In FT's gaia shader, ice caps form at +/- 76.5 latitude plus an fBm noise factor. No, you can't change it.

In FT's climate shader, ice caps form at an annual average temperature of -10C. No, you can't change it. Other climate types appear according to an internal lookup table ( http://www.ridgenet.net/~jslayton/climateinfo.gif ). The image climate shader lets you put an arbitrary picture over that climate image to determine the coloring.

I am more likely to play with the "Specify Base Temperature" options in FT to control the worldwide temperature than I am to try to use the "Physically-based Base Temperature" options.