Radiative-convective models of the atmospheres of Uranus and Neptune: heating sources & seasonal effects Gwenael Milcarecka,b, Sandrine Guerleta,c, Franck Montmessinb, Aymeric Spigaa, Jeremy Leconted, Ehouarn Milloura, Noe Clementd, Leigh N. Fletchere, Michael T. Romane, Emmanuel Lellouchc, Raphael Morenoc, Thibault Cavalied,c, Oscar Carrion-Gonzalezc a Laboratoire de M´et´eorologie Dynamique/Institut Pierre-Simon Laplace (LMD/IPSL), Sorbonne Universit´e, CNRS, ´Ecole Polytechnique, Institut Polytechnique de Paris, ´Ecole Normale Sup´erieure (ENS), PSL Research University, 4 place Jussieu BC99, 75005 Paris, France b Laboratoire Atmosph`eres, Milieux, Observations spatiales (LATMOS), IPSL, Observatoire de Versailles St-Quentin-en-Yvelines, Universit´e de Versailles St-Quentin-en-Yvelines, CNRS, 11 boulevard d’Alembert, 78280 Guyancourt, France c Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS, Sorbonne Universit´e, Universit´e Paris-Diderot, Meudon, France d University of Bordeaux, CNRS, LAB, UMR 5804, Pessac, France e School of Physics & Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom Abstract The observations made during the Voyager 2 flyby have shown that the stratosphere of Uranus and Neptune are warmer than expected by previous models. In addition, no seasonal variability of the thermal structure has been observed on Uranus since Voyager 2 era and significant subseasonal variations have been revealed on Neptune. In this paper, we evaluate different realistic heat sources that can induce sufficient heating to warm the atmosphere of these planets and we estimate the seasonal effects on the thermal structure. The seasonal radiative-convective model developed by the Laboratoire de M´et´eorologie Dynamique is used to reproduce the thermal structure of these planets. Three hypotheses for the heating sources are explored separately: aerosol layers, a higher methane mole fraction, and thermospheric conduction. Our modelling indicates that aerosols with plausible scattering properties can produce the requisite heating for Uranus, but not for Neptune. Alternatively, greater stratospheric methane abundances can provide the missing heating on both planets, but the large values needed are inconsistent with current observational constraints. In contrast, adding thermospheric conduction cannot warm alone the stratosphere of both planets. The combination of these heat sources is also investigated. In the upper troposphere of both planets, the meridional thermal structures produced by our model are found inconsistent with those retrieved from Voyager 2/IRIS data. Furthermore, our models predict seasonal variations should exist within the stratospheres of both planets while observations showed that Uranus seems to be invariant to meridional contrasts and only subseasonal temperature trends are visible on Neptune. However, a warm south pole is seen in our simulations of Neptune as observed since 2003.