  One of the most prominent climate tipping elements is the Atlantic Meridional
Overturning Circulation (AMOC), which can potentially collapse due to the input
of fresh water in the North Atlantic. Although AMOC collapses have been induced
in state-of-the-art Global Climate Models (GCMs) by strong freshwater forcing,
an AMOC tipping event has so far not been found. Here, we show results of the
first AMOC tipping event in such a GCM, the Community Earth System Model,
including the large climate impacts of the associated AMOC collapse. Using
these results, we develop a new physics-based and observable early warning
signal of AMOC tipping: the minimum of the AMOC induced freshwater transport at
the southern boundary of the Atlantic. Available observational and reanalysis
data indicate that the present-day AMOC is on route to tipping. The new early
warning signal is a useful alternative to classical statistical ones which,
when applied to our simulated AMOC tipping event, turn out to be sensitive to
the analysed time interval before tipping.
