Calmodulin (CaM) in complex with Ca(2+) channels constitutes a prototype for Ca(2+) sensors that are intimately colocalized with Ca(2+) sources. The C-lobe of CaM senses local, large Ca(2+) oscillations due to Ca(2+) influx from the host channel, and the N-lobe senses global, albeit diminutive Ca(2+) changes arising from distant sources. Though biologically essential, the mechanism underlying global Ca(2+) sensing has remained unknown. Here, we advance a theory of how global selectivity arises, and we experimentally validate this proposal with methodologies enabling millisecond control of Ca(2+) oscillations seen by the CaM/channel complex. We find that global selectivity arises from rapid Ca(2+) release from CaM combined with greater affinity of the channel for Ca(2+)-free versus Ca(2+)-bound CaM. The emergence of complex decoding properties from the juxtaposition of common elements, and the techniques developed herein, promise generalization to numerous molecules residing near Ca(2+) sources.