  We compute grids of radiative-convective model atmospheres for Jupiter,
Saturn, Uranus, and Neptune over a range of intrinsic fluxes and surface
gravities. The atmosphere grids serve as an upper boundary condition for models
of the thermal evolution of the planets. Unlike previous work, we customize
these grids for the specific properties of each planet, including the
appropriate chemical abundances and incident fluxes as a function of solar
system age. Using these grids, we compute new models of the thermal evolution
of the major planets in an attempt to match their measured luminosities at
their known ages. Compared to previous work, we find longer cooling times,
predominantly due to higher atmospheric opacity at young ages. For all planets,
we employ simple "standard" cooling models that feature adiabatic temperature
gradients in the interior H/He and water layers, and an initially hot starting
point for the calculation of subsequent cooling. For Jupiter we find a model
cooling age 10% longer than previous work, a modest quantitative difference.
This may indicate that the hydrogen equation of state used here overestimates
the temperatures in the deep interior of the planet. For Saturn we find a model
cooling age 20% longer than previous work. However, an additional energy
source, such as that due to helium phase separation, is still clearly needed.
For Neptune, unlike in work from the 1980s and 1990s, we match the measured
Teff of the planet with a model that also matches the planet's current gravity
field. This is predominantly due to advances in the equation of state of water.
This may indicate that the planet possesses no barriers to efficient convection
in its deep interior. However, for Uranus, our models exacerbate the well-known
problem that Uranus is far cooler than calculations predict, which could imply
strong barriers to interior convective cooling.
