Resource over-provisioning is common practice in network infrastructures. Coupled with energy unaware networking protocols, this can lead to periods of resource underutilization and constant energy consumption irrespective of the traffic load conditions. Driven by the rising cost of energy - and therefore OPEX - and increasing environmental consciousness, research in power saving techniques has recently received significant attention. Unlike the majority of previous work in the area, which has focused on centralized offline approaches, in this paper we propose an online approach by which the capacity of the network can be adapted in a decentralized fashion. Based on the modular architectures of modern IP routers, adaptation is achieved by configuring individual line cards to enter sleep mode. Re-configuration is performed periodically by intelligent ingress nodes that coordinate their actions in order to control the traffic distribution in the network, according to the actual demand. We evaluate our approach using real network topologies and traffic traces. In the case of the GEANT network, the proposed approach can, on average, reduce the energy to power the required line cards by 46% for a maximum utilization below 65%, and by 18% under heavily loaded conditions.