In this review, we present the different measures of early warning signals
that can indicate the occurrence of critical transitions in complex systems. We
start with the mechanisms that trigger critical transitions, how they relate to
warning signals and the methods used to detect early warning signals (EWS) for
sudden transitions or tipping. We discuss briefly a few applications in real
systems in this context, like transitions in ecology, climate and environment,
medicine, epidemics, finance and engineering. Towards the end, we mention the
issues in detecting EWS in specific applications and our perspective on future
trends in this area, especially related to sudden transitions in the dynamics
of connected systems on complex networks.Comment: 35 pages, 11 figure