You are correct, it's very near to zero. It's a "magical balance" that something very small could shift meaningfully. Those shifts have occured in the past, ice ages, etc. for other reasons (we think). Today, we're altering our atmosphere by producing a lot of CO2 (and other stuff like methane) that COULD be enough to affect this balance in a bad way. I think it's very complex and difficult to model and also think the models COULD be underestimating the impact of CO2.
If they are, we're cooked.
