Much of our work involves measuring the changing concentration and isotopic composition of O2 in air on various timescales, and using the results to characterize the fertility of ecosystems and to study Earth's climate history.
      We measure the O2 concentration of air in samples collected for us biweekly at remote sites between Antarctica and Alaska. Rapid mixing in the atmosphere allows us to use O2 concentration changes as measures of rates of net production (photosynthesis minus respiration) on the scale of a hemisphere. Seasonal variations in the O2 concentration primarily reflect the productivity of the ocean biosphere. The change in the O2 concentration from one year to the next reflects the fate of fossil fuel CO2 which does not remain in the atmosphere: how much dissolves in the oceans, how much is incorporated into land plants? O2 is relevant because it is produced when plants grow, but not when CO2 dissolves in seawater.
      Changes in the isotopic composition of O2 in air are global. We can use the isotopic composition of O2 to correlate ice cores from different areas, placing their climate records on a common timescale. The results provide information about the sequence of climate changes when Earth warms or cools. Sequence in turn informs us about cause. One result of this work has been to show that increasing concentrations of CO2 in air warm the Earth as ice ages end.
      Many aspects of our work involve the interpretation of data using simple or complex mathematical models of atmospheric circulation, climate, and large-scale biological processes.