Motivated by applications of sensor networks, there has been growing interest in monitoring large scale distributed systems. In these applications, we usually wish to monitor a global system condition defined as a function of local network elements parameters. In this paper, we study Reactive Monitoring in sensor networks, which has the benefit of operating in a decentralized manner. Our primary concern in adopting such a monitoring paradigm is reducing the communication cost which is the dominant factor of energy drain in wireless sensor networks. In this study, we address the reactive aggregate monitoring problem by casting the underlying threshold assignment as an optimization problem. This allow us to propose a distributed algorithm to set local thresholds on each sensor node to be adapted to the statistics of the events measured by spatially scattered sensor nodes. Through simulation, we illustrate that the proposed threshold assignment technique can significantly reduce the communication overhead of the monitoring mechanism in sensor networks.