Only recently have the possible economic ramifications of global warming been considered; the scientific uncertainties have been researched and discussed for a longer time, but progress has been slow. Although climate modeling has been given a high priority, more research on corroborating evidence and past climates is needed to calibrate the models and verify the associated assumptions (Eos, October 22, 1991, p. 465).
Proxy methods such as tree-ring studies and isotope studies on glacier ice generally give very good indications of past changes in temperature, especially over periods of a few years. Annual temperatures estimated from tree-ring data for the eastern United States [Fritts and Lough, 1985] and high-latitude North America [Jacoby and D'Arrigo, 1989] are at a minimum in the mid- to late-19 century and increase into the 20th century. Guiot's  reconstructed annual temperatures over 35-year intervals for central Canada generally are low in the mid 1800's and increase dramatically in the 20th century. Variations in temperature derived from tree-ring data for Russia [Graybill and Shiyatov, 1992] and Tasmania [Cook et al., 1991] also show that a cold period preceded the recent warming.