  We present a theory for Casimir--Polder forces acting on greenhouse gas
molecules dissolved in a thin water film. Such a nanosized film has recently
been predicted to arise on th surface of melting ice as stabilized by repulsive
Lifshitz forces. We show that different models for the effective polarizability
of greenhouse gas molecules in water lead to different predictions for how
Casimir--Polder forces influence the extraction of CH$_4$ and CO$_2$ molecules
from the melting ice surface. In the most intricate model of a finite-sized
molecule inside a cavity, dispersion potentials push the methane molecules
towards the ice surface whereas the carbon dioxide typically will be attracted
towards the closest interface (ice or air). Previous models for effective
polarizability had suggested that CO$_2$ would also be pushed towards the ice
surface. Release of greenhouse gas molecules from the surface of melting ice
can potentially influence climate greenhouse effects.
