Academic libraries use Library of Congress Subject Headings to describe materials. Think of these as pre-defined hashtags that cannot be modified.
Subject headings can be useful for discovering books and materials quickly, so it is good practice to look at the subject headings that are assigned to the books that you are interested in.
However, relying on subject headings alone can be very limiting. For example, they cannot describe multi-disciplinary or complex topics, and it is impossible to apply a subject heading to every topic that is covered in a given source.
In the column at right I've listed a few examples of Subject Headings that might be useful for your ENGL 197 assignment.
Fantasy fiction -- History and criticism
Science fiction -- History and criticism
Climate change
Environmental protection -- Fiction
Environmentalism in literature
Climatic changes -- In literature
Speculative fiction -- Women authors -- History and criticism
Speculative fiction -- Black authors -- History and criticism
Speculative fiction -- 20th century -- History and criticism
Global warming -- In literature
Environmental literature -- History and criticism