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Thee Ice Caps : the memory of the Earth's climate in the ice Among the methods used: measuring the quantity of subsistent radiocarbon contained in the air trapped in bubbles of gas - these fully-enclosed millimetric bubbles are veritable micro-atmospheres sealed as though in an airtight coffer - measuring the radioactivity of lead-210 (produced by the disintegration into the atmosphere of the radon gas emanating from the earth' s crust), and analysing the seasonal fluctuations of the composition of the ice (the summer layers are thinner than the winter ones) and the points of reference of known dates: if a bore-hole encounters the trace of significant layers of ash and one estimates that the sampler is approaching the end of the eighteenth century, one would know that the ash in question more than probably came from the eruption of the Krakatoa Volcano in 1883. The fallout of radioactive elements produced by the thermonuclear tests of 1954 has also been locked into the ice particles: such traces have been found in Greenland and across the whole of the Antarctic. A final chronological measurement is the isotopic clock. For a better understanding of what follows, remember that atoms are constituted from a nucleus around which gravitate x number of electrons: the atomic nucleus, for its part, is composed of the same x number of protons carrying a positive charge and z number of neutrons that have zero charge. Atoms of the same name (that is to say having the same number of protons) can have a different number of neutrons: these atoms then have different masses and therefore different physical properties, while keeping the same chemical properties: they are described as isotopes (from the Greek isos meaning equal, and topos meaning place) because they occupy the same box in Mendeleïev's celebrated classification of the elements. Since ice is mainly water and since water is composed of hydrogen and oxygen atoms (who cannot remember the famous H2O?) it is towards these two elements that the eyes of glaciologists will be turning. It has been shown that the relationship between the isotopes of oxygen 18 (O18) and those of oxygen 16 (O16) in the snow falls that affect the Antarctic is a direct function of temperature; we therefore have at our disposal, thanks to the isotopic compositions of the oxygen in the ice, a remarkable palaeothermometer. Because as elsewhere, there is a difference in temperature between winter and summer, the isotopic oxygen composition of the snow will be different according to the seasons. The seasonal levels can therefore be counted in a similar way to the counting of the years from the rings in a tree trunk, and, just as the age of the tree can be read from such concentric circles, so the age of the ice can be calculated in the same manner. The isotopic composition of the ice layers is not the only information retained by scientists from their glacial bore-holes. There is also the gas trapped in the air bubbles occluded in the ice. In order to analyse them, the latter has to be melted in a vacuum (the volume obtained is of the order of 10cm³ of gas for every 100 grams of ice) in order to liberate the imprisoned air and to make the gas appear. "These atmospheric microcosms reveal to us the composition of the atmosphere at the moment of the formation of the ice that imprisoned them", writes Roland Souchez in his book Polar Ice (1). "That is to say well after the snow crystals accumulate on the ground. Thus, at Siple Station in Antarctica, 100 years elapsed between the fall of snow and the occlusion of the gas bubbles. These gas bubbles also contain a mine of information about the origin of the ice and about the changes of thickness of the continental glaciers which come to confirm the hypotheses on climatic changes developed from isotopic profiles."
It would take too long to trace the steps of what is required to calculate the thickness of the ice of a continental glacier from these tiny bubbles of gas. It should nevertheless be known that because of the fact that the total gas content is essentially a function of the altitude at which the ice formed, scientists have been able to prove that the configuration of the frozen continent has been considerable modified throughout the ages: 400 metres less in Eastern Antarctica along the coasts and about 150 metres more in Terre Adélie are the currently cited figures. "The increase in the rate of accumulation during deglaciation would not be foreign to the thickening of the ice at the centre of the Antarctic since the end of the Last Ice Age" writes Roland Souchez further. "The thinning of the ice in the coastal regions since the Last Ice Age at the most would for its part result in a rise in sea level and a rise in temperature. The causes of this great warming that marked the end of the Last Ice Age are still barely known. But for the want of definitive conclusions, analysis of the gasses present in the ice provides us with the essential data on the variations of the key parameters of our environment during the establishment of glaciation or during deglaciation, and accordingly sheds light on the causes of the climatic changes." (2) If one for a moment takes an interest in C2O on its own, the research carried out by glaciologists have brought to light an important fact in the evolution of our atmosphere: the increase of carbon dioxide in the atmospheric content (25 to 30% in the last two centuries and 100% in a hundred years' time) would not have triggered but merely amplified the great postglacial global warming that the world is experiencing today. Of course the hypothesis has to be verified. But the question is now asked: is it warmer today because of C20, or is C20 the result of a global warming of the atmosphere? The latest information of interest to glaciology concerns the molecules of various substances contained in the bubbles and conveyed by the general circulation of the atmosphere: dust coming from a volcanic eruption or deserts, sea spray, meteorite dust, and pollution dust released by human activity. Imagine that a toxic cloud coming from the North found itself above the Antarctic and that it started to snow at that very moment: the crystals would carry in their memory for all time traces of the passage of that toxic cloud.
(1) Roland Souchez, Polar Ice, Brussels University publications, collection "Understanding Progress", 1988, p 129. (2) Roland Souchez, id above, p 131.
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