  It is widely believed that exact global symmetries do not exist in theories
that admit quantum black holes. Here we propose a way to quantify the degree of
global symmetry violation in the Hawking radiation of a black hole by using
certain relative entropies. While the violations of global symmetry that we
consider are non-perturbative effects, they nevertheless give $\mathcal{O}(1)$
contributions to the relative entropy after the Page time. Furthermore, using
"island" formulas, these relative entropies can be computed within
semi-classical gravity, which we demonstrate with explicit examples. These
formulas give a rather precise operational sense to the statement that a global
charge thrown into an old black hole will be lost after a scrambling time.
  The relative entropies considered here may also be computed using a replica
trick. At integer replica index, the global symmetry violating effects manifest
themselves as charge flowing through the replica wormhole.
