|MadSci Network: Earth Sciences|
There is disagreement over which photosynthetic organisms, land plants or phytoplankton and algae in the oceans, provide the majority of the earth's photosynthesis. Some experts ascribe more than half of current global photosynthesis to the oceans. Salisbury and Ross (1985) estimated one third of photosynthesis in oceans and two-thirds on land. They presented 1975 estimates that rainforests accounted for 22% of global photosynthesis and the oceans 32%. We know that oxygen definitely comes from photosynthesis and rain forests do produce a significant but decreasing amount because of all the rainforest destruction. Keep in mind that the accuracy of such estimates is not completely known as the Woods Hole website states "Unfortunately, the global rates of photosynthesis and respiration are neither known nor measured well enough to determine annual changes in carbon storage." There is a great deal of interest in computer modeling the global carbon-oxygen patterns because of global warming/climate change concerns. Oxygen makes up about 21% or 210,000 ppm of the atmosphere. Atmospheric carbon dioxide is now at about 365 ppm, and Salisbury and Ross (1985) state that only about 10% of the carbon dioxide in the atmosphere is used each year in photosynthesis. If that figure is a good estimate than you can see that a very tiny percentage of the earth's total atmospheric oxygen is produced each year by photosynthesis, i.e. 36.5 ppm is only 0.017% of total atmospheric oxygen. Despite the tiny percentage, annual global photosynthesis is immense. The second website cited says annual photosynthesis fixes carbon each year equivalent to "a line of railroad cars filled with coal stretching between Earth and the Moon 45 times." Right now, of course, the earth's oxygen level is falling every so slightly as the carbon dioxide concentration increases about 1.5 parts per million (ppm) each year. The carbon dioxide increase/oxygen decrease is due to deforestation and burning of fossil fuels. Thus, we are using some of the atmospheric oxygen added by ancient photosynthesis. Fossil fuels, such as coal, oil and natural gas represent carbon fixed by ancient photosynthesis. Fossil fuels can also be considered to represent photosynthetic oxygen placed in the atmosphere tens-of- millions to hundreds-of-millions of years ago. Therefore, you can consider that you are breathing some oxygen originally added to the atmosphere by giant lycopods, tree ferns and giant horsetails from the Carboniferous era 360 to 286 million years ago. References Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth. Woods Hole Research Center Carbon Cycle Earth's First-Ever Global Biological Record Documented from Space Global Terrestrial Ecosytem Carbon Model The Carboniferous Carboniferous Forests
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