I'll take a first shot at this.
Lower carbon prices will drive lower emissions reductions. Higher carbon prices will drive higher emissions reductions. It isn't a question of yes or no, or whether emissions reductions are effective or ineffective. It's a question of degree and of scale.
If the threshold is any emissions reductions at all, then that assertion is incorrect. Even low carbon prices drive emissions reductions, as we've seen in B.C. If it's a question of achieving the target and achieving the national emissions reductions we've set for ourselves, then, as some of the others have said, higher carbon prices or more stringent policies will be required to get those deeper emissions reductions.