The worry about a tipping point would be that models show that global warming will freshen the North Atlantic and that might go so far as to shut down the conveyor altogether. Mathematically, this is what's called a bi-stable system. It has two different ways that it can be in a given set of conditions - you're not going to get it back up just by changing things back a little bit. A big impact though is with something called the Intertropical Convergence Zone which is basically where the Northern Hemisphere's weather and the Southern Hemisphere's weather meet. Those movements will have effect in Central America where models suggest incredibly severe drought.