Studies of Recent Changes in Atmospheric O₂ Content

A very close coupling exists between changes in atmospheric O and CO concentrations, owing to the chemistry of photosynthesis, respiration, and combustion. The coupling is not perfect, however, because CO variations are partially buffered by reactions involving the inorganic carbon system in seawater, which has no effect on O. Measurements over the past two decades document variations in O on a range of space and time scales, including a long-term decrease driven mostly by fossil fuel burning and seasonal cycles driven by exchanges with the land biosphere and the oceans. In this chapter, these and other features seen in the measurements are described, also discussing variations in the tracer 'atmospheric potential oxygen,' which is a linear combination of O and CO designed to be insensitive to exchanges from the land biosphere and thereby sensitive mostly to oceanic processes. Challenges associated with measuring variations in O are addressed, and various applications of the observations are discussed, including quantifying the magnitude of the global land and ocean carbon sinks and testing ocean biogeochemical models. An updated budget for global carbon sinks based on O measurements from the Scripps O program is presented for the decades of the 1990s and 2000s.