilibrium between careful release of land to the seas, and the protection of large human conurbations from flooding. 


The options of management should be decided after the specific study of varying lengths of differently affected  coastline. This point is supported when the various isostatic sea level changes of the United States of America and the United Kingdom are considered:


The USA
New York: ~ 3mm/year rise, caused by eustatic sea level rise and small scale isostatic subsidence
Texas: ~6mm/ year rise, caused by substantial isostatic sinking, due to oil and groundwater extraction
North Oregon: near constant , up lift and sea level rise are nearly equal
Alaska: Sea level fall, caused by rapid isostatic, glacial rebound 


The UK
North, e.g. Aberdeen: <0.5 mm/year rise, cause by isostatic, glacial rebound
South, e.g. Thames Estuary: 1.9 mm/ year subsidence 


Such differences make the use of various approaches applicable along a single coastline; a nation-wide Shoreline Management Plan is a blindingly obvious, unsustainable option. Fortunately this has been realised in modern defence planning for example, England's current Shoreline Management Plan is divided along the countries littoral (sediment) cells. These cells are known to have little cell-to-cell interaction, allowing unique management to occur in each, with little worry of any knock-on effects (figure 5).

Figure 5: UK Littoral Cells [12]

As the predictions of future sea level rise are full of uncertainties, due to various methods of measurement, misunderstanding of cause and effects and the unknowns of future anthropogenic influences on the earth, decisions made today must be flexible and spatially variable, allowing adaptation to any future changes. Management schemes should reach a balance between the all out defence of some areas, say of high economic value, and the unpopular, but seemingly necessary option of managed realignment and limited intervention. If such a balance is not made and humans decide to go to war with the sea, the chances of success against such a relentless opponent are slim to none. 


Whilst the options available to Shoreline Management Plan policy makers are varied, one option we seem not to have available is whether or not to take action. The effect of storm surges, rising sea levels causing erosion and tsunamis are evident across the globe, with the economic, social and environmental cost of restoration after an event far exceeding that of pre-event defence construction and managed land loss.


It seems therefore, logical to prepare for the worst predictions made; those created from the observations of the Ocean Surface Topography Mission, conducted by NASA and CNES , which were publicised in the 2009 Copenhagen Diagnosis. As has been seen before, in events such as Hurricane Katrina, failure to prepare for the maximum level event leaves human populations open to wide scale disruption and destruction. However whilst Hurricane Katrina's effects were confined to New Orleans and the Louisiana state, the effects of maximum sea level rise could stand cause global scale destruction, a point that should encourage a  well planned, integrated response to the risk of future sea level rise. 


References

[1] D, Willie, 2005. Hurricane Katrina Pulls Its Punches in New Orleans [online]. Available at: http://news.nationalgeographic.com/news/2005/08/0829_050829_hurricane.html [01.05.2011]

[2] I.L. van Heerden, 2007. The failure of the New Orleans levee system following Hurricane Katrina and the pathway forward. Public Administration Review 67, 24-35

[3] Sweet, W. (2007) Protecting the big easy from the next big one. IEEE Spectrum  44(3), 10-12

[4] 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.

[5] Blogger.com, 2008. Coastal Zone Management [online]. Available at: http://www.jamo-czm.blogspot.com/ [01.05.2011]

[6] West Dorset District Council, n/d. Consideration of the Risk and Coastal Defence Options [online]. Available: http://www.dorsetforyou.com/2072 [01.05.2011]

[7] Geographical Association, 2009. Wallasea Island Case Study: How did the Wallasea Island Project Develop? [online]. Available at: http://www.geography.org.uk/resources /flooding/wallaseaisland/development [01.05.2011]

[8] Parliament Office of Science and Technology, 2009. Coastal Management, [online]. Available at: http://www.parliament.uk/documents/post/postpn342.pdf [01.05.2011]

[9] L. Carl, 2010. The New Ecology of Risk: Balancing Flood Risk Mitigation with Economic Development in the Humber Region [online]. Available at: http://www2.hull.ac.uk/science/ geography/prospective_students/phd/research_students/lewis.aspx [01.05.2011]

[10] P. Sistermans & Nieuwenhuis, O (n/d). Koge Bay, Denmak, [online]. Available at: http://www .eurosion.org/shoreline/6kogebay.html [01.05.2011]

[11] M. Vahtar, n/d. Slovenian Coast, Slovenia [online]. Available at: http://www.eurosion. org/shoreline/46sloveniancoast.html [01.05.2011]

[12] Coasts and Country Projects Limited, n/d. What is a Shoreline Management Plan? [online]. Available at: http://www.coastandcountryprojects.co.uk/whatsmp.html [02.05.2011]

Current Coastal Flood Defences From Around the World

Coastal flooding is not a new problem and so some defences are already in place to protect human settlements from inundation; this post documents two of the most interesting examples.

Thames barrier, London

Opened in 1982, the Thames barrier is one of the worlds largest movable flood barrier [1], the 520 meter barrier spans the River Thames and protects some 125 square kilometres of London from flooding [1]. 

Such flooding would be the result of a storm surge travelling up the river Thames, the type of which hit London, during the Great North Sea Flood of 1953 and was the actual cause of the construction of the Thames Barrier. The North Sea Flood killed a total of 307 people across the South East of the UK [2], a death toll that would have been much higher, had the force of the wave not been deflected by the Netherlands, on its way to the Thames Estuary. Please see the following video, for a BBC view on the matter.

The use of the barrier, which cost £0.5 billion pound to construct and consists of 10 steel gates that can be raised to prevent high tides and storm surges from inundating London [1], has been increasing on a annual basis, since it became operational [3] (figure 1).

Figure 1: Number of annual closures of the Thames Barrier to protect London [3]

As can be seen from figure 1, apart from a few years of lag, the number of closures have been generally increasing since the beginning of operations. Such a trend raises questions of the barriers ability to protect London in the future and what, if any changes should be made to the protection of the UK's capital. The next post will discuss such matters. 

The Netherlands

The Netherlands is in a unique and hugely compromising physical position, in relation to the sea. 26% of the country is below sea level, 70% of the country would be flooded without coastal defence and 70% of the GDP of the Netherlands is produced in areas below sea level [4]. It is because of this, that the Netherlands coast is lined with man made dykes, sea walls, coastal dams and flood gates, in an effort to stop the sea. 

One example of such defences can be seen in the village of Petten, which is currently protected from the sea by a 13 meter high and 46 meters thick sea wall [5]. This wall has been increased in height by the Dutch authorities, since its construction in 1976 [5] and is another example of the relentless and increasing threat of rising sea levels.

A second example of Dutch coastal flood defences can be seen in the city of Rotterdam; a city that has to compromise between maintaining a prosperous port and preventing the flooding of a vulnerable area. A resolution of this problem was found in a swinging gate system that can be closed across the shipping channels when a storm surge is approaching the city. 

Please see the following video, for a detailed explanation of the system and altered channel floor, I have included Dutch subtitles, for my international fans. 

As the video explains, the efforts that have been taken to prevent flooding of human settlements have advanced, both technologically and in terms of costs. With sea level rise potentially accelerating, we must now ask ourselves how much longer we can hold off the advancing seas.  

The examples given during this post document the extreme lengths governments are willing to go to to protect areas of land from inundation. There is however a growing train of though, promoting managed realignment of coasts to allow natural defences to reform. The next post will document such thoughts and ask what changes, if any, should be made to the current system.

Reference List

[1] The Environment Agency, 2011. The Thames Barrier [online]. Available at:  http://www.environment-agency.gov.uk/homeandleisure/floods/38353.aspx [26.4.2011]

[2] Henry, E., 2007. Sever Flooding 'Could Put Lives at Risk' [online]. Available at: http://www.martinfrost.ws/htmlfiles/nov2007/1953flood.html [26.4.2011]

[3] King, D.A. ,2004. Climate change Science: Adapt, Mitigate or Ignore? Science. 303. 176-177

[4] Waterland Information Network, (n/d). Dutch Flood Control and Protection [online]. Available at: http://www.waterland.net/index.cfm/site/Water%20in%20the%20Netherlands/pageid/E3B3B416-FB4E-0AB8-2FB6E2B271F1BD6E/index.cfm [26.4.2011]

[5] Woodard, C., 2001. Netherlands Battens Its Ramparts Against Warming Climate [online]. Available at: http://news.nationalgeographic.com/news/2001/08/0829_wiredutch.html [26.4.2011]

Is Sea Level Rise Accelerating?

Figure 1: IPCC sea level rise projections and more recent satellite observations [1] Figure 1 portrays the root of the debate; recent satellite observations have place observed sea level rise in the uppermost echelons of the IPCC sea level rise predictions, at a gradient that indicates acceleration in sea levels during the late 20th century and throughout the 21st century. However, the debate is ongoing, with conclusions ranging according to the type and length of study undertaken and some claims of the impossibility of a global prediction of present and future sea level change [2]. The reasons for the continuing debate and the varying conclusions of sea level rise are outlined below. Varying Lengths of Study  An example of the problems with varying lengths of study can be seen with the use of tide gauge data to predict sea level rise. Short term tide gauge data can be easily affected by short term changes, due to weather events, such as El Nino and isostatic sea level change, caused by tectonic movement or glacial rebound [2]. This issue has led researchers to study tide gauge data of 60 years or more in length [3], in an effort to avoid any short term changes obscuring the general trend. However, this increased length of observation created the need to average each data set, in an effort to remove the noise of short term, tidal changes. Such an averaging has been found to obscure any acceleration that may be evident [4] and has led to the general assumption that there was no acceleration in sea levels, during the 20th century [4].  As well as the accusations of the loss of any meaningful data, there have also been claims that the study of tide gauge data alone is not long enough to identify any acceleration that may have occurred [4 & 5], this has forced other researchers to combine tide gauge data and satellite observations, an effort that has produced more intriguing results (figure 2).  Figure 2: Sea levels between 1880 and 2001, showing an increase in the rate of rise after 1930 [4]   Figure 2 shows the merits of studying longer periods of time and combining methods of measurement; the sharpening of the incline after 1930 may have gone unnoticed if the length of study was shortened or the data of both tide gauges and satellites were not used.  Ice Sheet and Glacier Knowledge Many of the predictions of accelerated sea level rise are based around future ice sheet and glacier contributions. Claims have been made that half the present sea level rise can be attributed to the melting of terrestrial ice [6] and that glacier and ice cap melting makes up 60% of the ‘new water’ component of sea level rise (1.8 mm/year) [7]. A second point regularly made in the literature concerns the rise in sea levels, as a result of temperature rise and increased ice melt [7 & 8]; sea levels have been predicted to increase by 3.4 mm/ year per degree Celsius increase in temperature [8]. Such claims may make more of an impact, if the uncertainties of ice sheet dynamics and the response to warming were not so wide ranging and well publicised.  Such uncertainties include a lack of understanding of ice sheet dynamics, polar ice thickness, depth of bed rock, speed of movement towards the sea, the potential of mountain glacier disappearance and Antarctic contribution to sea level rise [6 & 8]. Such a spectrum of unknowns has prevented the creation of solid conclusions of the future increase in sea levels and the potential of acceleration (figure 3). Papers are forced to conclude with very open statements, covering the possibilities of no change, continued present day speed and rapid sea level rise [5, 6, 7 & 8].  Figure 3: Ranging predictions of sea level rise by 2100, upper most and lowest predictions include error bars, further increasing the range of possibilities [5]   The compounding factors tested in each scenario of figure three and the uncertainties of ice sheet reactions to heating have caused sea level predictions for 2100 to range between 0.1 and 0.7 meters. Despite the uncertainties and on going debates of future acceleration and sea levels, the current evidence does point towards 21st century acceleration in increase and generally higher sea levels across the globe (figure 1). With the advent of satellite observation, the global coverage, quality and temporal resolution of observations are only going to increase, making any future predictions and observation much more reliable.  After consideration of the findings and in full knowledge of the problems with any predictions, we must now decide how to react to the chance of sea levels rise, the steps we are willing to take to protect ourselves and cities and what, if any, change we should make now, in an  effort to prevent the potential sea level rise.  The remaining posts will focus on such matters. 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] Douglas, D.C., M.S. Kearney, S.P. Leatherman eds. (2001) Sea Level Rise: History and Consequences, Volume 1 London: Academic [3] Douglas, B.C., (1997) Global sea rise: a redetermination. Surveys in Geophysics, 18, 279-292.  [4] Church, J.A., N.J. White (2006) A 20th Century Acceleration in Global Sea-level Rise. Geophysical Research Letters, 33, L01602 [5] Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. Van Der Linden, X. Dai, K. Maskell, C.A. Johnson eds. (2001) Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.  [6] Thomas, R., E. Rignot, G. Casassa, P. Kanagaratnam, C. Acun, T. Akins,H. Brecher, E. Frederick, P. Gogineni, W. Krabill, S. Manizade, H. Ramamoorthy, A. Rivera, R. Russell, J. Sonntag, R. Swift, J. Yungel, J. Zwally (2004) Accelerated Sea-Level Rise from West Antarctica. Science 306, 255  [7] Meier, M.F. M.B. Dyurgerov, U.K. Rick, S. O’Neel, W.T. Pfeffer, R.S. Anderson, S.P. Anderson,A.F. Glazovsky (2007) Glaciers Dominate Eustatic Sea-Level Rise in the 21st Century. Science 317, 1064    [8] Rahmstorf, S. (2007) A Semi-Empirical Approach to Projecting Future Sea-Level Rise. Science 315, 368

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

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