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Climate reconstructions over the last thousand years employ several proxy measures in addition to drill cores. These include tree rings, coral, glacial lengths, and bore hole. The following describes each of these approaches and shows climate reconstructions derived from them. Tree rings, the field of Dendrochronology, is the study of tree rings to evaluate past environmental conditions. It is based on the phenomenon that each year a tree adds a layer of wood between old wood and bark. When growing conditions are favorable, a tree produces large cells. As conditions worsen, growth slows, and cell size decreases until unfavorable conditions, growth, and cell division ceases. The contrast between the small cells formed late in the season, one year, and the large cells formed early in the following season appear like a ring whend observed in a cross section. Trees from the same region develop a similar pattern of ring widths during a given time period. Comparing these patterns, matching them ring for ring, and averaging over multiple trees establishes a chronology. Wood from ancient structures when aligned with chronologies from living trees, extends a chronology even further back. One chronology for river oaks from South Germany extends for more than 10,000 years and another for bristlecone pines in the White Mountains of California in the United States extends for more than 8,500 years. Of course, as one goes further back in time, appropriately aged samples become fewer in number and farther between. And so chronologies become less certain. To sample a tree, one screws a tool called an inclement borer into the trunk, extracts the core about five milometers in diameter, and sections the core, polishes the sections, and examines it under a microscope. This procedure does not permanently injure the tree. The width of growth rings, especially for trees found near their altitudinal or latitudinal limits increases with air temperatures. Temperatures during the middle of a growing season have the strongest influence on ring width. A compilation of tree ring data from the Northern hemisphere indicates that air temperatures today are as warm as anytime during the last 1200 years. Another proxy measure involves corals. Corals are colonial marine animals that form exo-skeletons of calcium carbonate that produce reefs in clear shallow waters. These animals generate denser layers in their exo-skeleton during months with severe weather. And less dense layers during months with more benign weather. As a result, corals develop discernible annual bands that can be counted to establish the age of a sample. The delta eighteen oxygen in carbonates of marine organisms as discussed earlier decreases with the temperatures of the surrounding sea water. Cores from the coral reefs at the Great Barrier Reef in Australia dating as old as 1565 show that temperatures near the corals during the last half of the twentieth century are as warm as they have ever been.