The problem is on July 25, 1997 he voted against the Kyoto Protocol. The Senate passed the Byrd/Hagel Resolution rejecting Kyoto by a vote of 95-0.
The Byrd/Hagel Resolution says in part that the Senate would not approve anything that would result in serious harm to the economy of the United States. Apparently Kerry does not care about that anymore as he pursues his self-aggrandizing political career.
Dr. Tim Ball has a PhD in Climatology from the University of London, England. He is the author of The Deliberate Corruption Of Climate Science and has a web page at http://drtimball.com/