Please help to explain "When the polar caps melt"

[ice_breaker]

Hi everyone,

During learning some idioms, I'm stuck with this idiom-when the polar caps melt. Pls kindly explain it. Thx

[roses]

"When the polar caps melt" is a scientific reference. The Arctic and Antarctic ice sheets are both likes "caps" which means coverings, rather like pen caps" on pens. These caps are at the "polar" regions near the North and South Poles. Hence, if temperatures increase around the world these masses of ice could thaw and melt.

[riverclarin]

This idiom suggests that it will be a long time before something happens, e.g: "I will take you out to dinner when the polar caps melt". :)