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The Mediterranean region under climate change

 | 
Jean-Paul Moatti
, 
Stéphane Thiébault

Part 1. Mechanisms, observed trends, projections

Sub-chapter 1.1.2. Improving knowledge on the climate and environmental context of past Mediterranean societies

Marie-Alexandrine Sicre, Paulo Montagna, Laurent Dezileau, Nathalie Combourieu-Nebout, Julien Azuara, Dominique Genty, Nejib Kallel, Bassem Jalali, Maria-Angela Bassetti et Laurent Li

Texte intégral

Introduction

1The Holocene, our present interglacial, has long been considered as a stable climate period. However, paleoclimate reconstructions produced in recent decades have shown that the Holocene climate has been variable at millennial to centennial time scales. Several centennial scale relapses of sustained cold and/or dry climate conditions have been documented superimposed on the long-term cooling observed in the extra-tropics since the beginning of the Holocene (Wanner et al. 2011). Some of these abrupt cold climate regimes would have been synchronous with societal collapses, such as that of Akkadian cultures (Weiss and Bradley, 2001). With the recent rise in global temperatures, more rapid and intense events including floods, droughts or heat waves have been observed and are likely to be more frequent with climate change and to have more severe societal impacts.

2Understanding natural climate variability and the role of human activities in current climate change requires long pre-instrumental records. Indeed, direct observations of climate variables (i. e. temperature, precipitation) are usually limited to the last hundred years, which is too short to put the observed variability into perspective to see how it compares with the range of pre-industrial variability. The shortness of instrumental records also limits our understanding of low frequency climate variability as well as of gradual trends or sustained periods of floods, droughts and storms. Reconstructions of the pre-industrial climate rely on indirect information based on climate sensitive bio-indicators and geochemical tracers, called proxies. The improvement of sampling techniques, the development of new proxies and more robust calibrations has indisputably led to major advances and produced high quality reconstructions of the recent past climate. Proxies have been commonly measured in continental archives such as tree-rings, speleothems or ice cores to estimate past temperatures and precipitation but it is only recently that marine sediments have been explored to produce decadal time scale records of the temperature of the surface ocean, the primary heat reservoir of the Earth (McGregor et al., 2015). Yet accurate dating to assess rates of changes or the synchronicity between records is still problematic. As a complement to sediments, corals yield discontinuous but best dated and seasonally/annually resolved reconstructions thanks to their growth rates that are much larger than sedimentation rates.

3The study of the last millennium climate is of particular interest because this period encompasses the most recent pre-industrial warm climate interval known as the Medieval Climate Anomaly followed by the coldest centuries of the Little Ice Age interrupted around 1850 by the industrial era. In parallel with major progress in generating proxy signals, model simulations of the last millennium climate using state-of-the-art coupled ocean-atmosphere models within the Coupled Model Intercomparison Project (CMIP) allow cross-analyses between proxy and model data to explore the physical mechanisms at play and the role of external factors like solar activity, volcanism, land use and greenhouse gases in climate variability. Nevertheless, comparison of proxy data with GCM model simulations is still a critical first step in linking global climate and regional signals with the information collected across site(s) reflecting the impact of climate change on ecosystems and the response of human societies.

Climate and extreme events in the recent past

4The Mediterranean Sea is a unique region to explore the complex interactions between climate, environment and human activity at decadal to millennial timescales (Roberts et al. 2011). Sea level rise and the expected increase in extreme climatic events such as coastal flooding and storm surges are major concerns for populations living in the coastal regions of the Mediterranean. Understanding the frequency of intense storms in the past several centuries to millennia is important to better predict future vulnerability and economic loss. Because extreme events are rare and therefore difficult to observe in a human lifetime, proxy reconstructions are essential to trace their recent history and place them in a longer temporal context.

5Within the framework of the international MISTRALS/PaleoMeX program, new approaches have been developed using lagoon sediments distributed across the western Mediterranean Sea (in France, Spain, Morocco, Algeria and Tunisia) to document the recurrence of floods and storms that have stressed the coastline in the past (Figure 1AB). Proxy data have shown that during the Little Ice Age (from ca. 1400 to 1850) storms were more intense and frequent, thus contrasting with the low storm activity in the Medieval Optimum. In lagoons in southern France, stratigraphic data revealed an increase in catastrophic category 3 or more storms during the second half of the Little Ice Age (Dezileau et al. 2011). Longer records covering the past thousand years indicate that enhanced storminess in the western Mediterranean coastal region was coeval with known cold periods in the North Atlantic Ocean and in Europe. During the past 100 years, no major intense storm has directly struck the western Mediterranean area, a situation that has resulted in inadequate policies plus the subsequent construction of buildings and infrastructure (dams, recent harbors) well within the zone of possible storm tide flooding. The population residing on the coast has increased by a factor of 10 since 1700 with a dramatic rise since the 1970s (Figure 1C) and the number of residential or business buildings now threatened by flooding has been rising and will continue to rise.

6The last few centuries, and notably the abrupt termination of the Little Ice Age and onset of industrial era warming saw a regime shift in the occurrence of storms along the coast of the western Mediterranean Sea. While we acknowledge that the Little Ice Age climate is different from the climate we are experiencing today, little is known about the future of extreme events in the context of sea level rise and warming Mediterranean surface waters. The mechanisms that cause regime shifts are still not sufficiently well understood to allow accurate predictions of extreme events, or to assess the risk of exposure of human populations in the context of rapidly increasing urbanization and tourism along the Mediterranean coast.

Figure 1
A. Aerial photograph of washover fans (Palavasian lagoons, France). B. The Uwitec platform used in the different lagoons to collect sediment cores. C. The resident population on the coast of the French part of the Mediterranean area has increased by a factor of 15 since 1709 with a dramatic increase since the 1970s. Today, 150 000 people live on the sandy barrier all year round. The area had a 0.2% average probability of being struck by one catastrophic storm per year in the last 2,000 years. This estimate was higher (2%) during the latter half of the Little Ice Age when the risk increased by a factor of 10. The last few centuries have seen a regime shift in the occurrences of storms crossing the coast in the northwestern Mediterranean area (Dezileau et al. 2011).

7The recent reconstruction of sea surface temperature in the NW Mediterranean Sea (Gulf of Lion) revealed strong decadal scale fluctuations (over ~1 °C) associated with cold extremes followed by steep warming of over ~1 °C during the Little Ice Age, i.e. prior to the industrial area (Jalali et al. 2016) (Figure 2A). ~1 The occurrence of the severe conditions is thought to be linked to North Atlantic blocking regimes leading to intensified cold Mistral winds blowing in southern France and the NW Mediterranean Sea. According to model simulations, low solar activity during the Little Ice Age created favorable conditions for blocking regimes. During the Medieval Climate Anomaly (1000 – 1200 AD) surface temperatures in the north-western Mediterranean Sea were ~1 °C lower than those at the end of the 20th century.

Figure 2
A. Changes in the sea surface temperature in the north-western Mediterranean Sea (Gulf of Lion) over the last 2,000 years derived from alkenones as a temperature proxy. B. Comparison between the proxy reconstruction and instrumental data over the last century.

8The steep rise of ~2 °C in temperature over the last century seen in the proxy reconstruction is in agreement with instrumental data (Figure 2B). This rate of warming is higher than the 0.6 °C observed in the Northern Hemisphere during the same time interval or during the last deglaciation in the western Mediterranean (ca 0.06 °C/century). The post-industrial warming reversal of the pre-industrial long-term cooling, which is also observed in the global ocean surface temperature (McGregor et al., 2015), emphasizes the influence of human activities on climate.

9During the MISTRALS/PaleoMeX project, changes in deeper water temperatures were estimated for the first time from proxy analyses conducted on precisely dated deep water corals collected at ~ 400 m in the central and western Mediterranean Sea using a remotely operated vehicle (ROV) (Figure 3AB). According to our results, temperatures were ~2 °C lower during the Little Ice Age but close to modern values in the 19th century. Annually resolved temperature reconstructions between 1950 and 2000 obtained from the shallow water coral Cladocora caespitosa in the Tyrrhenian Sea revealed an increase in sea surface temperature of ~+0.023 °C/year, which is in very close agreement with instrumental measurements and the alkenone sea surface temperatures in the Gulf of Lion (see above).

Figure 3
A. Living cold water corals Madrepora oculata and Desmophyllum dianthus growing on a deep-sea giant oyster Neopycnodonte zibrowii collected by a remotely operated vehicle (ROV) in the Sardinia Channel during the RECORD cruise in 2013.
B. Hunting for deep sea corals in the Mediterranean Sea on board the R/V Urania. The ROV is maneuvered from the research vessel down to the seafloor, where it searches for samples of coral that preserve the history of climate change.

10Increasing CO2 emissions do not only result in global warming. Penetration of CO2 into the ocean through gas exchange with the atmosphere causes the progressive acidification of both surface and deeper waters. New research on Mediterranean corals to evaluate changes in pH beyond the period of direct observations was also conducted as part of the MISTRALS/PaleoMeX project. Coral derived pH proxy time series over the past 50 years indicate an acidification rate of the Mediterranean surface waters of ~ -0.0014 pH units per year, which is similar to the trend calculated from measured seawater data between 1986 and 2001. This finding confirms the ability of corals to assess changes in the pH of the ocean since the increase in CO emissions needed to estimate the beginning of acidification of the Mediterranean Sea, which represents a major threat for marine calcifying organisms.

Hydroclimate and vegetation changes in the Mediterranean region

11Vegetation is closely linked to regional climate and the pollen produced by plants and tree species has been used by palynologists to reconstruct past air temperature and precipitation. Pollen records have shown that forests recolonized the whole of Europe including the Mediterranean about 10,000 years ago. Over the last 5,000 years, temperate forest progressively retreated northwards or upwards in altitude with the overall increase in dryness over the northern Mediterranean as recorded in southern France, Spain and Italy. In southern France, recurrent swings between beech and oak abundances in the Languedoc hinterland characterized this trend. In the same period in southern Tunisia, the desert progressively expanded (Figure 4AB).

Figure 4
A. Map showing the vulnerability of the west Mediterranean region to desertification (from the Natural Resources Conservation Service).
B. An Olive tree submerged by desert sands near Sebkha Boujmel (southern Tunisia).

12Examination of changes in vegetation through pollen assemblages preserved in the sediments clearly shows that long term dryness continues today over the western Mediterranean and even more significantly in the southern Mediterranean borderlands. The species composition of the vegetation cover has been impacted by human activity through the deforestation of northern Mediterranean area. The expansion of desert landscape along the southern rim of the Mediterranean Sea was also notably enhanced by farming, cultivation and grazing from the Bronze Age (4,000 cal BP) to the end of the Iron Age (around 2,000 cal BP), and has further intensified since the beginning of the 20th century (Azuara et al. 2015; Jouadi et al. 2016). The hydrological activity of major Mediterranean rivers was also deeply modified, in particular during deforestation phases when high rates of erosion were revealed by sediment fluxes and by the subsequent increase in the occurrence of floods, as evidenced in the Rhone River basin (Bassetti et al. 2016). Modeling studies using pollen data estimated a decrease of about 50% in the arboreal pollen input in the western Mediterranean lowlands between 6,000 years ago and today (Collins et al. 2012). Furthermore, land registers indicate low forest cover (3% to 13%) in southern France at the end of the 19th century (Koerner et al., 2000). Quantifying vegetation changes is critical to evaluate feedback mechanisms on climate and their consequences for the environment.

13Information about past continental temperature and precipitation is also provided by speleothems that form in caves from precipitation waters depending on the location and the weathering of the host rocks. Oxygen isotopes are the most commonly analyzed chemical in speleothems to derive environmental and climatic factors. In some caves, climate information can be linked to human occupation, for example in Gueldaman cave in northern Algeria, one of the few examples in which it is possible to link human history and climate. In this cave, archaeological layers contained numerous prehistoric remains including pottery, bones and charcoal dated by radiocarbon dating (Kherbouche et al. 2014) (Figure 5). Stable oxygen isotopes in the stalagmites in the cave revealed a drought that lasted several centuries between 4,400 and 3,800 years ago, leading to the abandonment of the cave around 4,403 years ago after several thousand years of occupation. This study provides an example of the role that climate may have played in societal reorganization. Several stalagmites that grew in the cave during the Holocene indicate other periods of past climate variations that appear to be synchronous with human occupation.

Figure 5
Comparison of evidence of ancient human occupation and past climate change in Gueldaman Cave (from Ruan et al. 2015).

14While the current climate change has encouraged interdisciplinary collaboration to better understand the role that climate may have played in the development of past Mediterranean societies, effective collaboration between disciplines is still a challenge (Izdebski et al. 2016). The study of the occupation of Gueldaman Cave can be seen as a first step towards more integrated efforts between scientists, archeologists and historians the MISTRALS/PaleoMeX project is trying to promote, in the knowledge that, to be efficient, interactions need to be thought out and discussed at the very early stage of the process of the co-construction of the research project. Identifying target sites and designing the strategy has been the guideline for the second phase of MISTRALS/PaleoMeX science plan structured in Transects to favor exchanges between fields of expertise.

Bibliographie

References

Azuara, J., Combourieu-nebout, N., Lebreton, V., Mazier, F., Müller, S. D., Dezileau, L. (2015)
Late Holocene vegetation changes in relation with climate fluctuations and human activity in Languedoc (southern France), Climate of the Past, 11, 1769-1784, doi: 10.5194/cp-11-1769-2015, 2015.11, 1769-1784, 2015, 11, 1769-1784.

Bassetti M. A, Berné S., Sicre M. A., Dennielou B., Alonso Y., Buscail R., Jalai B., Hebert B., Menniti C. (2016)
Holocene hydrological changes in the Rhone River (NW Mediterranean as recorded in the marine mud belt, Climate of the Past, doi: 10.5194/cp-2016-8.

Collins, P. M., Davis, B. A., Kaplan, J. O. (2012)
The mid&# 8208; Holocene vegetation of the Mediterranean region and southern Europe, and comparison with the present day. Journal of Biogeography, 39(10), 1848-1861.

Dezileau L., Sabatier, P., Blanchemanche, P., Joly, B., Swingedouw, D., Cassou, C., Martinez, P., Van Grafenstein, U. (2011)
Increase of intense storm activity during the Little Ice Age on the French Mediterranean Coast. Palaeogeogr., Palaeoclimatol., Palaeoecol., 299, 289-297.

Izdebski, A., K. Holmgren, E. Weiberg, S.R. Stocker, U. Büntgen, A. Florenzano, A. Gogou, S. A.G. Leroy, J. Luterbacher, B. Martrat, A. Masi, A.-M. Mercuri, P. Montagna, L. Sadori, A. Schneider, M.-A Sicre, M. Triantaphyllou, E. Xoplaki (2016)
Realising consilience: how better communication between archaeologists, historians and geoscientists can transform the study of past climate change in the Mediterranean, Quaternary Science Review, 136, 5-22.

Jalali B., Sicre M.-A., Bassetti, M.-A. Kallel N. (2016)
Holocene climate variability in the North-Western Mediterranean Sea (Gulf of Lions), Climate of the Past, 12, 91-101, 2016, doi: 10.5194/cp-12-91.

Jaouadi, S., Lebreton, V., Bout-Roumazeilles, V., Siani, G., Lakhdar, R., Boussoffara, R., Dezileau, L., Kallel, N., Mannai-tayech, B., Combourieu-nebout, N. (2016)
Environmental changes, climate and anthropogenic impact in south-east Tunisia during the last 8 kyr, Climate of the Past, 12, 1339-1359, doi: 10.5194/cp-12-1339-2016..

Kherbouche F., Hachi S., Abdessadok S., Sehil N., Merzoug S., Sari L., Benchernine R., Chelli R., Fontugne M., Barbaza M., Roubet C. (2014)
Preliminary results from excavation at Gueldaman Cave GLD1, Quaternary International, 320, 109-124.

Koerner, W., Cinotti, B., Jussy, J. H., Benoît, M. (2000)
Evolution des surfaces boisées en France depuis le début du XIXe siècle: identification et localisation des boisements des territoires agricoles abandonnés. Revue forestière française, 52 (3), 249-270.

McGregor, H. V., M. N. Evans, H. Goosse, G. Leduc, B. Martrat, J. A. Addison, P. G. Mortyn, D. W. Oppo, M.-S. Seidenkrantz, M.-A. Sicre, S. J. Phipps, K. Selvaraj, K. Thirumalai, H. L. Filipsson, V. Ersek (2015)
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What drives social collapse? Science, 291:609-610.

Table des illustrations

Légende Figure 1A. Aerial photograph of washover fans (Palavasian lagoons, France). B. The Uwitec platform used in the different lagoons to collect sediment cores. C. The resident population on the coast of the French part of the Mediterranean area has increased by a factor of 15 since 1709 with a dramatic increase since the 1970s. Today, 150 000 people live on the sandy barrier all year round. The area had a 0.2% average probability of being struck by one catastrophic storm per year in the last 2,000 years. This estimate was higher (2%) during the latter half of the Little Ice Age when the risk increased by a factor of 10. The last few centuries have seen a regime shift in the occurrences of storms crossing the coast in the northwestern Mediterranean area (Dezileau et al. 2011).
URL http://books.openedition.org/irdeditions/docannexe/image/22983/img-1.jpg
Fichier image/, 211k
Légende Figure 2A. Changes in the sea surface temperature in the north-western Mediterranean Sea (Gulf of Lion) over the last 2,000 years derived from alkenones as a temperature proxy. B. Comparison between the proxy reconstruction and instrumental data over the last century.
URL http://books.openedition.org/irdeditions/docannexe/image/22983/img-2.jpg
Fichier image/, 152k
Légende Figure 3A. Living cold water corals Madrepora oculata and Desmophyllum dianthus growing on a deep-sea giant oyster Neopycnodonte zibrowii collected by a remotely operated vehicle (ROV) in the Sardinia Channel during the RECORD cruise in 2013.B. Hunting for deep sea corals in the Mediterranean Sea on board the R/V Urania. The ROV is maneuvered from the research vessel down to the seafloor, where it searches for samples of coral that preserve the history of climate change.
URL http://books.openedition.org/irdeditions/docannexe/image/22983/img-3.jpg
Fichier image/, 160k
Légende Figure 4A. Map showing the vulnerability of the west Mediterranean region to desertification (from the Natural Resources Conservation Service).B. An Olive tree submerged by desert sands near Sebkha Boujmel (southern Tunisia).
URL http://books.openedition.org/irdeditions/docannexe/image/22983/img-4.jpg
Fichier image/, 261k
Légende Figure 5Comparison of evidence of ancient human occupation and past climate change in Gueldaman Cave (from Ruan et al. 2015).
URL http://books.openedition.org/irdeditions/docannexe/image/22983/img-5.jpg
Fichier image/, 190k

Auteurs

Palaeoenvironment, Sorbonne Universités, (UPMC, Univ. Paris 06)-CNRS-IRD-MNHN, LOCEAN, Paris, France.
Marie-alexandrine.sicre@locean-ipsl.upmc.fr

ISMAR, CNR, Italy

Paleoclimatology-Geochemistry, Geosciences Montpellier, UMR 5243, Montpellier 2 University, CNRS, Montpellier, France
laurent.dezileau@gm.univ-montp2.fr

Palynology, UMR 7194 CNRS, Histoire naturelle de l’Homme Préhistorique, Département de Préhistoire, Muséum national d’Histoire naturelle, Institut de Paléontologie humaine, Paris, France.
nathalie.nebout-combourieu@mnhn.fr

UMR 7194 CNRS, Histoire naturelle de l’Homme Préhistorique, Département de Préhistoire, Muséum national d’Histoire naturelle, Institut de Paléontologie humaine, Paris, France.
julien.azuara@mnhn.fr

Speleothems, Laboratoire des Sciences du Climat et de l’Environnement, Gif-sur-Yvette, France
Dominique.Genty@lsce.ipsl.fr

University of Sfax, GEOGLOB, Tunisia

Palaeoclimate, Université de Sfax, Faculté des Sciences de Sfax, Laboratoire GEOGLOB, Sfax, Tunisia
bassemjalali@yahoo.fr

Geology, CEFREM UMR5110 CNRS, Université de Perpignan, France.
Maria-angela.bassetti@univ-perp.fr

Archaeology, Geoarchaeology, GEODE-UMR CNRS 5602, Toulouse 2 Jean Jaurès University, France
carozza.laurent@wanadoo.fr

© IRD Éditions, 2016

Conditions d’utilisation : http://www.openedition.org/6540