How do you even go about measuring "total energy in the system?"

Intuitively, adding greenhouse gasses to the atmosphere should, in isolation, increase energy in the system. But what if we really were heading for a new ice age as some feared in the 70s. And the only thing that prevented us from freezing to death, was the concerted effort of those notoriously concerned and altruistic SUV drivers taking one for the team, by revving up their Hummers.