Progress and uncertainties in global and hemispheric temperature reconstructions of the Common Era

Global and hemispheric temperature reconstructions provide an important means of placing recent anthropogenic temperature trends in the context of preindustrial climate variations and evaluating their causes. As new reconstructions have been developed and estimates of past climate have been refined, results continue to show that by the late 20th century temperatures very likely exceeded those of any time in at least the last millennium. Despite progress over the last two decades, however, there remain persistent uncertainties with regard to, inter alia, first millennium temperatures at global and annual scales, the magnitude of multidecadal to millennial-scale changes and their causes, and the surface temperature response to volcanic eruptions. We review the strengths and limitations of existing global and hemispheric paleoclimate temperature reconstructions and highlight likely sources of extant uncertainties, all in the context of the recent Sixth Assessment Report from Working Group I of the Intergovernmental Panel on Climate Change. Based on our review of these factors, we provide recommendations for using, interpreting, and improving large-scale temperature reconstructions.