20 Abstract. We present an analysis of uncertainties in global measurements of the column averaged dry-air mole fraction of CO2 (‘XCO2’) by the NASA Orbiting Carbon Observatory-2, (‘OCO-2’). The analysis is based on our best estimates for uncertainties in the OCO-2 operational algorithm and its inputs, and uses simulated spectra calculated for the actual flight and sounding geometry, with measured

25 atmospheric analyses. The simulations are calculated for land nadir and ocean glint observations. We include errors in measurement, smoothing, interference, and forward model parameters. All types of error are combined to estimate the uncertainty in XCO2 from single soundings, before any attempt at bias correction has been made. From these results we also estimate the ‘variable error’ which differs between

30 soundings, to infer the error in the difference of XCO2 between any two soundings. The most important error sources are aerosol interference, spectroscopy, and instrument calibration. Aerosol is the largest source of variable error. Variable errors are usually < 1 ppm over ocean and ~0.5 – 2.0 ppm over land. The total error due to all sources is ~1.5 – 3.5 ppm over land, ~1.5 – 2.5 ppm over ocean.

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