Is there a dispensation for ritual bathing (ghusl) in times of extreme cold and frost?

Whoever is obligated to perform ghusl, must bathe with water. The mere coldness of the weather is not a reason to forgo its use. The Prophet, peace and blessings be upon him, gave glad tidings to those who perform ablution thoroughly despite difficulties, promising elevation in ranks and remission of sins.

If it is possible to heat the water, then that is obligatory. If one is unable to heat it and fears harm from using it, then it is permissible for him to perform tayammum.