Fossil fuel combustion also consumes oxygen and as a result of fossil fuel use, the oxygen content of the atmosphere is slowly declining. Between 1989 and 1994, the oxygen content of the atmosphere dropped at an annual rate of 2 ppm out of 210,000 ppm (Broecker 1996). Thus, reducing fossil fuel use in the management of urban forests not only reduces emission of carbon dioxide, but also the consumption of oxygen. If fossil fuels are consistently used to maintain an urban forest, the net effect of the forest and its management will be carbon dioxide production and oxygen consumption. The point in the future when carbon emissions from maintenance exceed carbon sequestration by the forest varies by tree species and intensity of fossil fuel-based management (Nowak et al. 2002b).