Information and communication technologies (ICT) are increasingly permeating our daily life and we ever more commit our data to the cloud. Events like the COVID-19 pandemic put an exceptional burden upon ICT infrastructures. This involves implementation and utilization of data centers, which increasingly contribute to energy and environmental impact. In this work, we address strategies to improve energy efficiency of data centers, to enhance sustainability towards energy resource and the environment. Among key strategies are innovative cooling technologies, use of natural resources, automation techniques, low-power electronics, and equipment with extended thermal limits. Research perspectives are identified and estimates of improvement opportunities are presented.