Unlike the other factors that have been invoked to account for mass extinctions, climate change is manifest to us all, whether we travel from the tropics to the poles or experience the seasonal cycle. Over a longer timescale, the issue of global warming in the recent past and likely future, and its probable consequences for other aspects of the environment, has occupied a considerable amount of media attention. Those people who are unaware of the likely consequences of the burning of fossil fuels cannot count themselves as well educated. Over a longer timescale, geologists have been aware for many decades of significant climatic changes on a global scale leading to the appearance and disappearance of polar ice caps on a number of occasions. Steve Stanley, the distinguished palaeobiologist at Johns Hopkins University in Baltimore, has actively promoted the view that episodes of climatic cooling are the most likely cause of mass extinctions. However, we must consider also the significance of global warming, and for the continents, at any rate, the possible effects of changes in the humidity–aridity spectrum. Before examining the relationships between climatic change and mass extinctions we need to examine the criteria from the stratigraphic record that geologists use to determine ancient climates, or palaeo-climates. The most obvious way of detecting cold conditions in the past is to find evidence of the presence of ice. At the present day the sedimentary deposits associated with glaciers and ice sheets, which occur where melting ice dumps its rock load, range in grain size from boulders and pebbles to finely ground rock flour. Such deposits are known as boulder clay or till, and ancient examples consolidated into resistant rock as tillites. The surfaces of hard rock that have underlain substantial ice sheets bear characteristic linear striations indicating the former direction of ice movement, such as glaciers moving up or down a U-shaped valley. The striations are produced by pebbles embedded in the ice, and are a unique marker for glacial action. In the 1830s Louis Agassiz, the great Swiss naturalist, extrapolated from his knowledge of the margins of Alpine glaciers to propose that the whole of northern Europe had been covered by one or more ice sheets in the recent geological past.
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