The Collaborative International Dictionary of English v.0.48:

Alarmist \A*larm"ist\, n. [Cf. F. alarmiste.]
   One prone to sound or excite alarms, especially, needless
   alarms. --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alarmist
    n 1: a person who alarms others needlessly