The reason the oxygen production value of urban trees is insignificant has to do with the large amount of oxygen within the atmosphere (approximately 21% of the atmosphere’s volume is oxygen). As stated by Miller (1979): “We have a large number of serious ecological problems, but suffocation from lack of oxygen is not one of them (Broecker 1970; SCEP 1970). The oxygen content of the atmosphere remains essentially constant with the oxygen consumed by all animals, bacteria, and respiration processes roughly balanced by the oxygen released by land and sea plants during photosynthesis. The present atmospheric oxygen content seems not to have changed since 1910 (SCEP 1970). Furthermore, because air is about 20 percent oxygen, the total supply is immense (Broecker 1970).” Our atmosphere has such an enormous reserve of oxygen that even if all fossil fuel reserves, all trees, and all organic matter in soils were burned, atmospheric oxygen would only drop a few percent (Broecker 1996). Also, waters of the world are the main oxygen generators of the biosphere; their algae are estimated to replace ≈90% of all oxygen used (Encyclopaedia Britannica 1994). Thus, although urban trees do produce significant amounts of oxygen, it is not a significant ecologic benefit given the global nature of oxygen and the sheer volume of oxygen in the atmosphere.