That's great you feel that way engineering it's a good degree, I don't have the brains or motivation to complete a degree but you will make bank when you graduate.
That being said, I would say pretty much all disciplines of engineering can be directly tied to sustainability. You could become a nuclear engineer and mess around with cold fusion, you could become an environmental engineer (pretty self-explanatory), materials engineer where you work with making more efficient polymer chains.
Really you need to narrow down what you mean by sustainability because, in my mind, the options are endless with what you could do with that and an engineering degree.
"You come to Utah on vacation and leave on probation" - James K.