and organizations have produced an extensive body of literature on the importance of open access to scientific data and on the related guidelines for data archiving and data access (e.g., NRC 1995). Paleoclimate research would benefit if individual researchers, professional societies, journal editors, and funding agencies continued to improve their efforts to ensure that these existing open-access practices are followed.

Tree ring researchers have recognized the importance of data archiving since 1974, when the International Tree Ring Data Bank was established to serve as a permanent repository for tree ring data (measurements, chronologies, and derived reconstructions). Its holdings are available online via the World Data Center for Paleoclimatology, as are a number of other proxy data from ice cores, corals, boreholes, lake and ocean sediments, caves, and biological indicators. As proxy datasets become increasingly available on the Web, all researchers are given the opportunity to analyze data, test methods, and provide their own interpretation of the existing evidence via recognized, peer-reviewed scientific outlets.

What might be done to improve our understanding of climate variations over the last 2,000 years?

Surface temperature reconstructions have the potential to further improve our knowledge of temperature variations over the last 2,000 years, particularly if additional proxy evidence can be identified and obtained. Additional proxy data that record decadal-to-centennial climate changes, especially for the period A.D. 1–1600, would be particularly valuable. New data from the Southern Hemisphere, the tropics, and the oceans would improve our confidence in global temperature reconstructions, while additional data from regions that have already been sampled would help reduce the uncertainties associated with current reconstructions. In addition, many existing proxy records were collected decades ago and need to be updated in order to perform more reliable comparisons with instrumental records. Better data coverage would also make it possible to test whether or not past temperature changes had the same pattern as the warming during the last century.

New methods, or more careful use of existing ones, may also help circumvent some of the existing limitations of large-scale surface temperature reconstructions based on multiple proxies. Each individual proxy provides a record of environmental change, but the process of combining these signals into a spatially averaged temperature signal requires careful statistical evaluation. It might be possible to circumvent some of the limitations associated with these reconstructions by employing a number of complementary strategies in analyzing the proxy data, including using them to constrain climate models, and by attempting to calibrate the proxy data against climatic variables in different ways.

Finally, some of the most important consequences of climate change are linked to changes in precipitation, especially the frequency and intensity of droughts and floods, as opposed to temperature alone. Changes in regional circulation patterns, snowfall, hurricane activity, and other climate elements over time are also of interest. Hence, it would be valuable to see both regional and large-scale reconstructions of changes in precipitation and other climate variables over the last 2,000 years, to complement those made for temperature. Efforts to improve the reliability of surface temperature reconstructions also need to be complemented by efforts to improve our understanding of the forcings that have contributed to climate variability over the past 2,000 years.

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