Volcanic eruptions caused weakening AMOC during the preindustrial past millennium

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of our climate system. However, the evolution history and the forcing mechanism of AMOC in the last millennial has remained uncertain. Here, combining available paleo AMOC-sensitive indicators with multiple climate models and paleo Data Assimilation, we show, for the first time, that the AMOC transport exhibits a slow declining trend into the Little Ice Age, with a reduction of about -0.7\,± 0.4Sv. (~4%) from AD 850-1850. This weakening is mainly caused by explosive volcanic eruptions. After a rapid strengthening in the initial decade following a volcanic eruption, the AMOC tends to show a slow weakening of centennial timescale, driven by stored cooling in the deep Atlantic and its feedback on North Atlantic climate. A combined model-data study shows that volcanic eruptions caused the weakening of the Atlantic Meridional Overturning Circulation by ~0.7 Sv. during the past millennium before industrial revolution.