Abstract
Climate change risks triggering abrupt weakening in two climatically important North Atlantic Ocean circulation elements, the subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). Loss of AMOC stability has been inferred from slowing recovery of temperature and salinity fluctuations over time. However, observational datasets, constructed from records with sparse spatial and temporal coverage, may introduce substantial biases in stability indicators. Alternative records are therefore needed for reliable stability assessments. Here, using bivalve-derived environmental reconstructions, we show that the subpolar North Atlantic has experienced two destabilization episodes over the past ~150 years. The first preceded the rapid circulation changes associated with the 1920s North Atlantic regime shift, suggesting that a tipping point may have been crossed in the early 20th century. The second and stronger destabilization began around 1950 and continues to the present, supporting evidence of recent stability loss and suggesting that the region is moving toward a tipping point.