Michael E. Mann, Malcolm K. Hughes, & Raymond S. Bradley
Department of Geosciences, University of Massachusetts, Amherst, Massachusetts
Laboratory of Tree Ring Research, University of Arizona, Tucson, Arizona
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Reconstructions of annual global surface temperature patterns over several centuries are now possible, based on the multivariate calibration of widely distributed high-resolution proxy climate indicators. These reconstructions provide insight into both the spatial and temporal nature of climatic variations during the past six centuries. Time-dependent correlations of these temperature reconstructions with time series representing greenhouse gases, solar irradiance, and volcanic aerosols, suggest that each of these forcings has played a role in the climatic variability of the past several centuries, with greenhouse gases appearing to emerge as the dominant forcing during the 20th century. Northern hemisphere mean annual temperatures for three of the past eight years are warmer than any other year since (at least) 1400 AD, at a greater than 99.5% level of confidence.

Knowing both the spatial and temporal patterns of climatic change over the past several centuries remains a key to assessing a possible anthropogenic impact on post-industrial climate. In addition to the possibility of warming due to enhanced greenhouse gases during the past century, there is evidence that both solar irradiance and explosive volcanism have played an important part in forcing climate variations over the past several centuries. The unforced 'natural variability' of the climate system may also be quite important on multidecadal and century timescales. If a faithful empirical description of climate variability can be obtained for the past several centuries, a more confident estimation can be made of the roles of different external forcings and internal sources of variability on past and recent climate.

Because widespread instrumental climate data are available for only about one century, we must use proxy climate indicators combined with any very long instrumental records that are available to obtain such an empirical description of large-scale climate variability during past centuries. A variety of studies have sought to employ a 'multiproxy' approach to study long-term climatic variations, in which a widely-distributed set of proxy and instrumental climate indicators are analyzed to yield insights into long-term global climatic variations. Building on such past studies, we take a new statistical approach to reconstructing global patterns of annual temperature back through the 15th century, based on the calibration of multiproxy data networks by the dominant patterns of temperature variability in the instrumental record.

Using these statistically-verifiable yearly global temperature reconstructions, we analyze the spatiotemporal patterns of climatic change over the past half-millenium, and then take an empirical approach to estimating the relationship between global temperature changes, variations in volcanic aerosols, solar irradiance, and greenhouse gas concentrations during the same period.

Revised:  April 28, 1998