A method and a system to remove relatively pure carbon dioxide directly from ambient air. The method comprises generating process heat, to co-generate substantially saturated steam; alternately and repeatedly exposing a sorbent to a flow of ambient air, at substantially ambient conditions, to sorb, and therefore remove, carbon dioxide from said ambient air, and exposing the CO2-laden sorbent to a flow of the co-generated steam, at a temperature in the range of not greater than about 130° C, to release the carbon dioxide, thereby regenerating the sorbent, and capturing relatively pure carbon dioxide. To render this process more efficient, admix with the air a minor amount of a pre-treated effluent gas containing a higher concentration of carbon dioxide than in the atmosphere. The captured carbon dioxide can be stored for further use, or sequestered permanently. The purified carbon dioxide is useful for agriculture or chemical processes.