Rising CO2 Levels: Earth's Changing Atmosphere
An important impact of climate change is a rise in the level of carbon dioxide and a decline in oxygen levels. Atmospheric concentration of CO2 has risen from about an average of 0.027% (270 ppm) in preindustrial times to 0.039% at present and is anticipated to reach between 0.05% and 0.10% by the end of the century. These concentrations exceed any that have occurred during the last 20 million year. Direct effects of such changes on organisms can be profound.
The atmospheric CO2 concentration anticipated during this century is only a small fraction of the concentrations that cause respiratory distress. Human beings suffer loss of mental acuity at a CO2 concentration between 2% and 7.5%, loss of consciousness at between 5% and 10%, and loss of life at between 20% and 30%. The National Research
Council recommends that long-term exposure to CO2 in submarines be kept below 0.8% (National Research Council 2007).
Processes that alter atmospheric CO2 concentration generally produce nearly equal but opposite changes in atmospheric oxygen (O2) concentration. The actual ratios of CO2 to O2 vary among processes.
• When plants conduct photosynthesis, they release between 1.0 molecule and 1.2 molecules of O2 for every molecule of CO2 that they consume, depending on the extent to which they are synthesizing sugars versus converting nitrate (a form of inorganic nitrogen) into protein. 
• Combustion of natural gas, crude oil, and solid carbon consume 1.95 molecules, 1.44 molecules, and 1.17 molecules of O2, respectively, for every molecule of CO2 released. 
• Ingesting large quantities of fats (e.g., an Atkins Diet), leads to an inhalation of 1.4 molecules of O2 for every molecule of CO2 that one exhales; on the other hand, if you ingest mostly carbohydrates (e.g., an anti-Atkins Diet), you will inhale 1.0 molecule of O2 for every molecule of CO2 you exhale.
• When plants respire, they consume 1.1 molecules of O2 for every molecule of CO2 released if they are using ammonium (another form of inorganic nitrogen) as a nitrogen source and 1.4 molecules of O2 for every molecule of CO2 released if they are using nitrate.  Exceptions to these relatively fixed ratios between O2 and CO2 fluxes occur at the air–sea interface, where substantial O2 exchange may be independent of CO2 exchange and vice versa. In the summer, atmospheric O2 concentration rises as enhanced algal photosynthesis releases more O2 to the air. Deep waters tend to be slightly deficient in O2 because respiration and other chemical reactions consume O2, while limited mixing with the atmosphere and limited photosynthesis because of low light levels fail to replenish it. When these deep waters, low in O2, up well to the surface in the wintertime, more atmospheric O2 dissolves into the oceans, and the atmospheric O2 concentration falls. Fluctuations in atmospheric concentration of CO2 are smaller in amplitude than those of O2 for two reasons: CO2 is more soluble in water than O2, and oceans can absorb large amounts of CO2 as bicarbonate (HCO3–) and carbonate (CO32–) ions. As a consequence, terrestrial CO2 exchanges, but not oceanic ones, are proportional to atmospheric O2 fluctuations.  A comparison between CO2 and O2 fluctuations, therefore, provides an estimate of the relative amounts of CO2 sequestered in the ocean versus that sequestered on land .
Rates at which CO2 is sequestered in biomass on land or dissolved in oceans. Units are in 109 metric tons of carbon equivalents per year (carbon equivalent is the carbon content of CO2 where 1 g carbon equivalent = 3.67 g CO2). Based on changes in CO2 and O2 concentrations at Barrow, American Samoa, and Cape Grim. [After Bender et al. 2005.]
Computer models predict, based on the anticipated changes in atmospheric CO2 concentration and the ratios between O2 and CO2 fluxes, that atmospheric O2 concentration will decrease in the future — but this will not leave people gasping for breath. Seasonal O2 oscillations are a mere 0.003% (30 ppm) on a background of 20.946%. The decline in average global O2 concentrations from 1993 to 2003 of about 25 ppm is smaller than the one a person experiences in an elevator when it ascends one or two floors. On top of Mt. Everest, where everyone gasps for breath, O2 concentration is about 7%.
 Rachmilevitch, S., A. B. Cousins, and A. J. Bloom (2004) Nitrate assimilation in plant shoots depends on photorespiration. Proceedings of the National Academy of Sciences of the United States of America 101:11506-11510.
 Bender, M. L., D. T. Ho, M. B. Hendricks, R. Mika, M. O. Battle, P. P. Tans, T. J. Conway, B. Sturtevant, and N. Cassar (2005) Atmospheric O2/N2 changes, 1993-2002: Implications for the partitioning of fossil fuel CO2 sequestration. Global Biogeochemical Cycles 19: doi:Gb4017.
 Bloom, A. J., S. S. Sukrapanna, and R. L. Warner (1992) Root respiration associated with ammonium and nitrate absorption and assimilation by barley. Plant Physiology 99:1294-1301
 Keeling, R. F. and H. E. Garcia (2002) The change in oceanic O2 inventory associated with recent global warming. Proceedings of the National Academy of Sciences of the United States of America 99:7848-7853.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents