Corals: Storehouse for Temperature Data
Corals are marine animals that form external skeletons (exoskeletons) of calcium carbonate (CaCO3). Colonies of corals produce reefs in clear, shallow waters. These animals generate denser layers in their exoskeletons 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 ratio of heavy to light oxygen isotopes (represented by ?18O) in shells of marine organisms decreases with the temperature of the surrounding seawater. In shallow waters where corals grow, the ?18O values of the seawater and, thereby, of the corals, also changes with rainfall, evaporation, and river input. Therefore, the ?18O record is often supplemented with other proxy measures of temperature, such as the strontium-to-calcium ratio (Sr/Ca).
Strontium (Sr) can substitute for calcium (Ca) in certain biological processes because both elements dissolve in water to form ions with the charge +2, and these ions are of similar size. The Sr/Ca ratio in coral skeletons decreases with temperature for reasons that may involve discrimination between the two elements during their transport from seawater into the coral. 
The world's largest coral reef, the Great Barrier Reef in Australia. Temperatures around the reef are as warm as they've been in the last 400 years.
Cores from coral reefs at the Great Barrier Reef, Australia dating as far back as 1565 show that temperatures near the corals during the last half of the twentieth century are as warm as they have been in over 400 years. Contrasting the Sr/Ca and ?18O data suggests that changes in the ?18O values of coral exoskeletons derive from both rising temperatures and increasing input of fresh water from human activities onshore, particularly after 1850.
 Corrège, T. (2006) Sea surface temperature and salinity reconstruction from coral geochemical tracers. Palaeogeography Palaeoclimatology Palaeoecology 232:408-428.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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