The Atlantic Meridional Overturning Circulation (AMOC) is a prominent climate tipping element that can collapse due to freshwater input in the North Atlantic. This study uses the Community Earth System Model (CESM) to simulate an AMOC tipping event and develop a physics-based early warning signal for AMOC collapse. The simulation shows that the AMOC collapses abruptly after a gradual weakening due to increasing freshwater forcing. The key indicator of AMOC stability is the minimum of the AMOC-induced freshwater transport at 34°S, which occurs 25 years before the tipping point. Reanalysis products indicate that the present-day AMOC is on a trajectory towards this tipping point. The minimum of the freshwater transport is a reliable early warning signal, while classical statistical indicators are less reliable. The study highlights the importance of sustained observations and future measurements to monitor the AMOC's stability and predict potential tipping events.The Atlantic Meridional Overturning Circulation (AMOC) is a prominent climate tipping element that can collapse due to freshwater input in the North Atlantic. This study uses the Community Earth System Model (CESM) to simulate an AMOC tipping event and develop a physics-based early warning signal for AMOC collapse. The simulation shows that the AMOC collapses abruptly after a gradual weakening due to increasing freshwater forcing. The key indicator of AMOC stability is the minimum of the AMOC-induced freshwater transport at 34°S, which occurs 25 years before the tipping point. Reanalysis products indicate that the present-day AMOC is on a trajectory towards this tipping point. The minimum of the freshwater transport is a reliable early warning signal, while classical statistical indicators are less reliable. The study highlights the importance of sustained observations and future measurements to monitor the AMOC's stability and predict potential tipping events.