Box I I The concept of ice ages historical background

Environmental change is a continuous process where dynamic systems of energy and material operate on a global scale to cause gradual and sometimes catastrophic changes in the atmosphere, hydrosphere, lithosphere and biosphere. During most of the Earth's history the agents in the environmental system have been the natural elements (wind, ice, water, plants and animals). Some 2-3 million years ago, however, a new and perhaps the most powerful generator of environmental change, the hominids, emerged. The earliest testament to this are the cave paintings in many parts of the world. The first written accounts came with the rise of the ancient Mediterranean civilizations in Greece and Rome. Ideas changed little during the Middle Ages, when European scholars returned to the concept of a flat Earth. The bipartite nature of geography, first intimated in Stra-bo's work, involved human and physical divisions. This concept was formalized by Varenius (ad 1622-1650) who originated the ideas of regional or 'special' geography and systematic or 'general' geography. The deductive and mechanistic philosophy earlier advocated by Newton (1642-1727) was continued in the work of Charles Darwin (1808-1882). In his classic work The Origin of Species (1859) he advanced theories of evolution and suggested a relationship between environment and organisms. By the end of the nineteenth century, the theory of the biblical flood as a major agent in shaping the face of the Earth was questioned.

The earliest descriptions of glaciers are in Icelandic literature and date from the eleventh century. During the Little Ice Age, glaciers around the world expanded considerably. In the Alps and in Norway the glacier advance led to destruction of pastures and property. The ice-age theory was developed during the nineteenth century. The main spokesman for the theory of ice ages in the early nineteenth century was Louis Agassiz, the influential president of the Swiss Society of Natural Sciences, who has been regarded as the 'Father of Ice Ages'. Agassiz was, however, not the first to believe that glaciers had previously been more extensive, and he himself was sceptical for several years. Perhaps the first to document the evidence for more extensive glaciers was the Swiss minister Kuhn. In 1787 he interpreted erratic boulders below the glaciers near Grindelwald as evidence for a more extensive glaciation. Scot Hutton, one of the leading contemporary geologists, published in 1795 his 'Theory of the Earth' in which he described how ice had transported great boulders of granite into the Jura Mountains. A Swiss mountaineer and hunter named Perraudin argued in 1815 that glaciers had flowed into the Val de Bagnes in the Alps, and tried to convince Carpentier, who later became an advocate of the glacial theory, of his views. Perraudin also tried to persuade the Swiss engineer Venetz three years later, but he too was sceptical of the theory. However, Venetz began to accept the hypothesis and in 1829 he argued from the distribution of moraines and erratics that glaciers had covered the Swiss plain, the Jura and other regions of Europe. In 1824 the Norwegian geologist Esmark had already argued that glaciers in Norway had been much more extensive than at present. It was, however, the German poet Goethe who promoted the idea of an ice age (Eiszeit) in the novel Wilhelm Meister (1823).

Meanwhile, Carpentier accepted Venetz's theory of more extensive ice, and started to collect evidence in favour of this hypothesis. At that time it was believed that the biblical flood explained the distribution of the erratics, and resistance was therefore strong to the ice age concept. In 1833 several researchers had accepted the view of Lyell, the leading British geologist of the day, that boulders had been deposited by icebergs, a theory (the 'drift' theory) developed in 1804

by the German mathematician Wrede. Darwin supported Lyell, and in a series of papers he advocated the theory until his death in 1882.

During a field trip to Bex, Agassiz was convinced by Carpentier of the truth of the glacial theory, which for the first time had a strong, forceful and influential spokesman. By now Agassiz and Carpentier were familiar with Goethe's great ice age theory, but researchers ignored it because of his lack of scientific style. Unfortunately, Agassiz developed the glacial theory beyond available evidence, and when he presented the theory to the Swiss Society of Natural Sciences at Neu-chatel in 1837, he was met with great opposition. Agassiz published his work in 1840 in the book Etudes sur les Glaciers. The ice age theory substituted the Great Flood, of which Buckland, a professor of mineralogy and geology at Oxford University, was a great spokesperson. Buckland joined Agassiz on a trip to the Alps, but Buckland was still not convinced. After Buckland had discussed glacial deposits in Scotland and northern England with Agassiz, Buckland finally became convinced about the glacial theory. Agassiz moved to the US in 1847 as professor at Harvard University. Many researchers had already accepted his theory, but his appointment speeded up its acceptance, and when Agassiz died in 1873, only a few scientists had not yet accepted the ice age theory. Subsequently, tillites were found as evidence of ancient glaciations. Around the turn of the century, evidence for four ice ages were found in North America, the European Alps, Scandinavia, Britain and New Zealand. The first deep-sea sediment cores, covering most of the Quaternary, were be a response to a step-like warming of the Arctic in the early twentieth century since the end of the Little Ice Age. Maritime Scandinavian and Icelandic glaciers, on the other hand, show increasingly positive mass balance due to increased precipitation during the accumulation season (Pohjola and Rogers, 1997a).

obtained in the 1950s. Oxygen isotope studies of planktonic and benthic foraminifera were used to estimate palaeotemperatures and ice volumes. In the early 1970s it was assumed that the period of glaciations was equivalent to the Quaternary period (ca. 2.5 million years). In 1972, long cores retrieved from the Antarctic continental shelf in the Ross Sea showed evidence of glaciations 25 million years ago. Cores obtained in 1986 showed evidence of glaciations as far back as 36 million years ago (Oligocene). The precise timing of the onset of Cenozoic glaciation in Antarctica remains to be determined.

As evidence of environmental change accumulated, attention also focused on the underlying cause of climate change. The French mathematician Adhemar was the first to involve astronomical theories in studies of the ice ages. In 1842 he proposed that orbital changes may have been responsible for climatic change of such magnitude. The Scottish geologist James Croll advanced a similar approach in 1864, suggesting that changes in the Earth's orbital eccentricity might cause ice ages. In the book Climate and Time he explained the theory in full. Due to the inability to date and test Croll's hypothesis, his theory was not seriously considered until Milutin Milankovitch, a Serbian astronomer, revived the theory during 1920-40. The Milankovitch theory, or the astronomical theory of ice ages, has become widely accepted since the 1950s with evidence from the deep-sea records. The late eighteenth and early nineteenth centuries witnessed the establishment of new methods, mainly based on biological remains, thus establishing the field of palaeoecology.

A degree-day glacier mass-balance model was applied by Johannesson et al. (1995) to three glaciers in Iceland, Norway and Greenland, where mass-balance data for several years are available. The model results corresponded reasonably well with measured variations in the mass balance with elevation for each glacier. A similar degree-day model approach was used by Braithwaite (1995) to study ablation on the Greenland ice sheet.

At the margin of the eastern North Greenland ice sheet, Konzelmann and Braithwaite (1995) studied variations in ablation, albedo and energy balance. Their results showed that net radiation is the main source for ablation energy, and turbulent fluxes are about three times smaller energy sources, while heat flux into the ice is a substantial heat sink which reduces the energy available for ice melt. Studies show that small-scale albedo variations must also be evaluated carefully in large-scale energy balance calculations.

1.3 The study of glaciers and past glacier fluctuations in the context of present and future environmental change

The past 2-3 million years (the Quaternary Period) have been characterized by periodic climatic variations. During cold periods, glaciers and ice sheets became more extensive than today, while in milder intervals, the glacier extent was much less. One of the main achievements of the earth sciences has been the demonstration that the sequence of glacials and interglacials are primarily driven by Earth's orbital parameters (Imbrie and Imbrie, 1979; Berger, 1988; Imbrie et al, 1992, 1993a,b). This external forcing mechanism causes responses and chain reactions in the internal elements (atmosphere, oceans, the hydrological cycle, vegetation cover, glaciers and ice sheets) of the Earth (e.g. Bradley, 1985). Changes in one element of the Earth's system can cause responses in other elements because they are coupled in a linked system. These can lead to feedback reactions which can amplify the original signal. Glaciers and ice sheets play an important role in the global climate system. Glacier advance and retreat may therefore be both a consequence and a cause of climate change (Imbrie et al, 1993a,b).

Ice sheets normally take longer to grow than to decay, but most information is available about their decay phases. This is mainly due to the fact that geomorphological and stratigra-phical evidence of glacier retreat has a much better preservation potential than evidence from glacier build-up (e.g. Clark et al., 1993).

Models of ice-sheet growth are largely theoretical because of little data. Flint (1971) proposed the highland origin, windward growth model for the Laurentide ice sheet in North America (Fig. 1.1). Alternative models suggest the coupling of regional-scale topography and climate. Ives et al. (1975) refined a model referred to as instantaneous glacierization for the growth of the Fennoscandian and Laurentide ice sheets. This model infers that snow accumulating on mountain plateaux produces plateau glaciers which may expand and coalesce to produce a multi-domed ice sheet.

The growth of ice sheets with large marine-based components was explained by Denton and Hughes (1981) in the marine ice transgression hypothesis. In this model, sea ice expands to inter-island channels and large embay-ments. The albedo increases, thus reducing the temperature and lowering the snowline. Snow accumulates to build ice shelves which eventually ground to form a marine ice dome.

There is a growing body of data from both terrestrial and marine environments that the large northern hemisphere ice sheets were characterized by relative instability during the last glacial cycle. Samples from the Hudson Bay area (e.g. Clark et al., 1993) and sediment cores from the North Atlantic contain layers of lithic fragments ('Heinrich layers') interpreted as ice-rafted debris carried by icebergs (e.g. Heinrich, 1988). These ice-rafting events record episodes of ice break-up along the eastern margin of the Laurentide ice sheet, and indicate that even large ice sheets are able to respond rapidly to climatic and/or dynamic forcing and undergo large volume changes over a few thousand years.

When ice sheets are established, they influence regional climate and commonly create their own weather system. On minor glaciers and ice caps, precipitation normally increases with increasing altitude. On larger ice sheets, however, precipitation increases with altitude around the outer margins of the ice sheet.

THE STUDY OF GLACIERS AND PAST GLACIER FLUCTUATIONS 7

Figure I. I The model of highland origin, windward growth of ice-sheet inception. Adapted from Flint (1971), Ives et al. (1975) and Benn and Evans (1998).

THE STUDY OF GLACIERS AND PAST GLACIER FLUCTUATIONS 7

Over the central parts, a high-pressure zone is the prevailing situation, which reduces precipitation considerably. In Antarctica, for example, the precipitation around the margins is about ten times that of the interior. High albedo and high elevation of the central parts of the ice sheets make the winter temperatures fall to around -70°C in Antarctica and -40°C over Greenland. Because an ice sheet reduces the absorption of solar radiation, the thermal contrasts between polar and equatorial regions are accentuated. Increased equatorial-pole temperature gradients during glaciations will increase the strength of the zonal (east-west) and meridional (north-south) circulation. High-latitude ice sheets can persist in a state of disequilibrium with existing climate because ablation rates are low. The West Antarctic ice sheet, which is grounded in the sea and has ice shelves in the Ross and Weddel seas, however, responds mainly to changes in glacial-interglacial sea-level fluctuations. The Antarctic ice sheet has remained relatively stable since the Miocene more then 20 Ma ago (Denton et al., 1993). Global climate variations produced only minor changes at the margins of the Antarctic ice sheet compared with the oscillations of the Laurentide and the Eurasian ice sheets. The Greenland ice sheets have fluctuated considerably more than the Antarctic ice sheet, as demonstrated by the results from the GRIP and GISP2 ice cores (see Chapter 3).

Glaciers are sensitive to climate changes of various magnitudes and different time-scales. They therefore constitute an important source of palaeoclimatic data. In addition, their widespread geographical distribution makes them suitable for establishing climate proxy data and for evaluating the nature of global climate fluctuations (e.g. Porter, 1981a,b).

Because glaciers respond to changes in their climatic environment by growing or shrinking, they can be used as sensitive palaeoclimatic indicators (winter precipitation, summer temperature, and prevailing wind direction). Studies of ice cores give one of the best available indicators of climate variations back in time in polar and alpine regions (e.g. Johnsen et al., 1992). Glacial deposits may form the basis for discontinuous time series of glacier fluctuations. However, glacier surges not related to climate, lags in the dynamic response of the glacier front to climatic variations, together with chronological uncertainties related to dating problems, make assessment of glacial geological data difficult.

The early to middle Holocene was, in general, a time of glacial retreat and warmer climate, termed the 'hypsithermal' (Deevey and Flint, 1957). The late Holocene witnessed the rebirth and readvance of most alpine glaciers throughout the world, an event termed Neoglaciation (Porter and Denton, 1967). During the Little Ice Age (last four to five centuries) there is widespread evidence for repeated glacier fluctuations throughout the world (e.g. Grove, 1988). During this time, the equilibrium line altitude (ELA) was lowered by approximately 100-200 m, equivalent to about 15 per cent of the ELA lowering at the last glacial maximum.

Historical observations going back to the eighteenth or nineteenth centuries help in reconstructing glacier fluctuations, together with terminal moraines dated by lichenometry and/or dendrochronology. Analyses of sediment cores from lakes downstream of glaciers may provide continuous records of Holocene glacier fluctuations. The relative abundance of minerogenic and organic content in lacustrine sediments is interpreted to be an indicator of glacier activity in the catchment area. Greater minerogenic content is generally indicative of more extensive glaciation (e.g. Karlen, 1976, 1981). Detrital organic matter, tephra and varves usually provide age control, and lake sediments are commonly in agreement with moraine records (e.g. Leonard, 1986).

Information about the Earth's climate in the past can help us to predict the direction and magnitude of future climatic change. This palaeoclimate information is inferred from a diverse array of biological, chemical and geological indicators (e.g. pollen, shells of marine micro-organisms, and glacial landforms).

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