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Glaciers form over land where temperatures are sufficiently low so that, during the course of a year, more snow falls than melts. Successive snow accumulations over many years compact the snow, which slowly recrystallizes into ice. Under the influence of gravity, the ice begins to move, and a glacier is formed. A mere 18,000 years ago, Earth was in the grip of a cold spell, with alpine glaciers extending down river valleys and continental glaciers covering vast areas of North America and Europe. The ice measured several kilometers thick and extended as far south as New York and the Ohio River Valley. Perhaps glaciers advanced ten times during the past two million years, only to retreat. In the warmer periods between glacier advances, average global temperatures were slightly higher than they are at present. The geological evidence left behind by advancing and retreating is one factor suggesting that global climate has undergone slow but continuous changes. To reconstruct past climates, scientists must examine and then carefully piece together all the available evidence. Unfortunately, the evidence only gives a general understanding of what the climate was like. For example, fossil pollen of a tundra plant collected in a layer of sediment in New England and dated to be 12,000 years old suggests that the climate of that region used to be much colder. Other evidence of global climatic change comes from core samples (taken by deep drilling) from ocean-floor sediments and ice from Greenland. Thousands of meters of ocean-floor sediment obtained with a hollow-centered drill were analyzed. The sediment contains the remains of calcium carbonate shells of organisms that once lived near the surface. Because certain organisms live within a narrow range of temperature, the distribution and type of organisms within the sediment indicate the surface-water temperature. In addition, the oxygen-isotope ratio of these shells provides information about the 【sequence】 of glacier advances. For example, most of the oxygen in seawater has an atomic weight of 16 because its nucleus contains eight protons and eight neutrons. However, about 1 out of every 1,000 oxygen atoms contains an extra two neutrons, giving it an atomic weight of 18. When ocean water evaporates, the heavier oxygen 18 tends to be left behind. In periods of cold, the evaporated water, rich in oxygen 16, falls as snow and becomes part of growing glaciers rather than returning to the oceans. Consequently, during periods of glacier advance, the oceans have less water and a higher concentration of oxygen 18. Since the shells of marine organisms are constructed from the oxygen atoms existing in ocean water, determining the ratio of oxygen 18 to oxygen 16 within these shells yields information about how the climate may have varied in the past. A higher ratio of oxygen 18 to oxygen 16 in the sediment record suggests a colder climate, while a lower ratio suggests a warmer climate. Using data such as these, scientists have been able to reconstruct Earth's ocean surface temperatures. Vertical ice cores extracted from glaciers in Antarctica and Greenland provide additional information on past temperature patterns. Since ice is composed of hydrogen and oxygen, examining the oxygen-isotope ratio in ancient cores provides a past record of temperature trends. Generally, the colder the air when the snow fell, the richer the concentration of oxygen 16 in the core. Moreover, bubbles of ancient air trapped in the ice can be analyzed to determine the past composition of the atmosphere.Still other evidence of climatic change comes from dendrochronology, the study of annual growth rings of trees. As a tree grows, it produces a layer of wood cells under its bark. Each year's growth appears as a ring. The changes in thickness of the rings may indicate climatic changes that have taken place from one year to the next. Tree rings are only useful in regions that experience an annual weather cycle and in trees that are stressed by temperature or moisture during their growing season. The growth of tree rings has been correlated with precipitation and temperature patterns for hundreds of years into the past in various regions of the world.
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