The accuracy with which emissions of CO₂, CH₄ and CO from a complex city can be estimated from geostationary orbit is assessed via numerical experiment. Sources of the gases, meteorology, clouds and aerosols over the city are simulated, as are spectra of reflected sunlight in absorption bands of the gases. Gas concentrations are estimated from the spectra, and source distributions from the concentrations. Comparison of estimated and true sources measures the accuracy of the observing system.