So why don't we tax water, which makes as much sense as taxing CO2?  Without water, all life on earth would perish.  The same is true of CO2, as below about 100 ppm, all plant life would perish due to its inability to engage in photosynthesis.  Burn this formula into your brain: 6CO2 + 6H2O + Light = C6H12O6 (Glucose) + 6O2.  It translates to 6 molecules of CO2 plus 6 molecules of water, in the presence of light energy, produces one molecule of glucose (a fundamental plant nutrient) plus 6 molecules of oxygen.  This is the most basic formula for life on earth.  If you want to eat or breathe, you'd best be thankful for CO2 and water, in equal proportion.