  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.
