  In the spirit of Richardson's original (1948) study of the statistics of
deadly conflicts, we study the frequency and severity of terrorist attacks
worldwide since 1968. We show that these events are uniformly characterized by
the phenomenon of scale invariance, i.e., the frequency scales as an inverse
power of the severity, P(x) ~ x^-alpha. We find that this property is a robust
feature of terrorism, persisting when we control for economic development of
the target country, the type of weapon used, and even for short time-scales.
Further, we show that the center of the distribution oscillates slightly with a
period of roughly tau ~ 13 years, that there exist significant temporal
correlations in the frequency of severe events, and that current models of
event incidence cannot account for these variations or the scale invariance
property of global terrorism. Finally, we describe a simple toy model for the
generation of these statistics, and briefly discuss its implications.
