Conservation

A great survivor

Australia’s coral barrier reef keeps dying and coming back

THE Great Barrier Reef, which runs for 2,300km along the coast of Queensland, is one of the icons of environmentalism. Conservationists constantly worry that human activity, particularly greenhouse-gas-induced global warming, will harm or even destroy it. Such fears are not foolish, but they do reflect a view of the reef’s permanence that is at variance with the truth. For, a mere 10,000 years ago, the coral-covered seabed that now forms the Great Barrier Reef was dry land—a fact lamented in the songs, tales and dances of indigenous people living along the coast, which speak of homelands being drowned by incoming waters.

The reality of the Great Barrier Reef’s existence is that it is a movable feast. Reef-forming corals prefer shallow water so, as the world’s sea levels have yo-yoed during the Ice Ages, the barrier reef has come and gone. The details of this have just been revealed in a paper published in Nature Geoscience by Jody Webster of the University of Sydney and her colleagues. The authors examined cores drilled through the reef in different places. They discovered, as the chart shows, that it has died and then been reborn five times during the past 30,000 years. Two early reefs were destroyed by exposure as sea levels fell. Three more recent ones were overwhelmed by water too deep for them to live in, and also smothered by sediment from the mainland. The current reef is therefore the sixth of the period.

The barrier reef’s ability to resurrect itself is encouraging. But whether it could rise from the dead a sixth time is moot. The threat now is different. It is called bleaching and involves the tiny animals, known as polyps, which are the living part of a reef, ejecting their symbiotic algae. These algae provide much of a polyp’s food, but also generate toxins if the temperature gets too high, in which case the polyp throws them out. That causes the coral to lose its colour.

Polyps can tolerate occasional bleaching, but if it goes on too long, then they die. In the short term, therefore, global warming really does look a serious threat to the reef. It would, no doubt, return if and when the sea temperature dropped again. But when that would be, who knows?