The problem is not whether there are extinctions or not, or if the Earth endured global warming before or not, of course it did, and 99% of the organisms that once lived on Earth are now extinct.
Having said that, I think we are contributing (and may have created) the current global warming period. If you look back to the last 10 interglacial (warm) periods, they are all cooler than present temperatures, and they are usually shorter, the next ice age (and not global warming) is overdue.
The problem with the present global warming and why this is so bad for coral reefs is not the warming per-se. Corals have endured many warm periods before, but they always had somewhere to go. Now, there is not only global warming, but also pollution, sediment runoff, spread of diseases via canals and ship ballast water, physical damage to the reefs, all of which are caused by humans. The combination of all these factors is what concerns me, not just global warming.
This present report is a clear example of it. 2005 was the warmest year on the history of climatic record, yet the corals were recovering, but disease killed them off. These things are happening too fast.
Justifying that it is a natural phenomenon is an excuse used by many that don’t want to deal with it. Oh well, extinction is a natural phenomenon, the meteor that killed the dinosaurs 65 million years ago also killed the majority of the organisms on the planet, yet we came back. Why then protect the coral reefs now? Or the rain forest? Or the everglades? Or the bald eagle? Or the jewfish?
Think about it…