How Can Ocean Currents Be Bad For The Environment?

They aren't necessarily bad for the environment, but they can be bad for certain life forms in the environment.
when currents change they have an affect on weather patterns and throw everything off. For instance, the polar ice caps melting are starting to disrupt the ocean currents changing weather patterns elsewhere. Where rain has been plentiful, there's droughts, there's been unusual flooding elsewhere. Hurricane strengths are increasing. Winters are producing more snow and getting colder in some areas but in others they're unseasonably mild.
some say it's cooling down again. That may be. I hope so. It's still too early to tell. Give it a few more years and we'll see.