"No one has ever been able to prove that glaciers in the Arctic region are melting because of global warming. Therefore, glaciers in the Arctic region must be melting on their own." Which fallacy is committed in the above statement?

The reasoning treats absence of proof as proof of the opposite, which is the appeal to ignorance.

The premise only says no one has shown global warming causes the melting.

Lack of a successful proof for a claim does not establish its denial.

From "not proven that X causes Y" the argument leaps to "Y happens on its own".

A gap in evidence is here wrongly converted into positive evidence for a rival conclusion.

This shifting of the burden onto missing knowledge is argumentum ad ignorantiam.