How can ice core samples preserve air when it's hot outside?

Ice core data has certainly been consistient and reproduceable. I’m not wondering about that. But how can an ice sample trap air bubbles during warm spells when the ice is more prone to melt and not be ice in the first place?



Obviously ice core samples can only preserve air bubbles if the ice has remained frozen continuosly since the ice was first frozen.

So you have to get your ice cores from places like Greenland that have had a permanent ice cap since the Pleistocene. Mountain glaciers aren’t good enough, since even if a glacier has been on the mountain for thousands of years it isn’t the same ice, the ice flows down the mountain and is replaced by new ice.

OK, got it.

Follow up question:

How do we know that the ice has been accumulating for a half million years?

I assume it’s similar to how we know how stuff is underwater for the same period of time, but I’m still curious.

Dust accumulates on the surface of the glacier in the summer, then is covered by new snow in the winter. Much like counting the rings on a tree, ice cores can be dated by counting the dust layers. Other techniques are also employed: