CO2 was chosen to be a climate "boogie man" because of its inability to fight back.
From "Rules for Radicals":
Rule 11: Pick the target, freeze it, personalize it, polarize it. Don't try to attack abstract corporations or bureaucracies. Identify a responsible individual. Ignore attempts to shift or spread the blame.
THAT's why they picked carbon dioxide -- it's an essential atmospheric compound of life, but it's also an ideal target upon which to thrust blame.
How can CO2 "defend itself"?
A brilliant move by the greenunists to create an "atmospheric Goldstein"...