Conservatives have long suspected that this is a cynical attempt to gain control of the world economy. To test leftists' sincerity, we can consider their support for alternatives to regulation of the global economy. One big alternative is nuclear energy, which has zero carbon emissions.
Even if this isn't the left's ideal solution, it is one that most on the right would accept. Are climate change hysterics advocating the expansion of nuclear energy? If not, why should anyone believe that they are sincere?