Just chiming in as a coral scientist, because these threads always make my head hurt:
- Ocean acidity isn't a problem for corals, at all. It will become a problem for coral growth rates decades from now but it's not even clear whether it will have a negative impact on bleaching or subsequent mortality (some studies have actually shown that increased acidity reduces bleaching susceptibility). OA is not the most urgent or actionable threat to coral reefs.
- One half of corals on the GBR didn't die. One third did. Edit: the article quotes Dr Hughes as saying half. It’s been a common misquote where scientists say half of the northern GBR died and popular article say half the reef is dead, and the nature study this article is based on didn’t quantify that. But I didn’t see that quote - if that’s not a misquote then it’s right, Dr Hughes would know.
- Local impacts like sunscreen, fertilizer runoff, dredging, overfishing, and on and on can all devastate reefs - and they're far more manageable than global processes like climate change. They should not be ignored because 'the corals are all going to die anyway', as these gloom and doom popular articles imply.
- The GBR didn't actually bleach because of climate change either. Climate change MAY have exacerbated it, but we don't know that yet. The high temperatures were caused by a severe El Nino that scientists had been predicting for years. It was the worst event since 1998, when similar bleaching and mortality was seen in the Indo-Pacific. Those reefs subsequently recovered. The question now is whether the slowly creeping baseline temperatures will effect which species of corals may recover in Australia and how fast.
Edit2: just want to re-iterate my analogy on the El Nino point - it is like a cancer patient being hit by a bus. We can say the cancer patient may have had slower reflexes or may have been weakened to the impact, but the patient was still killed by a bus. The El Nino causality is that clear - and we'd known it was coming for many years, it was not unexpected.
And that's not to minimize the problem of climate change, which is certainly responsible for the 3rd NOAA-declared Global Bleaching Event.
But it's important that people not think that climate change has already killed half the GBR because ANYONE would expect that if that's the case, then we are about to imminently lose the rest in the next few summers. And that's not the case. That El Nino event was historically bad, but there's still much we can do to help the GBR as it recovers in the next few years.
