  We study predictions of reversals of Earth's axial magnetic dipole field that
are based solely on the dipole's intensity. The prediction strategy is,
roughly, that once the dipole intensity drops below a threshold, then the field
will continue to decrease and a reversal (or a major excursion) will occur. We
first present a rigorous definition of an intensity threshold-based prediction
strategy and then describe a mathematical and numerical framework to
investigate its validity and robustness in view of the data being limited. We
apply threshold-based predictions to a hierarchy of numerical models, ranging
from simple scalar models to 3D geodynamos. We find that the skill of
threshold-based predictions varies across the model hierarchy. The differences
in skill can be explained by differences in how reversals occur: if the field
decreases towards a reversal slowly (in a sense made precise in this paper),
the skill is high, and if the field decreases quickly, the skill is low. Such a
property could be used as an additional criterion to identify which models
qualify as Earth-like. Applying threshold-based predictions to Virtual Axial
Dipole Moment (VADM) paleomagnetic reconstructions (PADM2M and Sint-2000)
covering the last two million years, reveals a moderate skill of
threshold-based predictions for Earth's dynamo. Besides all of their
limitations, threshold-based predictions suggest that no reversal is to be
expected within the next 10 kyr. Most importantly, however, we show that
considering an intensity threshold for identifying upcoming reversals is
intrinsically limited by the dynamic behavior of Earth's magnetic field.
