Systems at a roughly stable equilibrium can be surprisingly easy to shift out of that equilibrium by pushing them ever so slightly outside a local minima. Nobody right now is saying this particular situation is going to result in catastrophe, but we should exercise some caution when we are causing observable effects to the one planet we inhabit.
"If you are in a flying 747 flying directly at another one, then it appears as a very very tiny spec in the sky, until you get close enough to see it, and then it gets really really big, really really fast, and then you are dead."
He also said this about stable equilibrium: You can roll a glass in a circle on a table, and it can roll in a circle for a long time, but if it falls off the table, it is going to take some effort to get back to equilibrium.
We have little idea what equilibrium is, and we also have less idea about what it will take to get back to it, after leaving it.
See: "The Day After Tomorrow." (2004)