Quantitative attribution of Northern Hemisphere summer temperaturesover the past 2000 years

Quantitative assessment of natural internal variability and externally forced responses of Northern Hemisphere (NH) temperatures is necessary for understanding and attributing climate change signals during past warm and cold periods. However, it remains a challenge to distinguish the robust internally generated variability from the observed variability. Here, large-ensemble (70 member) simulations, Energy Balance Model simulation, temperature ensemble reconstruction, and three dominant external forcings (volcanic, solar, and greenhouse gas) were combined to estimate the internal variability of NH summer (June–August) temperatures over the past 2000 years (1–2000 CE). Results indicate that the Medieval Climate Anomaly was predominantly attributed to centennial-scale internal oscillation, accounting for an estimated 104% of the warming anomaly. In contrast, the Current Warm Period is influenced mainly by external forcing, contributing up to 90% of the warming anomaly. Internal temperature variability offsets cooling by volcanic eruptions during the Late Antique Little Ice Age. Ultimately, this study indicates that the dominant internal climate factor driving centennial-scale fluctuations in NH summer temperatures over the last two millennia has been the AMOC, and that the primary external forcing agent is volcanic activity. These findings have important implications for the attribution of past climate variability and improvement of future climate projections.