The greenhouse effect developed from burning materials that contain carbon, such as coal and oil. Judith Zeeman has created Carbon Age: three differently sized clocks made from charcoal that display carbon emissions in various periods in history. The hands point at the speed with which the carbon is emitted every year. Carbon is made from plant material that has been gassed instead of burnt. The carbon in charcoal is inert: when it is buried, it cannot be absorbed into the environment, and the CO2 is prevented from entering the atmosphere for at least a thousand years. By burying these clocks in the garden at the end of their life, users will get the opportunity to contribute to reducing the greenhouse effect. In addition, the charcoal will act as fertiliser for plants.