Roman Warm Period and Late Antique Little Ice Age in an Earth System Model Large Ensemble

Changes in climate during the Roman Warm Period (RWP, 1–250 CE) and Late Antique Little Ice Age (LALIA, 536–660 CE) play a critical role in early societal evolution, but the climate differences between these two periods and the possible causes of the changes remain poorly explored. Here we use the LOch–Vecode‐Ecbilt‐CLio‐agIsM model Large Common Era Ensemble with 70 members to examine the climate change over these two intervals and compare the results of this ensemble with the latest temperature reconstructions from the Past Global Changes 2k network and the transient simulation for the past 2,000 years from the Community Earth System Model. Results from both proxy reconstructions and climate model simulations show warming in mid‐to‐high latitudes of the Northern Hemisphere (NH) during the RWP compared with the LALIA. This is likely linked with the increased radiative forcing associated with weaker volcanic eruptions in the RWP, which results in reduced sea ice area and pronounced high‐latitude warming through surface albedo and lapse‐rate feedbacks. This increases the upper ocean heat content over centennial time scales to maintain warming over the NH high‐latitude regions. Moreover, the RWP has drier (wetter) conditions in the eastern (western) equatorial Pacific than the LALIA, and this is related to the zonal sea surface temperature gradient in the equatorial Pacific through modification of the zonal circulation. Plain Language Summary Climate changes during the Roman Warm Period (RWP) and Late Antique Little Ice Age (LALIA) had large impacts on human migration, agricultural production, and political upheaval. However, little is known about the mechanism of climate change during these periods. We analyze the climate differences during the two periods using large ensemble climate model simulations and the latest proxy reconstructions. We find two consistent dominant features from model–data comparison: warming in mid‐to‐high latitudes of the Northern Hemisphere (NH) and drier (wetter) precipitation anomalies in the eastern (western) equatorial Pacific in the RWP than in the LALIA. Mid‐to‐high latitude warming may be related to the increased radiative forcing associated with weaker volcanic eruptions in the RWP, resulting in the decrease of sea ice area and Arctic warming amplification through positive surface albedo and lapse‐rate feedbacks. The precipitation differences may be caused by modification of the zonal circulation, which is forced by the atmo