Why is the Paris deal bad? It will have devastating economic costs to the United States and will essentially achieve nothing. 

According to the Heritage Foundation, a climate model by the National Center for Atmospheric Research indicates that, if the U.S. eliminated all artificial carbon dioxide emissions, the effect on global temperatures would be less than two-tenths of a degree on the Celsius scale. Eliminating all artificial carbon dioxide emissions throughout the entire industrialized world would have an effect of less than four-tenths of a degree.

http://www.heritage.org/environment/report/consequences-paris-protocol-devastating-economic-costs-essentially-zero