The fear is that there is indeed a critical threshold beyond which the release of methane will form a self-sustaining feedback loop - hence the "gun". Nobody kn

The fear is that there is indeed a critical threshold beyond which the release of methane will form a self-sustaining feedback loop - hence the "gun". Nobody knows for sure whether this can happen, or at what level, but it's a sufficiently terrifying possibility to warrant talking in terms of thresholds we should really try to avoid. 400 is just a nice round number. The next nice round number, 500, is quite nasty even without the clathrate gun (2-3 degrees of warming).