Decrease in atmospheric O2 has been detected in stations around the world for the past decade, a consistent downward trend that has accelerated in recent years.
The largest fall in O2 was observed in the study of Swiss research team led by Francesco Valentino at University of Bern, for data collected at high altitude research stations in Switzerland and France. The Jungfraujoch (JFJ) station in Switzerland (3 580 m above sea level, 46o 33’N, 7o 50’E) is located on a mountain crest on the northern edge of the Swiss Alps. The Puy de Dôme station (1 480 m above sea level, 45o46’N, 2o 58’E) is situated west of the Alps at the summit of Puy de Dôme.
The research team confirmed the general upward trend for atmospheric CO2 and a downward trend in atmospheric O2. But since 2003 for JFJ, and mid 2002 for at Puy, there is a significant enhancement of O2 and CO2 trends compared to previous years. At JFJ, the rate of CO2 increase shifted up from 1.08 ppm (parts per million) for the years 2001-2002 to 2.41 ppm/y for 2003-2006; while the increase in D(O2/N2) and APO (measures of oxygen concentration, see Box 1) shifted downwards to greater extents from –2.4 ppm/y and -1.5 ppm/y to -9.5 ppm/y and -6.9 ppm/y respectively.
For Puy, CO2 increase changed from 2.43 ppm/y for 2001-2002 to 1.07 ppm/y for 2003-2004, followed by 2.4 ppm/y for the years 2005-2006; while the changes in D(O2/N2) and APO were -6.1 ppm/y and -3.7 ppm/y for 2001-2002, to -10.4 ppm/y and -7.6 ppm/y for the years 2002-2006. Averaged over all years – by removing the trends and plotting correlations between CO2 and O2, an O2:CO2exchange ratio of -1.9+0.7 is found for JFJ, and -1.8+0.5 for Puy; both significantly different from the 1.1 assumed for land photosynthesis and respiration i.e., 1.1 mole of O2 generated per mole of CO2 fixed, and -1.4 for burning fossil fuels, or 1.4 mole of O2 used up when one mole of CO2 is produced.
Over time, the O2:CO2exchange ratio for JFJ, which is much less exposed to local or regional anthropogenic influence because of its elevation and location, was -2.1+0.1 for the years 2001-2002 and -4.1+0.1 for the years 2003-2006. At Puy, the ratio was -4.2+0.1 for the period 2001-2003, and -7.3+0.1 for 2003-2006. These ratios are completely out of line with what could be expected from fossil fuels, and other data indicate that there has been no significant change in fossil fuel emission rates during the period 2003-2006.
The researchers speculated that the large decrease in atmospheric oxygen since 2003 could have been the result of oxygen being taken up by the ocean, either due to a cooling of water in the North Atlantic, or water moving northwards from the tropic cooling, both of which would increase the water’s ability to take up more oxygen. However, it would require unrealistic cooling to account for the change in O2concentration. And all the indications are that the ocean waters have warmed since records began .
In a second study, atmospheric O2 and CO2 data collected from two European coastal stations between 2000 and 2005 were analyzed . Mace Head Ireland (53o20’N 9o54’W, 35 m above sea level), which serves as the marine background, relatively free from local fossil fuel consumption, and Station Lutjewad (53o24’N, 6o21’E) on the northern coast of The Netherlands 30 km to the northwest of the city of Groningen, which serves as a continental station receiving continental air with northerly winds. Similar trends were detected. Over the entire period at Lutjewad, CO2 increased by 1.7+0.2 ppm/y while oxygen decreased at -4.2+0.3 ppm/y; the corresponding figures for Mace Head were 1.7+0.1 ppm/y and -4.0+0.3 ppm/y. O2 is decreasing faster than can be accounted for by the rise in CO2. Furthermore, the decrease is not uniform throughout the entire period; instead it is much steeper between 2002 and 2005 at both stations, and is not accompanied by any change in the trend of CO2 increase. This sharp acceleration in the downward trend of atmospheric O2 from 2002-2003 onwards in Ireland and The Netherlands is in accord with the findings in Switzerland and France . And this cannot be explained by a realistic increase in fossil fuel use, or oxygen uptake by cooler ocean waters; if anything, oxygen level in the oceans has also been falling . So where and what is this oxygen sink that is soaking up oxygen?Mystery of the oxygen sink
One distinct possibility that has been considered is that an extra oxygen sink has opened up on land as the result of human activities.
James Randerson at University of California Irvine was lead author on a report published in 2006  pointing out that a decrease in atmospheric O2 could result if carbon within the land biosphere becomes more oxidized (sequestering more oxygen) through disturbance of natural ecosystems. This has changed the natural land cover, replacing it with plants that effectively remove more oxygen from the atmosphere.
Atmospheric exchange of O2 with land ecosystems is commonly expressed in terms of a net carbon flux from the atmosphere to the ecosystem (Fnet) and the net O2:CO2exchange ratio (Rnet):
dO2/dt = - Rnet Fnet (6)
By convention, positive sign indicates release into the atmosphere and negative sign sequestered in the land biosphere. The net rate is really a difference between two processes, one moving from atmosphere to biosphere, and the other in reverse, from biosphere to atmosphere, so eq. (6) can be written as follows.
dO2/dt = - (Rab Fab + RbaFba) (7)
where Fab is the atmosphere to biosphere carbon flux (the same as net primary productivity, NPP), Rab is the oxidative ratio related to NPP (moles O2 released per mole CO2 fixed), Fba is the biosphere to atmosphere return flux (a combination of respiration, fires and other losses), and Rba is the oxidative ratio related to the return flux (moles O2 consumed per mole CO2 released).
In an ecosystem at steady state (in dynamic balance), Fab and Fba will have the same magnitude. But the carbon in Fba is always offset in time from newly assimilated carbon in Fab because of carbon storage in the plant, dependent on plant tissue lifetimes, rates of litter and soil organic matter decomposition, and so on. Changes in Rab and Rba have the potential to cause relatively large changes in atmospheric O2, basically because of the time delays between fixation and the return flux due to carbon storage. The longer the carbon storage (turnover) time, the larger the effective offset between Fab and Fba; so O2 is consumed at a slower rate, and more of it remains in the atmosphere
Randerson and colleagues hypothesize that increasing levels of disturbance across natural ecosystems in recent decades has decreased Rab. This includes wide-spread deforestation and replacement of woody vegetation with pastures and crops in the tropics, an increase in fire activity and tree mortality and increasing the abundance of deciduous tree species and herbaceous plants in the boreal (northern) regions. Globally, this includes an increase in invasive species and increased disturbance of agricultural soils by plowing and grazing during the 20th century. All these activities increase the oxidation state of carbon in plant and soil organic matter. The increases in oxygen content of the resultant biomass causes a small sink for atmospheric O2 that has not been accounted for in atmospheric budgets.
Within a plant, lipids and lignin compounds have carbon that is more reduced, i.e., with more hydrogen and less oxygen; they have and large R values of 1.37 and 1.14 respectively, and are energetically more costly to build than compounds such as cellulose and starch, which have less hydrogen, more oxygen, and R value of 1.0. Thus, the expansion of agriculture and grazing during the 20th century has probably caused a decrease in the oxidative ratio of the plant biomass within these disturbed ecosystems. Using several simple models, the researchers showed that, indeed, small changes in Rab could lead to substantial decreases in atmospheric O2.
Another research team has raised the possibility that reactive nitrogen produced in making artificial fertilizers for agriculture could also be tying up more oxygen in plant tissue, soil organic matter and oceans in the form of nitrates .The importance of oxygen accounting in climate policies
Change in land use, and increased oxidation of nitrogen could explain the long term steady decline in atmospheric O2, and may well also account for the sharp acceleration of the downward trend since 2002 and 2003.
These years happen to coincide with record rates of deforestation. In Brazil, 10 000 square miles were lost mainly to pasture land, soybean plantations and illegal logging, a 40 percent rise over the previous year . Massive deforestation has continued in the Amazon and elsewhere, spurred by the biofuels boom ; it is estimated that nearly 40 000 ha of the world’s forests are vanishing every day.
The crucial role of forests and phytoplankton  in oxygenating the earth shows how urgent it is to take oxygen accounting seriously in climate policies. Reductionist accounting for CO2 alone is insufficient, and even grossly misleading and dangerous.
A case in point is the proposal of the International Biochar Initiative (IBI). ‘Biochar’ is charcoal produced to be buried in the soil that IBI has been promoting worldwide over the past several years  as a means of sequestering carbon from the atmosphere to save the climate and enhance soil fertility. It involves planting fast growing tree and various other crops on hundreds of millions of hectares of ‘spare land’ mostly in developing countries, to be harvested and turned into charcoal in a process that could produce crude oil and gases as low grade fuels. There are many excellent arguments against this initiative , but the most decisive is that it will certainly further accelerate deforestation and destruction of other natural ecosystems (identified as ‘spare land’). In the process, it could precipitate an oxygen crisis from which we would never recover  (Beware the Biochar Initiative, SiS 44).References
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