A. J. van Loon's Correspondence, “Corals resist extinction by global warming”, shows how two correct, but unrelated and incomplete, statements can lead to false conclusions1. Missing pieces in his reasoning include the different genetic base of corals past and present, the amount of temperature change to which corals can adapt, and the role of human-caused stresses.
The corals living during warmer conditions before the Pliocene–Pleistocene transition were different species from those alive today. The start of the Ice Ages saw a period of species extinction and origination2. Today's corals are descendants of the most cold-tolerant corals that survived glacial episodes, while corals adapted to high temperatures largely vanished.
The upper temperature limits of modern corals have been determined experimentally by many researchers, starting with Mayor3 and Yonge and Nicholls4. They showed that only a few degrees of warming above normal maximum temperatures bleached and killed most corals, and found no behavioural or physiological capacity to adapt to higher temperatures. Subsequent research has refined but not changed their key conclusions5,6,7,8,9.
Extinct species were once able to adapt to warmer conditions than extant corals can tolerate, and presumably modern corals could do so again if there were sufficient evolutionary time and sufficiently slow rates of change. But fossil corals evolved in a world without human-accelerated change. Coral temperature tolerance thresholds are now being exceeded worldwide, and rates of temperature change currently under way dwarf ‘natural’ rates.
It is hardly surprising that modern corals are incapable of rapidly re-evolving the tolerance of high temperatures that their ancestors appear to have lost millions of years ago. Exponentially increasing anthropogenic pollution — in the form of greenhouse gases, nutrients, pathogens, sediments and other stresses — makes it very unlikely that corals will be able to adapt to a ‘greenhouse world’ quickly enough to survive, despite wishful thinking in Nature.
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A decline in bleaching suggests that depth can provide a refuge from global warming in most coral taxa
Marine Ecology Progress Series (2018)