We present an analysis of uncertainties in global measurements of the column-averaged dry-air mole fraction of CO₂ (XCO₂) by the satellite OCO-2. The analysis is based on our best estimates for uncertainties in the OCO-2 operational algorithm and its inputs. From these results we estimate the "variable error", which differs between soundings, to infer the error in the difference of XCO₂ between any two soundings. Variable errors are usually < 1 ppm over ocean and ~ 0.5–2 ppm over land.