The Historical Development of Large-Scale Paleoclimate Field Reconstructions Over the Common Era

Climate field reconstructions (CFRs) combine modern observational data with paleoclimatic proxies to estimate climate variables over spatiotemporal grids during time periods when widespread observations of climatic conditions do not exist. The Common Era (CE) has been a period over which many seasonally- and annually-resolved CFRs have been produced on regional to global scales. CFRs over the CE were first produced in the 1970s using dendroclimatic records and linear regression-based approaches. Since that time, many new CFRs have been produced using a wide range of proxy data sets and reconstruction techniques. We assess the early history of research on CFRs for the CE, which provides context for our review of advances in CFR research over the last two decades. We review efforts to derive gridded hydroclimatic CFRs over continental regions using networks of tree-ring proxies. We subsequently explore work to produce hemispheric- and global-scale CFRs of surface temperature using multi-proxy data sets, before specifically reviewing recently-developed data assimilation techniques and how they have been used to produce simultaneous reconstructions of multiple climatic fields globally. We then review efforts to develop standardized and digitized databases of proxy networks for use in CFR research, before concluding with some thoughts on important next steps for CFR development. Few observational records of meteorological variables like surface temperature and precipitation extend beyond 100-150 years. The length of observational records is therefore insufficient for learning how climate varies over decades to centuries, or for estimating many climate extremes. In contrast, annually-resolved climate proxies, such as tree rings, ice cores, and corals, when used in concert with observational records, can provide information on how climate conditions have changed over decades to millennia. These proxies are also abundant enough over the last two millennia to create reconstructions in both space and time, or maps of climate conditions at seasonal or annual intervals. These kinds of reconstructions are called climate field reconstructions (CFRs) and we review their scientific history back to the 1970s when they were first attempted. We survey the underlying methods that are used to derive these reconstructions and how they have been applied, with a specific focus on the last two decades when the number of CFRs has greatly increased. We conclude with a survey of the data networks that are being cultivated for CFRs and some reflections on how the science can move forward. Reconstructions of climate fields are valuable paleoclimatic products that have been developed since the 1970sImportant similarities and differences exist across the approaches used to derive hydroclimate and temperature climate field reconstructions (CFRs) targeting the Common Era (CE)Controlled and systematic comparisons of CFRs should be a priority for the CE paleoclimate community