CLIMATE

Contemporary climate

The contemporary climate regime of Europe is spatially variable, because of its position between the Arctic and Mediterranean zones on the latitudinal extent, and the Euro-Asian continental mass and Atlantic coast zones on the longitudinal extent. Additional spatial climatic variability results from the topography of Europe, which is characterized by a combination of high mountain chains, low lying countries and a circuitous shoreline. The penetration of the sea into the continent causes the climate to become milder, in terms of temperature. Consequently, the climatic conditions in each region differ according to its position in relation to the more extreme conditions along the borders. Another influence is the warm Gulf Stream, which flows along the western shores of Europe. From the global point of view, the European continent lies within the westerlies wind system. Within this system, barometric pressures influence the storm regime. High-pressure, anticyclone system characterizes the eastern continental region, while the low-pressure cyclone system characterizes the west over the Atlantic.

The Pleistocene–Holocene transition period

The last glacial maximum occurred between 22 ka and 16 ka BP. A massive deglaciation started shortly thereafter, characterized by strong fluctuations and occurring between 15 ka and 8 ka BP. A warm period seems to have taken place from 13.5 ka to 11 ka BP. This is referred to as the “Windermere interstadial” in Britain and Ireland and the “Bölling interstadial” in Scandinavia.

At a rather early stage of the research to be reported in this book, it was decided to use the connections between climate changes, hydrological and socio-economic systems in the Levant in order to establish a basic reference sequence of climate changes during the Holocene. Once this had been accomplished, this sequence would be correlated with other regions over the globe. This decision was based on the following observations.

1. This region is a transition zone between two climate belts: the westerlies system and the sub-tropical or intertropical convergence zone (ITCZ) overlying the Arabian–Sahara desert belt. The rate of movement of these two belts north and south affects the mean annual quantity of rain, as well as its variability from year to year. Consequently, the positions in the past of these belts that affect the Mediterranean region's climatic regime and hydrological cycle may provide information reflecting global climate changes.

2. The Nile, which reflects the easterlies and the tropical climate regime over eastern Africa, reaches the Mediterranean and its sediments reflect the history of the climate changes over its watershed.

3. The relatively moderate size of the Mediterranean region, causing climate changes to be rather synchronous (although not absolute) over most of the area, enables establishment of a regional climate change chronology.

4. The long history of human societies in this region, the abundance of documents and archaeological excavations, all facilitate investigation of the impact of climate changes on past socio-economic systems.

SOUTHWESTERN USA

Contemporary climate

The precipitation regime of the southwestern USA (Fig. 5.1: south of latitude 40° N) is a function of the interplay between the westerlies system over the Pacific and the monsoonal system over the Gulf of Mexico. It has two rainy periods. The summer rains result mainly from monsoonal air masses, which originate in the Gulf of Mexico, and air masses coming from the Pacific. In addition, the height of the Colorado plateau magnifies the thermo-synoptic contrast between continent and sea, forcing air masses to rise towards the low-pressure area over the plateau.

California has a moderate climate with an average annual precipitation in excess of 1000 mm. During the summer, temperatures vary between 27 and 28 °C; in winter, between 7 and 8 °C.

On the Pacific coastal plain (in Mexico), the climate is dry and very hot. Here, cold winds from the northwest blow for about 8 months of the year. During the summer, southwesterly winds bring torrential rains.

The factors that play a role in deciding the relative influence of the two systems (i.e., the westerlies and the monsoons) are the circumpolar vortex (the strength) of the sub-tropical westerlies, and large-scale anomalies in temperatures of the sea surface, primarily those associated with the ENSO and the NAO (Hughes and Graumlich, 1996). The climate of California is especially influenced by the California current. This current is a branch of the southward flowing system of currents of the northeastern Pacific.

There is a general agreement among scientists that the surface temperatures of both oceans and continents are rising. It is also agreed that greenhouse gases like carbon dioxide and methane are increasing in the atmosphere and that this increase is a result of the continuous rise in human industrial and transportation activity, depending on the fossil fuels, i.e., coal and petroleum. There is still an ongoing debate whether all three phenomena are interconnected and whether part of the blame for the warming should be apportioned to natural processes, such as those that caused climate changes before the industrial revolution. The majority of scientists will not contest natural processes as a possible additional factor but will put the main blame on the emission of greenhouse gases, while admitting that there are some questions which still remain to be solved: such as what is the cooling effect of other products of industry emitted into the atmosphere (e.g., smoke and sulfurous particles, which may cause a shading layer with a cooling effect).

One of the most important tools for investigating the reasons for the global change, as well as for predicting future developments, is computerized general climatological models (GCM), which simulate the physical processes taking place in the atmosphere and beyond, and their impact on the temperatures of sea and land. However, as with all computer models, the correct output is a function of correct input, when input in this case involves data as well as procedures.

CLIMATE CHANGES DURING THE HOLOCENE: GLOBAL CORRELATION

In Fig. 6.1, representative time series from the regions discussed in the preceding chapters are correlated. From the correlation lines suggested in this figure, it can be concluded the main climate changes that occurred during the Holocene in the Levant can be traced in all other regions, although the range and nature of impact on the hydrological cycle differed from one region to another. As was discussed in Chapter 1, the archaeological stratigraphy in the Levant was by and large decided by the main climate changes. Moreover, archaeological investigations as well as historical documentation in this region are most extensive compared with other regions of the globe (Issar and Zohar, under revision). It is, therefore, suggested that the archaeological–paleo-climatic stratigraphy of the Levant should be adopted as the basic chronostratigraphy of the Holocene on a global scale.

The main chronostratigraphical divisions are listed in the key under Fig. 6.1 (from bottom to top) and shown on the diagram.

Some general conclusions can be drawn concerning the climate changes on a global scale demonstrated in Fig. 6.1.

1. During the Holocene, the global climate went through more than a few pronounced changes, which affected the hydrological cycle, the impacts of which were different from region to region on the time, temperature and humidity scales.

2. In the regions dominated by the westerlies, described earlier in this book (the circum-Mediterranean region, western and central Europe and western USA), cold climate brought more precipitation, which caused the hydrological systems to overflow. In the higher latitudes and altitudes, this cold climate caused glaciation. In these regions, warm climates, by comparison, led to less precipitation, causing desertification of the regions along the margins of the deserts.

3. […]

CHINA

Figure 3.1 shows the East Asian area.

Contemporary climate

The contemporary climate of west Asia is dominated in winter by the polar continental air mass (PCAM). During this period there is a northerly flow at the lower troposphere layer, which comes from cold and dry air of middle–high latitude. In the summer, the region is dominated by the tropical–sub-tropical oceanic air mass (TOAM) and the tropical continental air mass: a southerly monsoon dominates the lower troposphere layer, bringing oceanic warm and moist air. There are two types of summer monsoon, the southwestern and the southeastern, influencing different areas. Today, the southeastern monsoon dominates most of China and, in purely theoretical terms, it should have been so for the last 130 ka years (An Zhisheng et al., 1991a).

Climate changes during the Upper Pleistocene and Holocene transition period

During the transition from the Pleistocene to the Holocene, the climate in eastern Asia, especially in China, was different from one region to another, according to the climatic belt to which each region belonged. While the most northwestern region belonged to the westerlies belt, the rest of China was influenced by the monsoon regime. Thus, while the first region was cold and humid during the last glacial period and became warmer and drier as the glaciers melted, most other regions, especially the inner ones, were dry and cold during the Ice Age and warm and moist as deglaciation proceeded (Li Jijun, 1990).