An important long-term factor that determines whether glacial ice will form on a continent has likely been the proportion of which gas in the atmosphere?

The correct answer is carbon dioxide.

Explanation:

Because carbon dioxide levels continue to rise in the atmosphere, global temperatures increase.

This causes the glaciers to begin to melt or break, being able to form ice continents.

What causes CO2 is that large blocks of ice become vulnerable to cracking and manage to detach large parts of them. An example of this case is what happened with the Pine glacier, which cracked and when it came off formed another glacier the size of Berlin.

Currently the area occupied by glaciers on the planet is equivalent to the land occupied by Europe and North America.

And in a long-term, they could melt much faster than they currently do, covering most of the ground on our planet with water.

brave new world, aldous huxley (1932)