A.
“World government” and “global (environmental) governance” are practically the same thing. They suggest that there is a centrally placed international organization with global enforcement capacities that regulate environmental agreements.
B.
World government” implies one centrally placed international organization with global enforcement capacities and “global (environmental) governance” implies complex patterns of institutions and collaboration between state and non-state actors.
C.
“World government” and “global (environmental) governance” are similar concepts with the only difference that world government denotes the sort of organization needed to address urgent sustainability challenges.