Reconstructing Earth's surface temperature over the past 2000 years: the science behind the headlines

Abstract

The last quarter century spans the publication of the first assessment report of the Intergovernmental Panel on Climate Change in 1990 and the latest report published in 2013-2014. The five assessment reports appearing over that interval reveal a marked increase in the number of paleoclimate studies addressing the climate of the last 2000 years (the Common Era). An important focus of this work has been on reconstruction of hemispheric and global temperatures. Several early studies in this area generated considerable scientific and public interest, and were followed by high-profile and sometimes vitriolic debates about the magnitude of temperature changes over all or part of the Common Era and their comparison to 20th-and 21st-century global temperature increases due to increasing levels of atmospheric greenhouse gases. Behind the more public debates, however, several consistent themes of scientific inquiry have developed to better characterize climate variability and change over the Common Era. These include attempts to collect more climate proxy archives and understand the signals they contain, improve the statistical methods used to estimate past temperature variability from proxies and their associated uncertainties, and to compare reconstructed temperature variability and change with climate model simulations. All of these efforts are driving a new age of research on the climate of the Common Era that is developing more cohesive and collaborative investigations into the dynamics of climate on time scales of decades to centuries, and an understanding of the implications for modeled climate projections of the future. © 2016 Wiley Periodicals, Inc. How to cite this article:WIREs Clim Change 2016. doi: 10.1002/wcc.418 INTRODUCTIONThe farther backward you can look, the farther forward you are likely to see.-Winston Churchill T his well-known comment by Winston Churchill asserts that there are lessons of history that help us better understand the present and more clearly anticipate the future. Herein we review the application of this tenet to paleoclimatology, the scientific field that seeks to estimate, or reconstruct, Earth's past climatic states and interpret the causes of past variability. We principally address climate reconstructions of the past two millennia, a period of time known as the Common Era (CE), that use proxies for climate information prior to the instrumental record of the past two centuries or so. A climate proxy, by definition, is a surrogate for an instrumental observation such as temperature, precipitation, or solar irradiance. Proxies are selected principally on the basis of their sensitivity to changes in target quantities, their geographic distribution, and their temporal range and resolution. Each proxy is nevertheless an imperfect representation of past climate, a reality that is central to ongoing efforts to improve proxy acquisition, interpretation, and application for a better understanding of past and, consequently, future climate variability and change.Our focus in this review is sp...

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