At present, glaciers and ice sheets cover about 10 per cent of the Earth's surface, locking up about 33 million km3 of freshwater, which would correspond to a sea-level rise of 70 m. During the Quaternary glacial periods, ice sheets were more extensive, covering about 30 per cent of the Earth's land area. Glaciers and ice sheets are also sensitive to climate change, varying in response to winter precipitation and summer temperature. To answer the questions of why, where and when glaciers form, it may be helpful to recognize glaciers as systems, with inputs, outputs, and interactions with other systems. Mass and energy enter the system in the form of precipitation, gravity, solar radiation and geothermal heat, and leave the system in the form of vapour, water, ice, debris and heat. The mass and energy is transferred through the system at various speeds and periods of storage. The most important input to glacier systems is, however, snowfall, windblown snow and snow avalanches from adjacent valley sides. Glaciers grow where climatic and topographical conditions allow the input to be greater than the output (accumulation > ablation) and vice versa.
Due to the growing evidence for global climate change and ice-sheet variations, and as a result of the complexity of the different records, scientists are turning to simulation modelling for explanations of the causes of observed changes. Conceptual models aim to highlight the connections and feedbacks between different components of the system. These models can later be tested empirically (by observation) and by simulation modelling. Imbrie et al. (1992) developed a conceptual model to examine the response of Atlantic Ocean circulation to solar radiation changes. They proposed four circulation modes:
(1) During an interglacial, deep convection in the Atlantic is concentrated into three cells: in the Nordic Seas, in the open ocean, and in the Antarctic seas.
(2) During a pre-glacial episode following an interglacial, the Nordic heat pump is reduced in strength or stopped, while the Antarctic circulation cell is enhanced.
(3) During a full glacial, the open ocean heat pump operates at a maximum, while the Antarctic cell is stronger than during an interglacial.
(4) During déglaciation, the Nordic cell is restabilized, while the open ocean heat pump is stronger than during interglacial periods.
This model demonstrates how a change in one oceanic circulation component immediately affects the entire system. Reduced terrestrial radiation receipts as a result of Milankovitch forcing causes glacial ice cover and sea ice to expand. During this stage, the Nordic heat pump becomes gradually reduced and flow is concentrated in the open ocean. Evaporation from the ocean surface and lowering of the temperatures will occur simultaneously with a shift in the position of the westerly winds and more enhanced ocean circulation in lower latitude oceans. In turn this may effect the strength and location of the monsoon cells and the position of the intertropical convergence zone. Terrestrial and marine Quaternary proxy data support the models of late quaternary climate and ice-sheet evolution 161
evidence that changes in the North Atlantic circulation influence climatic changes in other parts of the world (e.g. Lowe and Walker, 1997).
Kukla and Gavin (1992) presented a model of how the Earth may respond to insolation variations as a result of Milankovitch forcing. Their model indicates that the three insolation components in combination drive the Earth's climate into an ice age. The main reasons for this are precessional changes (gradual decrease in insolation between July and November), reduced receipt of radiation in high latitudes as a result of obliquity variations, and reversed seasonal insolation variations in lower latitudes, leading to increased insolation during the spring. Their model suggests a gradual decrease in radiation, but feedback processes force the system into more rapid cooling. The cooling in higher latitudes combined with increased northern transport of water vapour causes build-up of glaciers. In their model, orbitally induced changes magnify short-term effects (such as changes in solar activity and volcanic eruptions) when they are in phase, and damp them when they are out of phase.
Oeschger (1992) presented a hypothesis for Late Quaternary climate change based on the interaction between changes in ocean thermohaline circulation and variations in atmospheric gases. His hypothesis included a change between a glacial state with reduced North Atlantic Deep Water (NADW) formation, and warmer climates when NADW formation was strengthened, sea-level was higher, and atmospheric concentrations of CH4 and C02 were at present levels. In Oeschger's model, changes in the distribution of solar irradiance cause changes in NADW formation. In addition, changes in ocean chemistry and the operation of the biological pump as a result of sea-level fluctuations cause variations in the C02 flux between the ocean and atmosphere. Atmospheric C02 concentrations may be buffered by a net flux from terrestrial biomass because of reduced vegeta tion cover during glacials. This may explain why the decline in atmospheric C02 content lagged behind the lowering of the global temperature at the end of the Eemian interglacial. The reduction in CH4, however, coincided with the climatic deterioration as a result of increased aridity and reduced areas of wetland, and therefore reduced CH4 flux to the atmosphere.
During glacial periods, NADW formation is reduced. Changes in critical boundary conditions like solar irradiance, sea-level change, and the extent of ice cover (albedo) may cause rapid switches in NADW formation. The Dansgaard-Oeschger cycles have been attributed to a combination of solar irradiance and damped effects of NADW formation. Changes in NADW formation, and thereby the biological heat pump in the oceans, have led to changes in the atmospheric C02 concentration of between approximately 200 (reduced NADW formation) and 240 ppm (enhanced NADW formation). Enhanced NADW formation at the beginning of the Holocene increased the heat transport to northern latitudes and increased melting of the northern ice sheets. The atmospheric C02 content paralleled the global sea-level rise.
Variation in insolation induced by Milankovitch forcing is considered to be the main driving force for glacial-interglacial cycles. Superimposed on these cycles are, however, a series of complex internal feedback mechanisms, such as ocean heat transfer, albedo and gas exchange. How these different components interact to cause climatic changes is, however, not well known. Of special interest are the complex relationships between atmospheric gas exchange, ocean thermohaline variations, variations in meltwater flux, and ice-sheet fluctuations. It is often difficult to establish precisely the order of events as a result of poor stratigraphie resolution and lack of precise dating and correlation. It is therefore difficult to solve the 'chicken and egg' dilemma.
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