Hi, Andrew—Like you, I think a revision is needed. I like your idea of using the future perfect (“will have determined”). However, I think a second edit is needed; I’d change “grows up in” to “has grown up in”:
“By the time she graduates from high school, in just seven years, the decisions the world makes (now) around carbon emissions will have determined the environment she has grown up in.”
We could imagine her future self looking back and saying, “Wow, the decisions the world MADE when I WAS in the fifth grade HAVE DETERMINED the world I HAVE GROWN UP in.”