Water, Environment and Society in Times of Climatic Change

Natural variations in atmospheric ¹⁴C, which are expressed as wiggles in the radiocarbon calibration curve (Stuiver et al, 1993) severely limit the possibilities for fine-resolution dating of changes in vegetation and climate as recorded in lake deposits and raised bogs (e.g van Geel, 1978; Magny, 1993a; Barber et al, 1994). Van Geel and Mook (1989) stressed the importance of ¹⁴C Wiggle-Match Dating (WMD) of organic deposits, given that WMD can reveal relationships between ¹⁴C variations and short-term climatic fluctuations caused by solar and/or geomagnetic variations. Kilian et al (1995) have shown that, by using this strategy, raised-bog deposits in particular can be dated more precisely. Hence the raised-bog archive can be compared effectively with other proxy data archives, the more so because WMD has shown that an unexpected ¹⁴C reservoir effect plays a role in dating raised-bog deposits (individual conventional radiocarbon dates appear to be 100 to 250 years too old). Wiggle-matching is an elegant way of identifying this effect, and of estimating its magnitude.

Discussions of future climate, whether in forums of science or policy, have come to be dominated by the debate over human-induced global warming. Typically, these discussions focus on the temperature departures that might result from continued anthropogenic emission of greenhouse gasses; on the response of the earth’s physical and biotic systems to those departures; and on the socio-economic impacts of those responses. Strong differences of opinion fuel these discussions. Many scientists, citing computerized simulations of an atmosphere with increased heat-trapping capacity, conclude that significant global warming is likely in the future, and perhaps already underway. Others, stressing the uncertainties of the computer models and questioning the assumptions on which the simulations are based, proclaim that predictions of human-induced global warming are premature, unscientific, and alarmist. The socio-economics of the debate are no less polarized. One side warns that failure to curtail fossil-fuel emissions could threaten the food and water supply of the Earth’s burgeoning human population; the other raises the specter that restricting fossil fuel consumption might lead to global economic collapse.

Most of those who have written sometime this century about the historical impact of climate change have been geographers or climatologists: C.E.P. Brooks, Ellsworth Huntington, Hubert Lamb.... Historians have tended thus far to be either blankly indifferent or somewhat scornful. Some notable exceptions are to be found among the community of French historians associated with Annales, a journal founded in 1929 and committed to forging links with other subjects though especially geography. Fernand Braudel was a doyen in this respect. Le Roy Ladurie, ‘by common consent the most brilliant of Braudel’s pupils’ (Burke, 1990:61) produced what may still be the most widely cited of all the climate-and-history studies. In Times of Feast, Times of Famine, he stressed how complex is the challenge of assessing the effect on crop yield of secular changes of mean air temperature that may well not exceed a degree Celsius. Also he discerned a disposition to want things both ways on the question of folk migrations, ‘The Teutons of the first millennium before Christ are supposed to have left their countries of origin because of the cold. The Scandinavians of the period before AD 1000 are supposed to have done the same thing for exactly the opposite reason—the mildness of the climate, stimulating agriculture and thus also population growth, is said to have led to the departure of surplus male warriors’ (Le Roy Ladurie, 1972).

A wide spectrum of data obtained as a result of palynological, hydrological, geological, paleontological and archaeological investigations is enabling us to reconstruct quite comprehensively and with high definition the climate changes which have occurred in the Mediterranean region during the last 10,000 years. One may start with indicators not influenced by human activity, like isotope ratios, or ancient Mediterranean and Dead Sea levels; proceed with such environmental proxy-data, as pollen time series; and then add to this archaeological and human historical data. One may thus obviate the ‘danger of arguing in circle’ which is the danger of ‘inventing climatic change to explain events in human history, which are then held to prove the occurrence of a change of climate’ (Lamb, 1982).

Eretz-Israel (The Land of Israel) is located at the south-eastern corner of the Mediterranean Sea, between the Sea and the Syrian Desert. It is the transition zone between the Euroasian and African continents, and between the Mediterranean and the arid climatic zones (Map 7.1). The historical-geographical name Eretz-Israel applies to the region between the Mediterranean Sea to the Jordan River and the Dead Sea Rift Valley, which includes the State of Israel and the Palestinian Authority and was known until 1948 as the British Mandate of Palestine. This land has been settled by Man since the early stages of his history upon earth, since the Early Pleistocene. Archaeological findings and historical records present the history of the Land and its inhabitants during the historical and pre-historical periods.

ʿUvda Valley, in the southern Negev, Israel, is an extreme desert area. It is characterized by summer temperatures above 40°C, low precipitation of 28 mm annual average, with an annual evaporation rate of 4,000 mm. The significance of these figures is more apparent when compared with those of the Negev Highland desert, 100 km to the north, with 100 mm average annual precipitation and 2,000 mm evaporation potential. Obviously, the water balance of the southern Negev is very negative. Since vegetation is restricted to wadi beds, the carrying capacity of the area is low for vegetation, for animals, and for man who subsists on both. Accordingly, only limited archaeological remains would be expected, i.e. the markers for human presence and activity in the past. Indeed, most of the southern Negev is characterized by a low density of ancient sites. Yet, contrary to expectations, archaeological remains in the southern-most Negev are exceptionally abundant. From Ueda Valley in the north to Eilat in the south, some 1,200 km², 1,600 sites are known today, even though only 16% of the area has been surveyed in detail.

Stalactites and stalagmites in the Soreq Cave, a well known tourist attraction in Israel, provide a very detailed climatic record for the Eastern Mediterranean region during the last 25,000 years (Bar-Matthews et al., 1997a) and 58,000 years (Bar-Matthews et al., 1997b). These studies indicate that the isotopic compositions of speleothems that are older than about 6,500 BP are significantly different from those of present-day speleothems; and that about 6,500 years ago the isotopic composition of the speleothems became similar to that of today. Thus, the climatic conditions that prevailed in the Eastern Mediterranean area before ~ 6,500 BP were very different from now. Only from that time have the conditions become similar to those of the present day. Soreq Cave is one of a series of karstic caves situated within the steeply westward dipping flank of the Judean Hill anticline. Its geological and hydrological setting and its environment are summarized by Asaf (1975), Even et al. (1986) and Bar-Matthews et al. (1991). The cave is located approximately 40 km inland from the Israeli Mediterranean coast, and is 400 m above sea level. Presently, the climate in the Soreq Cave area is typical of the semi-arid Mediterranean type, with an average annual air temperature of ~20.5°C, a cave water temperature varying between 18.0 and 20.5°C (and a mean annual rainfall of ~500 mm). The area is located in a narrow transition zone between humid and arid climates; and slight climatic variations, such as changes in the annual rainfall, would have affected the desert boundary and the inhabitants in the past.

Southeastern Anatolia is a region which is influenced by both Mediterranean and continental climatic factors. It encompasses both mountain and steppic zones, and has had a history of settlement throughout the Holocene which is characterized by fluctuations between simple village farming communities and complexly organized cities and towns. Archaeologically it is a region where there have been major fluctuations in population, some perhaps related to environmental factors, others related to social and historical processes.

Our understanding of past frequencies of extreme hydroclimatically-induced events such as floods and mass wasting is mainly based on direct or indirect interpretations of sediments, landforms and parallel biotic changes. These sources of information point to episodes with higher or lower frequency of extreme events, and indicate that some of these occurred under wetter and cooler conditions that permitted higher water storage (cf. Starkel, 1983). It also seems true that greater year-to-year and multi-year variability have characterized those times of transition when water storage or runoff drastically adjust (e.g. ice sheet decay in North America; Teller, 1995). In the arid zone, such instability can involve high frequency of cyclones, and thus wetter conditions; in the semi-arid zone, in contrast, greater instability may cause expansion of the desert. In the monsoonal areas, floods seem to become more prevalent during warming trends (e.g. the early Holocene; Kutzbach, 1983). Transitions and phases with frequent events have thus played a major role in shaping the physical and biotic environment during the Holocene.

Environmental reconstruction since the Last Glacial Maximum in Northwestern Tropical Africa has been severely limited by the incompleteness of the record, owing primarily to major discontinuities in lacustrine sediments preserved in a predominantly arid climate. Only two long and complete pollen sequences, each from deep crater lakes, have been obtained to register the whole deglacial and Holocene history of the humid equatorial forest (Maley, 1989). Other continental sites, all closely subject to local hydrogeological conditions (Lézine and Casanova, 1989) provide detailed ‘windows’ on short periods of the Holocene. However, comparison with marine sedimentary sequences that provide continuous, well dated records over longer time spans allows one to reconstruct changes in past atmospheric patterns over West Africa and in vegetation distribution on sub-continental as well as local scales. I present here a review of pollen data from West Africa and the nearby Eastern Atlantic undertaken to understand the vegetation response to past climatic and hydrological changes on different time scales. Interpretations of pollen diagrams are based on numerous studies on both modern pollen deposition from the Equatorial evergreen and semi-deciduous forests to the south (Brenac, 1988; Elenga, 1992; Reynaud-Farrera, 1995) to the Sudanian, Sahelian and Saharan driest ecosystems to the north (Lézine and Edorh, 1991; Lézine and Hooghiemstra, 1990; Maley, 1972, 1981; Ritchie, 1987) (Fig. 14.1). Land-sea correlations are also made.

Today the Sahara comprises an area of more than eight million square kilometers of the desertic geobiome (ecosystem). Vegetation is sparse, rare or absent. Lithosoils are dominant between large sand dunes areas (Erg) and organic carbon is low to absent in the soils. The total vegetation biomass (phytomass) and soil organic matter probably does not exceed 8–15 gigatonnes (10¹² kg) of carbon. A mean annual rainfall of 100 mm or 150 mm is usually taken as the edge of the Sahara, but wide areas receive less than 5 mm annual rainfall.

Upland Western Maharashtra is drained by the Krishna and Godavari rivers and their tributaries. These rivers have their source in the Western Ghats and flow into the Bay of Bengal. The region is semi-arid (800–400 mm rainfall) due to its location on the leeward side of the Western Ghats. The major rivers therefore derive much of their discharge from the high rainfall source region (up to 6,000 mm), and bring water to the semi-arid zone downstream. The landscape is dominated by flat denudational surfaces at 1,200, 1,000, 800 and 700 meters above sea level (Kale and Rajaguru, 1988) of pre-Quaternary age developed on 60 myr old Deccan Trap basalt. The rivers lack well-developed floodplains and alluvium is confined to à narrow belt less than 2 km wide. The non-alluvial part of the landscape is basalt bedrock, covered with varying thicknesses of weathered bedrock, locally called ‘murrum’ on which a soil has developed. This weathered mantle developed during the Tertiary, when the climate was comparatively humid. The thickness of the alluvium is generally less than 20 m. This alluvium includes several fills ranging in age from the Early Middle Pleistocene to the Late Holocene. The Holocene deposits are non-calcareous silts and sand and generally form an alluvial terrace 5–6 m high inset into an older Pleistocene alluvial fill terrace 10 to 15 m high. The Pleistocene alluvium is calcareous and dominated by sandy silt with lenses of gravels and fissured clays. Colluvium covers some footslopes.

This chapter is intended to provide an indication of how the contents of this book fit within the framework of the UNESCO International Hydrological Programme and associated partners (e.g. WMO, IGBP-BAHC, GEWEX) under the umbrella of WCP-Water (World Climate Programme-Water). To achieve this objective, this short contribution will select some of the cross-cutting issues which link some of findings from the paleoclimatic studies, with the problems of using outputs from atmospheric General Circulation Models (GCMs) in hydrology and water resources planning. Reference will also be made to contributions from the combined efforts of the micrometeorological-hydrology communities towards improving GCMs which forms a very active sub-project of the IHP. This latter aim requires my going into more detail so that the non-specialist has a better appreciation of the current difficulties in coupling hydrological models with GCMs. Subsequently the value in developing climatic variability scenarios based on paleoclimatic and historical evidence should therefore become apparent.