  Archival data from the WISE satellite reveals infrared flux variations of
tens of per cent around numerous dusty white dwarfs. Data spanning more than
seven years reveal more than half of known systems are varying in the 3.4
micron band, while the 4.6 micron data are challenging to interpret due to
lower signal-to-noise. The sparsely-sampled data limit interpretation, but the
heterogeneous light curves suggest each source may be idiosyncratic, where
there maybe competing processes operating on different time-scales. Collisions
are likely driving the observed decays in flux, and this finding suggests that
dust production is operating more often than indicated by previous
observations. The observed variation is at odds with the canonical flat disc
model in isolation, and underscores the need for infrared monitoring of these
evolved planetary systems to inform the next generation of theoretical models.
