I have read and watched a lot of info on global warming but by no means am I an expert. All my comments here are merely my own opinions. I feel the planet has been going through warming and cooling periods since it was formed and will continue to do so, with or without mans influence. Still I feel we should do all we can to live in harmony with nature and leave as little of a footprint as possible.
As far as the coral reefs go I think there is evidence that the recent warming of the oceans (manmade or not) and other factors are effecting the existing coral reefs, but I do not feel that reefs will cease to exist, rather they will move to areas that are more favorable. The coral is an adaptable, mobile organism. It reproduces in more than one way which gives it the ability to adapt/move. While a particular reef may be devastated by either man or nature, those corals when they were thriving were likely spawning regularly. When a coral spawns it puts its genetic material into the water column and ocean currents can carry it to who knows where. If conditions are favorable in the new area the coral may grow.
I feel if the oceans continue to warm we will just see the reefs move out from what are now the favorable areas near the equator. Who knows maybe someday we will have a great barrier reef off of the Washington coast.
On a side note the western Washington area could use a little of that global warming this spring! Here it is almost June and I still need a sweatshirt and jacket outside! Brrr . . .