Quantitative attribution of Northern Hemisphere temperatures over the past 2000 years
Feng SHI1,2(), Mingfang TING3, Zhengtang GUO1,2,4
1. Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2. CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044, China 3. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA 4. University of Chinese Academy of Sciences, Beijing 100049, China
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. These findings have important implications for the attribution of past climate variability and improvement of future climate projections.
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