A method and a system for removing carbon dioxide directly from ambient air, using a sorbent under ambient conditions, to obtain relatively pure CO2. The CO2 is removed from the sorbent using process heat, preferably in the form of steam, at a temperature in the range of not greater than about 130° C., to capture the relatively pure CO2. Increased efficiency can be achieved by admixing with the ambient air, prior to contacting the sorbent, a minor amount of a preferably pretreated effluent gas containing a higher concentration of carbon dioxide. The captured carbon dioxide can be stored for further use, or sequestered permanently. The system provides the sorbent substrate and equipment for carrying out the above method, and for obtaining purified carbon dioxide for further use in agriculture and chemical processes, or for permanent sequestration.