They say that in the last decade the ocean has become more acidic by 25% thus threatening the reef. In actual fact, the oceans are alkaline, with a very stable pH about 8.1, making up the world's largest buffer solution. It would require a huge inflow of acid (from volcanic eruptions etc) to make a detectable change in pH.
The GBR was a heap of rubble very recently, only 20,000 years ago, just before the end of the last ice age when the sea level was about 130 meters lower. The reef then was part of the mainland. Next the world had a short period of intense global warming (ca 12 degrees over about 4000 years) melting the northern ice sheets and causing the rapid rise in sea level. Human beings were thin on the ground at that time and seem not to be responsible.
Isn't it wonderful how the Great Barrier Reef survived the Ice Age and blossomed again after such a drastic change in climate!