The History of Oxygen
|So, how did it get this way?||Hewitt's Air
Oxygen is one of the 'master variables'
of the Earth System.
Rothschild and Lister
|Origin of Atmospheric
Free oxygen is never more than a trace component of most planetary atmospheres. Thermodynamically, oxygen is much happier when combined with other elements as oxides; the pressure of O2 in equilibrium with basaltic magmas is only about a ten millionth of an atmosphere. Photochemical decomposition of gaseous oxides in the upper atmosphere is the major source of O2 on most planets. On Venus, for example, CO2 is broken down into CO and O2. On the earth, the major inorganic source of O2 is the photolysis of water vapor; most of the resulting hydrogen escapes into space, allowing the O2 concentration to build up. An estimated 20 billion grams of O2 per year is generated in this way. Integrated over the earth’s history, this amounts to less than 3% of the present oxygen abundance. The partial pressure of O2 in the prebiotic atmosphere is estimated to be no more than 0.001 atm, and may have been several orders of magnitude less.
major source of atmospheric oxygen on the earth
carried out by green plants and certain bacteria.
Law of Conservation of Mass/Energy
matter is neither created or lost
in any chemical reaction.
|Temperature is rising. . .|
do we know that the
atmospheric build-up of greenhouse gases
is due to human activity?
|First, the nuclei of carbon atoms in carbon dioxide emitted by burning coal, oil, and natural gas (fossil fuels) differ in their characteristics from the nuclei of carbon atoms in carbon dioxide emitted under natural conditions. Coal, oil, and natural gas were formed deep underground tens of millions of years ago, and the fraction of their nuclei that were once radioactive has long ago changed to non-radioactive carbon. But the carbon dioxide emitted from natural sources on the Earth's surface retains a measurable radioactive portion. As carbon dioxide has been emitted through fossil fuel combustion, the radioactive fraction of carbon in the atmosphere has decreased. Forty years ago scientists provided the first direct evidence that combustion of fossil fuels was causing a buildup of carbon dioxide and thereby diluting radioactive carbon in the atmosphere by measuring the decreasing fraction of radioactive carbon-14 captured in tree rings, each year between 1800 and 1950.|
||Second, scientists began making precise measurements of the total amount of carbon dioxide in the atmosphere at Mauna Loa, Hawaii, and at the South Pole in the late 1950s. They have since expanded their observations to many other locations. Their data show convincingly that the levels of carbon dioxide have increased each year worldwide. Furthermore, these increases are consistent with other estimates of the rise of carbon dioxide emissions due to human activity over this period.|
|Third, ice buried below the surface of the Greenland and Antarctic ice caps contains bubbles of air trapped when the ice originally formed. These samples of fossil air, some of them over 200,000 years old, have been retrieved by drilling deep into the ice. Measurements from the youngest and most shallow segments of the ice cores, which contain air from only a few decades ago, produce carbon dioxide concentrations nearly identical to those that were measured directly in the atmosphere at the time the ice formed. But the older parts of the cores show that carbon dioxide amounts were about 25% lower than today for the ten thousand years previous to the onset of industrialization, and over that period changed little.|
evidence comes from the geographic pattern of carbon dioxide
measured in air. Observations show that there is slightly more carbon
dioxide in the northern hemisphere than in the southern hemisphere. The
difference arises because most of the human activities that produce carbon
dioxide are in the north and it takes about a year for northern hemispheric
emissions to circulate through the atmosphere and reach southern latitudes.
|No matter where you stand on the global warming issue, one particular conclusion from these data is irresistible: Carbon dioxide levels in the atmosphere have increased over 15 percent since 1958. This "modern" database can be combined with studies of CO2 trapped in polar ice cores, where CO2 concentrations are literally frozen in time. The data suggest that although concentrations ranged widely during 400,000 years of ice core record, the recent high levels of atmospheric CO2 are unprecedented.|
| One of these
thermometers is extremely common - it's trees, or more precisely,
tree rings. These rings show how much
a tree has grown each year, with each ring representing one year. In
warmer, wetter years, the tree will grow more so the ring for that year
will be wider than the ring for a colder year. By studying the
rings, scientists can learn about the changes in weather over the tree's
Just like trees, coral also builds up rings as it grows, but instead of being made of wood these growth rings are made of calcium carbonate - the hard white stuff that forms the 'skeleton' of a coral. Each year, the coral grows a little bit more. If the sea temperature is warm the coral will grow faster than if the temperature is cold, so warmer years will make wider growth rings and colder years will create thinner rings. By looking at these rings we can get an idea of what the sea, and therefore the surface temperatures, were like each year.