Importance of beginning industrial-era climate simulations in the eighteenth century

Abstract Climate simulations of the industrial era typically start in 1850, using the first fifty years as a baseline for ‘pre-industrial’ climate. However, the period immediately prior to 1850 is of particular interest due to early human influence and heightened volcanic activity, the latter of which led to cooler global temperatures than those observed in the subsequent historical period. In this study, we present a suite of Earth system model simulations (using UKESM1.1) that start in 1750 and span the entire industrial period. We compare these simulations to a new instrumental observation-based dataset, GloSATref, which provides global surface air temperature variations from 1781 onwards. We investigate the climatic changes during the early industrial period, separating the effects of natural and anthropogenic forcings. Model-simulated early-19th-century temperature patterns show substantial cooling relative to the long-term mean, particularly in low latitudes, which agree well with observed patterns. We find significant long-term differences between simulations initialized in 1750 and 1850, with lasting effects well into the 20th century, consistent with differences in vegetation and the substantial ocean cooling driven by high volcanic activity in the 1750 simulations. Our results indicate that an earlier start to historical simulations could lead to more representative climate simulations over the historical period, and deepen our understanding of early anthropogenic warming, natural climate variability, and the climate responses to future volcanic eruptions.