  The topology of center vortices is studied. For this purpose it is sufficient
to consider mathematically idealised vortices, defined in a gauge invariant way
as closed (infinitely thin) flux surfaces (in D=4 dimensions) which contribute
the n'th power of a non-trivial center element to Wilson loops when they are
n-foldly linked to the latter. In ordinary 3-space generic center vortices
represent closed magnetic flux loops which evolve in time. I show that the
topological charge of such a time-dependent vortex loop can be entirely
expressed by the temporal changes of its writhing number.
