This paper deals with sustainable energy technologies under restrictions on resources and environmental impact. We have developed a database to estimate life-cycle efficiencies and greenhouse-gas emissions. Next, we define sustainability limitations on resource depletion and emissions. Based on these concepts, the present world energy system is judged to be unsustainable. Mitigation measures such as disposal of CO2 and solar power generation are evaluated utilizing the concept. We show how sustainability influences the cost-effectiveness of measures taken. At the same time CO2 removal is shown to play a transitional but important role to realize sustainable energy scenario in ultra-long term.