OK before you all yell "EVAPORATION", I know that's the boilerplate answer, but why and how.
Back in high school we were taught water, and most other elements and compounds, have three states, solid, liquid, and gas. Which state it's in being dependent on temperature and pressure. OK that all makes sense.
To transition from one state to another sufficient energy has to be imparted to the substance to pass through that transition before it will continue to heat up. To get water from ice all the way up to a gas takes a lot of energy… ask all the distillers out there.
Still.. all good…
But my ice cubes disappear over time inside a closed freezer.
The water is evaporating, turning into a gas state, but where did the energy come from to take it all the way from a solid to a gas when it never left the freezer..
Best Answer
I had never really noticed that happening, so before I said "that seems odd, are you sure?" I did some research and surprised myself.
According to the Wikipedia page on freezer burn water will sublimate from the surface of ice if the air temperature is low enough and the air is dry enough.
The reduced vapour pressure of the dry air is enough to cause sublimation.
Hopefully someone can give a more mathematical approach, but this is somewhere to start.