I think balance will restore itself, but balance from an Ecological point of view, not a human one.
There have been several changes to the composition of the atmosphere if research is to be believed, over the last few hundred million years. At one stage, the atmosphere contained an awful lot more methane than it does now, though, tbh, this was back before even triploblasts emerged, so we're talking a more unsettled planet.
The thing is, we aren't talking about a harmony here, The Gaia theory has that one weakness, it assumes that there is an ideal state that the earth wants to be at and it will always return to it. That's not strictly true, what the Earth wants as such is to reach a point of stability, but that stability may not neccesarily be in the 'Gaia' environment from a human point of view, in fact, it does not need to even be life supporting.
Foruntately, life imposes itself on the environment, the more life there is, the more life there can be, and the more suitable the environment becomes, but suitable is not stable, at some point the change in the environment is going to promote a response from it. In many ways Environmentalism isn't about loving planet Earth, it's about finding ways to deal with its indifference to us.
Edit : In short, from a purely ecological point of view, an Ice Age would screw us, as size goes, we are in something like the top 15% largest animals on the planet, and they are the ones most at risk from environmental change because they place the highest demand on their environment.
But we're odd, as far as we are aware, for the first time, an Ice Age threatens a species that can sidestep a lot of the rules, because we can plan ahead and, just possibly, adapt to the environment faster than it can change, even if massive environmental change does come soon, I don't think it would be the end of humanity, though it would certainly be the end of a large percentage of it.
