How does ice absorb carbon dioxide?

It doesn't.

It traps atmosphere in small bubbles when it freezes. Air contains roughly 0.03% CO2 so it only collects very small amounts.

WATER, on the other hand, in our oceans does absorb CO2! The Ocean is the primary method we remove CO2 from the Atmosphere. That is why we see naturally higher CO2 levels when the temperature warms and why CO2 always follows temperature and never leads it.

Melting ice allows more surface area and allows us to absorb more CO2. That is a negative feedback loop often ignored by doomsday people that believe in the theory of man induced global warming.