But models are still estimates. There can still be un-knowns. Scientist have been writing about un-known, or difficult to model feedback loops for awhile. That is the movie-style doomsday, where we hit some un-foreseen tipping point and climate change takes off.

Instead view climate as a large dynamic system, you hit it hard enough, maybe it moves to a different stability point. Even with small systems this can be hard to estimate with a model.

Of course, this is the plot to the movie "Day After Tomorrow".

But just because it was the plot to a movie doesn't mean it can't be a real risk.