The global oxygen trend, measured over the last 20 years or so in air archived at Cape Grim, could be explained quantitatively by the amount of fossil fuel carbon known to have been burnt over this period. However, we know that large areas of forests have also been cleared and combusted, so the oxygen decrease from this cause must have been approximately balanced by the release of oxygen from a greater rate of photosynthesis of the remaining terrestrial biosphere. This is possibly a result of higher plant growth rates caused by higher CO 2 levels, or climate change. The biosphere over this period is said to be in mass balance. Measurements of trace gases are also possible from satellites.
Satellite images show quite clearly the ozone depletion in the stratosphere over recent decades.
However, satellite trace gas measurement is presently too imprecise to determine global exchanges, where regional differences are only 1 per cent or less. Satellite measurements may well be suited to detection of emissions 'hot spots', such as carbon monoxide from cities, or methane from biomass burning. Management of future atmospheric composition Observations like these, combined with theoretical and process studies, contribute to our ability to manage the planet s atmospheric composition. A similar process has already enabled some control over the global levels of ozone-destroying chlorofluorocarbons. Control of other gases, especially carbon dioxide, is also theoretically possible but could be much more difficult.