Alarmists didn't start the use of parts per million as a scare tactic.  It is a standard measure for the concentration of a gas in the atmosphere.  Your analogy is misleading, at best.  What you fail to include is the effect of what appears to be a relatively small increase in carbon dioxide.  It may well cause the Earth's average temperature to rise to an unsustainable level.  Of course, the Earth doesn't care.  It will get along just fine.  Humans, on the other hand, may find it a little challenging.