Acidification has been referred to be already affecting corals by inducing coral bleaching and diminishing calcification rate. At least there are evidences that bleaching is frequently currently killing corals by starving or by predisposing them to disease. What about the expected time scale for killing them through other ways, e.g. physiological stress induced by low pH?
Corals are very sensitive lower level organisms, according to many reports even one Degree increase of temperature can destroy the corals, now the combined effects of other pollutants the thermal tolerance level will decrease again. Marine disposal should be restricted in the sensitive areas. Slow increase of temperature is not a problem, sudden increase will affect the corals.
Sorry, I was not enough clear in my commnet about the question and even confounded by mentioning "thermal stress" instead of "noxious physiological effect of low pH" (lapsus mentis) . I really ask about the expected time scale for killing them through other ways like physiological effect of low pH. I was wrong to mention thermal stress when dealing with acidification.
In other words, I ask about the time scale at which coral mortaliy is expected to begin to take place: years? decades? centuries?
There is no simple answer to this question.
The first concerns for corals regarding low pH were to do with the dissolution of their skeleton or impediments to forming the skeleton rather than physiological effects per se. There is an abundance of literature showing a variety of responses by corals, some species can cope with low pH, others not so much. This website has a newsfeed for ocean acidification news and you will find many links to papers looking at the impacts of pH on corals including specific physiological impacts.
http://www.iaea.org/ocean-acidification
The time scale for negative impacts will depend on the rate of pH decline. it could be decades or centuries- depending on the rate of increase of atmospheric CO2 and consequent decline of seawater pH. If emissions slow down then the time of impact will be pushed back.
Surveys of corals around natural CO2 vents at Papua New Guinea show that corals still grow at pH 7.8 but the diversity is much reduced. Some species such as porites are able to grow, but the complex branching corals necessary for reef building are less abundant.
Also, be aware that while individual corals may still be able to grow at low pH as 7.8, a coral reef may not form. A coral reef is a build up of dead carbonate skeletons of corals and other reef calcifiers. If the coral fields at the PNG vents are a window into the future, then there will be no preservation of skeletal material at this pH even if corals keep growing. This means that coral reefs may not longer form even though individual corals can grow. Thus the answer is different for a coral versus a coral reef.
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