  The early detection of tipping points, which describe a rapid departure from
a stable state, is an important theoretical and practical challenge. Tipping
points are most commonly associated with the disappearance of steady-state or
periodic solutions at fold bifurcations. We discuss here multi-frequency
tipping (M-tipping), which is tipping due to the disappearance of an attracting
torus. M-tipping is a generic phenomenon in systems with at least two intrinsic
or external frequencies that can interact and, hence, is relevant to a wide
variety of systems of interest. We show that the more complicated sequence of
bifurcations involved in M-tipping provides a possible consistent explanation
for as yet unexplained behavior observed near tipping in climate models for the
Atlantic meridional overturning circulation. More generally, this work provides
a path towards identifying possible early-warning signs of tipping in
multiple-frequency systems.
