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The atmospheric pressure-temperature profiles for transiting giant planetscross a range of chemical transitions. Here we show that the particular shapeof these irradiated profiles for warm giant planets below 1300 K lead tostriking differences in the behavior of non-equilibrium chemistry compared tobrown dwarfs of similar temperatures. Our particular focus is H$_2$O, CO,CH$_4$, CO$_2$, and NH$_3$ in Jupiter- and Neptune-class planets. We show thecooling history of a planet, which depends most significantly on planetary massand age, can have a dominant effect on abundances in the visible atmosphere,often swamping trends one might expect based on Teq alone. The onset ofdetectable CH$_4$ in spectra can be delayed to lower Teq for some planetscompared to equilibrium, or pushed to higher Teq. The detectability of NH$_3$is typically enhanced compared to equilibrium expectations, which is oppositeto the brown dwarf case. We find that both CH$_4$ and NH$_3$ can becomedetectable at around the same Teq (at Teq values that vary with mass andmetallicity) whereas these "onset" temperatures are widely spaced for browndwarfs. We suggest observational strategies to search for atmospheric trendsand stress that non-equilibrium chemistry and clouds can serve as probes ofatmospheric physics. As examples of atmospheric complexity, we assess threeNeptune-class planets GJ 436b, GJ 3470b, and WASP-107, all around Teq=700 K.Tidal heating due to eccentricity damping in all three planets heats the deepatmosphere by thousands of degrees, and may explain the absence of CH$_4$ inthese cool atmospheres. Atmospheric abundances must be interpreted in thecontext of physical characteristics of the planet.

the astronomical journal

Beyond Equilibrium Temperature: How the Atmosphere/Interior Connection  Affects the Onset of Methane, Ammonia, and Clouds in Warm Transiting Giant  Planets