There's also speculation about recreating processes like limestone formation -which usually takes nature thousands of years- that eats carbon from the air.
Nanoarchitects are aiming to speed the process up to a matter of days. They believe it could be done simply by coating the walls of buildings with tiny droplets of engine grease. The grease would be laced with a common salt such as magnesium chloride. When the magnesium reacts with carbon dioxide in the air, a solid magnesium carbonate pearl begins to form.
This serves as the seed for the growth of white, wheatsheaf-shaped carbonate crystals. The large surface area of a droplet of grease maximises the interface between the magnesium and the atmospheric carbon, speeding up the rate of the reaction. Within days, the grease would be transformed into a sparkly crystalline coating similar in appearance to heavy frost or snowfall... A green city...would look like Narnia under the White Witch, crystal white and beautiful. The carbon choking our planet could become a harmless decorative feature.