produce quantitative reconstructions of past climates and use historical and archeological evidence, when it is available, to provide a consistency check.
Tree ring formation is influenced by climatic conditions, especially in areas near the edge of the geographic distribution of tree species. At high latitudes and/or at high elevations, tree ring growth is related to temperature, and thus trees from these sites are commonly used as a basis for surface temperature reconstructions. Cores extracted from the trees provide annually resolved time series of tree ring width and of wood properties, such as density and chemical composition, within each ring. In some cases, records from living trees can be matched with records from dead wood to create a single, continuous chronology extending back several thousand years.
Tree ring records offer a number of advantages for climate reconstruction, including wide geographic availability, annual to seasonal resolution, ease of replication, and internally consistent dating. Like other proxies, tree rings are influenced by biological and environmental factors other than climate. Site selection and quality control procedures have been developed to account for these confounding factors. In the application of these procedures, emphasis is placed on replication of records both within a site and among sites and on numerical calibration against instrumental data.
The annual bands in coral skeletons also provide information about environmental conditions at the time that each band was formed. This information is mostly derived from changes in the chemical and isotopic composition2 of the coral, which reflects the temperature and isotopic composition of the water in which it formed. Since corals live mostly in tropical and subtropical waters, they provide a useful complement to records derived from tree rings. Coral skeleton chemistry is influenced by several variables, and thus care must be taken when selecting coral samples and when deriving climate records from them. Thus far, most of the climate reconstructions based on corals have been regional in scale and limited to the last few hundred years, but there is now work toward establishing longer records by sampling fossil corals.
Oxygen isotopes measured in ice cores extracted from glaciers and ice caps can be used to infer the temperature at the time when the snow was originally deposited. For the most recent 2,000 years, the age of the ice can in most places be determined by counting annual layers. The isotopic composition of the ice in each layer reflects both the temperature in the region where the water molecules originally evaporated far upwind of the glacier and the temperature of the clouds in which the water vapor molecules condensed to form snowflakes. The long-term fluctuations in temperature