Ex situ mineral carbonation is likely to be an economical way to sequester carbon dioxide if waste products (such as concrete wastes or ash) are used as sources for metal carbonates. Most indirect methods result in uneconomical carbonation. If temporary sequestration is desired, the processes can be made profitable by selling the carbonates (particularly pure and ultrapure CaCO3), but the market for carbonates is much smaller than the scale which would be needed to capture a significant fraction of human carbon dioxide emissions. In situ methods are likely to be permanent methods for CO2 sequestration, requiring minimal monitoring, and are economical, but carbon dioxide sequestrated is difficult or impossible to reintegrate into the carbon cycle.