1 SEA LEVEL VARIABILITY AND COASTAL ...

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climate change is only a particular ~1100 periodic cycle of the natural climatic .... the Roman Empire, the medieval warm period and the next one that should be.
SEA LEVEL VARIABILITY AND COASTAL EVOLUTION Miguel A. Losada1, Asunción Baquerizo, Juan M. Santiago, Alberto Ávila, Isabel Moreno and Miguel Ortega-Sánchez Sea level variability and associated coastal evolution is analyzed through historical “facts” of the Spanish coasts. Those facts suggest that the usually named human-induced climate change is only a particular ~1100 periodic cycle of the natural climatic variability. This works shows that the last maximum was reached in the XII century and, accordingly, we should be presently beginning a new warming that will reach the maximum around the XIV century, with expected amplitude for the Spanish coast, at least, between 2-3m higher than the present mean water level. Applying Bruun’s rule, the coastal evolution implications would be shoreline retreat, occupation of estuaries and general management problems.

INTRODUCTION

This work is based on some open topics that are being discussed since last decades, and that can be summarized in the following items: 1. “Forced” climate change: since the middle of last century the sea level is raising at a higher rate, and different factors indicate a general Earth climate warming. Since the general definition of climate change is “any long-term significant change in the average weather that a given region experiences”, the international community established that we are starting a period of a climate change. Accordingly, it is generally being accepted that the human activities are producing an increase in the average measured temperature of the Earth's near-surface air and oceans since the mid-20th century; thus, human activities and its projected continuation are the main source of climate change. 2. On the contrary, some authors consider that this climate change is just a particular cycle of the natural climatic variability or, at least, that both processes can be simultaneous and compatible. 3. Current studies also indicate that radiative forcing by greenhouse gases is the primary cause of global warming. However, we will show that the same pattern was observed in the past without C02 effect. We do not neglect its influence, but suggest that the same pattern would be produced without C02 greenhouse effect, although it can reinforce the variability. 4. This work shows historical sea level variability data over the last few millennia indicating that the usually named “human-induced climate change” shows the same climate variability patterns observed during last 8000 years.

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Grupo de Dinámica de Flujos Ambientales, CEAMA-University of Granada, Avda. del Mediterráneo, s/n, Granada, Spain

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2 The knowledge and estimations about the future sea level variability are important for coastal management. Coastal evolution during last centuries has been mainly controlled by a slightly and maintained sea level raised with superimposed oscillations of different temporal scales. According to Bruun’s rule (Bruun, 1962), the beach erodes or accretes, and morphological features are generated or destroyed. Figure 1 shows the present configuration of a Spanish coastal lagoon that was massively built up during last century. The environment is clearly not sustainable with the predictable sea level rise that would be produced during the following centuries.

Figure 1. Example of a coastal lagoon and barrier at Spain (left image). Its final morphology was achieved in XVIII century. Since then, massive touristic development took place (right images).

HISTORICAL EVOLUTION

Along the history different climatic cycles of different amplitudes occurred (Masselink and Hughes, 2003). Particularly, during the late Pliocene the Earth has seen different cycles of ice sheets advancing and retreating on 40,000 and 100,000-years time scales. Those periods are usually named glacials (glacial advance) or interglacials (glacial retreat); we are currently in an interglacial. The main consequence from the last ice age (18000 years ago) is that the sea level has risen about 130m (so-called transgressive conditions). Figure 2 shows a general scheme of the amplitude of the climatic cycles, that were first studied by Milankovitch (1941), and that are mainly due to the Earth’s movements. It can be observed that the larger amplitude cycles correspond to the cycle length of 100.000 years, whereas there are other smaller amplitudes corresponding to smaller lengths. From an engineering point of view, it is interesting to establish

3 how important are those smaller cycles on the evolution of the sea level. This will be further addressed.

Figure 2. Amplitude of the climatic cycles (Milankovitch, 1941).

Figure 3. Changes on the sea level since the end of the last glacial episode (from http://www.wikipedia.org).

4 Figure 3 shows the evolution of the sea level since the end of the last glacial episode. It is based on collected data from various reports and adjusted them for subsequent vertical geologic motions, primarily those associated with postglacial continental and hydroisostatic rebound (Fleming et al. 1998, Fleming 2000, Milne et al. 2005). The first refers to deformations caused by the weight of continental ice sheets pressing down on the land, the latter refers to uplift in coastal areas resulting from the increased weight of water associated with rising sea levels. It should be noted that because of the latter effect and associated uplift, many islands, especially in the Pacific, exhibited higher local sea levels in the mid Holocene than they do today. Uncertainty about the magnitude of these corrections is the dominant uncertainty in many measurements of sea level change. The black curve is based on minimizing the sum of squares error weighted distance between this curve and the plotted data. It was constructed by adjusting a number of specified tie points, typically placed every 1 kyr but at times adjusted for sparse or rapidly varying data. A small number of extreme outliers were dropped. It should be noted that some authors propose the existence of significant short-term fluctuations in sea level such that the sea level curve might oscillate up and down about this ~1 kyr mean state. Others dispute this and argue that sea level change has largely been a smooth and gradual process. Thus, there have been fierce debate regarding the fine detail of the last part of the curve (Masselink and Hughes, 2003). As exposed before, there are two main theories: Fairbridge (1961) supported that there are significant fluctuations over the mean sea level, whereas Jelgersma (1961) and Shepard (1964) supported that there exist a gradual and smooth rise of the sea level. The disagreement is generally ascribed on the difficulties associated to the separation of eustatic from isostatic effects. This is important for coastal engineers, since they generally estimate coastal evolution assuming a gradual and smooth sea level rise. The actual debate on climate change reopens the question: Monotonic sea level rise? Vs Fluctuations over a mean value? RECENT EVOLUTION (LAST ~3000 YEARS)

This work supports the oscillation theory. Thus, during those last ~3000 years we would have suffered oscillations around the stabilized sea level with a temporal scale of ~1100-1300 years, and with amplitudes that depends on the location, but normally vary between 1-3.5m. According to this theory, the last maximum was achieved in the XII century; thus, we should expect that the next maximum would be produced in the XXIV century. However, there are no clear historical evidences of such cycles. Spain is located facing to both the Mediterranean Sea and the Atlantic Ocean, and was occupied by different civilizations along the history. We can find historical records of such occupations near the coast that can provide facts about those climatic oscillations with a temporal scale of aprox. 1100 years.

5 Spanish Historical Facts

Celts were a diverse group of tribal societies in Iron Age Europe that were in Spain around the 6th century B.C. Figure 4 shows the rest of Celtic Castro, a fortified pre-Roman Celtic village, usually located on a hill or some naturally easy defendable place. This image is from our present time. As it can be appreciated, the sea level should have been lower during their using time, otherwise they would have been easily flooded by the sea action. Thus, this can be considered as a fact of the lower sea level during this past time.

Figure 4. Rest of a Celtic Castro located in the north of the Spanish coast.

If we moved to the Roman Empire period (around 1 century B.C. to the 2 century A.C), there are many evidences of their constructions in the southern Spanish coast. Figure 5 shows the rest of a fishing factory that is at the present time too far onshore and approximately 5.5m over the mean sea water level. Also, the remains of a foundation of a pier are now emerged. Accordingly, those facts evidence that, during this time, the sea level should have been higher that it is now.

Figure 5. Present location of the rest of a fishing factory (left image) and rests of the city wall and the foundation of a pier (right image) at Carteia (Algeciras, Cádiz)

6 One of the best preversed Spanish roman ruins close to the shore are Baelo Claudia (Trafalgar Coast, Cádiz). The life of the inhabitants reached its greatest splendor during the 1st century BC and the 2nd century AD. In the middle of the 2nd century, however, the town suffered a great tidal wave which wiped out a large part. Figure 6 shows some of those rests; it is of particular interest the relative position of the fishing factory, which is too high and far onshore with respect to the present mean sea water level.

Figure 6. Rest of the main square of the city (left image) and location of the rest of the fishing factory at Baelo Claudia (Cádiz)

If we move to the XII century, usually known as the “medieval warm period” due to the increase in the relative Earth temperature, there are rest of tidal mills at the southern Spanish coast (figure 7). The location of those mills is at the present moment too far onshore and many of them are not affected by the tidal flow. Accordingly, for those mills to be operative, the sea level should have been higher during this past period.

Figure 7. Map with the present configuration of the coastline, over which the location of tidal mills existing in the XII century are indicated with numbers.

7 Reconstructed sequence and amplitude estimations

Considering the facts that have been shown and our historical knowledge, the following main historical sequence can be reconstructed: 1. 700-650 b.C: Migration of civilizations from middle-northern Europe to the Mediterranean area; sea level below the present; cold climate 2. 100-50 b.C: Beginning of Roman Empire. Historical development in northern Europe. Warm climate, higher sea level 3. 400-550: Ending of Roma Empire; migration of northern people to the region; cold climate; sea level below present 4. 1100-1150: Medieval warm period; cultural development in central Europe; nordic cultures develop; higher sea level 5. 1700-1850: Little ice age; technical development, cold climate; lower sea level 6. 1920: Warm period, increasing sea level 7. 2300? Prediction of the maximum? amplitude? This is schematically represented in figure 8: the curves maximum correspond to the Roman Empire, the medieval warm period and the next one that should be achieved in the XXIV century.

Figure 8. Last three 1100 years period climatic cycles (left graphs) that should define the fine form of the curve (right scheme).

From the previous data we can establish that cold periods persisted ~ 250300 years, warm periods persisted ~ 100-150 years and that the ascending or descending transitions persisted ~ 100-150 years. Thus, warm periods are pulses

8 every ~ 1100 years superposed a stabilized sea level (evidences for the Iberian Peninsula). Regarding the amplitude estimations, from the historical evidences that we found at the Spanish Coasts, the amplitude is between 2 and 3m with respect to the present mean sea water level that stabilized around 7000 years ago. There are general difficulties to properly estimate those values, particularly due to the subsidence and the human activities that can be presently modifying (increasing) the climate variability. Those data should be carefully applied to other places, since the amplitude depends on the local conditions; thus, only near geographical places with similar conditions data should be grouped. Regarding the influence of those results on the coastal evolution, the present shoreline shows a morphology associated to the last sea level fall (millenary scale) and that has been generated over the new coastline that formed about 8000 years ago due to the sea level rise after the last ice age. This morphology varies depending on the morphological response time, the celerity of the sea level changes and the sediment availability. Over this general behavior, century or annual oscillations can induce some additional effects. CONCLUSIONS

This work studied the sea level variability and its possible influence on the coastal evolution with historical facts found at the Spanish coasts. From those facts and with our historical background, we suggest that the named “forced climate change” has the same patterns observed during last 9000 years: over a constant or slightly increasing sea level position reached after the last ice age, there exist climatic oscillations of different scales. It is probable that the Earth begins a warming period since last 50 years, and the sea level is rising due to the natural climatic variability. If the historical cycle repeats, sea level at the Spanish Coast should be 2-3m higher by ~2300. Future coastline evolution should be affected by those results. Just applying the Bruun’s rule (Bruun, 1962) we will suffer occupation of estuaries, shoreline retreat and serious management problems. This work also intend to be used as a confirmation of the future sea level rise, pointing out that this rise would be faster and higher that the general expectations, even if we stop the human-induced climate warming.

REFERENCIAS

Bruun, P. 1962. Sea-level rise as a cause of shore erosion. Proceedings of the American Society of Civil Engineers, Journal of the Waterways and Harbors Division 88 (1962), pp. 117–130. Fairbridge, R. 1961. Eustatic changes in sea level. Physics and Chemistry of The Earth, vol. 4, pp. 99-185. Fleming, K, P. Johnston, D. Zwartz, Y. Yokoyama, K. Lambeck and J. Chappell. 1998. Refining the eustatic sea-level curve since the Last Glacial

9 Maximum using far- and intermediate-field sites. Earth and Planetary Science Letters 163, 327-342. Fleming, K. M. 2000). Glacial Rebound and Sea-level Change Constraints on the Greenland Ice Sheet. Australian National University. PhD Thesis. Jelgersma, S. 1961. Holocene sea level changes in the Netherlands. Meded. Geol. Sticht., Ser. C IV 7, 1–101. Masselink, G. and M.G. Hughes. 2003. Introduction to coastal processes and geomorphology. Hodder Arnold, London. Milankovitch, M. 1998. Canon of Insolation and the Ice Age Problem. Agency for Textbooks, 636pp. Milne, G. A., A. J. Long and S. E. Bassett. 2005. Modelling Holocene relative sea-level observations from the Caribbean and South America. Quaternary Science Reviews 24, 1183-1202. Shepard, F.P. 1964. Sea Level Changes in the Past 6000 Years: Possible Archeological Significance. Science, 143 (3606), 574 - 576

10 KEYWORDS – ICCE 2008 SEA LEVEL VARIABILITY AND COASTAL EVOLUTION Miguel A. Losada, Asunción Baquerizo, Juan M. Santiago, Alberto Ávila, Isabel Moreno and Miguel Ortega-Sánchez Abstract 157 Sea level Climate change Climatic variability Coastal evolution Historical facts