We demonstrate coherent dual frequency-comb spectroscopy for detecting variations in greenhouse gases. High signal-to-noise spectra are acquired spanning 5990 to 6260 cm -1 (1600 to 1670 nm) covering ~700 absorption features from CO 2 , CH 4 , H 2 O, HDO, and 13 CO 2 , across a 2-km open-air path. The transmission of each frequency comb tooth is resolved, leading to spectra with <1 kHz frequency accuracy, no instrument lineshape, and a 0.0033-cm -1 point spacing. The fitted path-averaged concentrations and temperature yield dry-air mole fractions. These are compared with a point sensor under well-mixed conditions to evaluate current absorption models for real atmospheres. In heterogeneous conditions, timeresolved data demonstrate tracking of strong variations in mole fractions. A precision of <1 ppm for CO 2 and <3 ppb for CH 4 is achieved in 5 minutes in this initial demonstration. Future portable systems could support regional emissions monitoring and validation of the spectral databases critical to global satellitebased trace gas monitoring.