Other risks:

Without the economic benefits made available from utilization of fossil fuels, there will be less capital to invest in the development of alternative energy sources, and new carbon absorption techniques.

The good intentions of 1 billion or so souls in the western developed world may be insufficient to wield significant influence on the planet's climate, and may be worse than useless if those good intentions undermine the economic foundation of necessary technological development.

The deterrent effect of carbon taxes may be considerably less than is currently hoped for.  For example, European fossil fuel combustion rates are not significantly different from North American rates, despite being more than twice as expensive in most European jurisdictions.  

Politicians may squander the extra carbon tax collected just as destructively here, as they have in Europe, where the main outcome has been more government, and a resultant level of discontent approaching boiling point.