The abstract of the article Declining Coral Calcification on the Great Barrier Reef states:
The causes of the decline remain unknown; however, this study suggests that increasing temperature stress and a declining saturation state of seawater aragonite may be diminishing the ability of GBR corals to deposit calcium carbonate.
The scientists Glenn De'ath, Janice M. Lough, Katharina E. Fabricius of the Australian Institute of Marine Science report that their data suggests that the decrease in calcification of 14.2% since 1990 is unprecedented in the past 400 years.
Aragonite is CaCO3 - a form of calcium carbonate. It makes sense that this is important to coral growth. But what might be reducing the saturation of aragonite in seawater around the Great Barrier Reef? On further research I found that CO2 reacts with H2O and CaCO3 to make Ca and H2CO3 - carbonic acid! Aha - so that is why the oceans are becoming more acidic.
So what happened to corals much more than 400 years ago? The Earth was at least as warm, some say warmer, in the Medieval times. Coral was around then, so how did it cope? Maybe the answer is in slower growth, not killing. The coral is growing more slowly, but will recover again when the temperatures reduce with the next Little Ice Age. What is happening with coral in other locations around the world?
What will happen/ has happened to coral when the planet cools?
Coral seems to be pretty resilient. It has survived Crown of Thorns plagues, bleaching episodes and massive quantities of superphosphate washing out of Queensland rivers. I would like to see more studies of what has happened to corals over time with other unfavourable events.
As for the scientists claim that this is due to global warming from man-made CO2 (as at least one stated on camera), I didn't see any reference to their proof that CO2 is increasing in the atmosphere due to man made causes, nor that this is causing global warming.
All that can be concluded is (possibly) that the rate of growth of one type of coral in the Great Barrier Reef has been slower recently than it has in the past.
Am I correct in understanding that the coral is STILL GROWING, just 14.2% more slowly?