unsustainablyadverb
uk/ˌʌn.səˈsteɪ.nə.bli//ˌʌn.səˈsteɪ.nə.bli/in a way that cannot continue over a period of time:
Prices have been set unsustainably high for years.
in a way that causes damage to the environment and is therefore not able to continue for a long time:
Water is being used unsustainably and is running out.