The heatwave in Western Australia last summer broke records. As marine scientists, we were deeply concerned about whether Ningaloo’s corals would survive. We were prepared for the worst, but what my colleague and I found when we returned to assess the damage months later was still a shock.

It was the longest-lasting, largest and most intense underwater heatwave ever recorded in WA. And it hammered the coral at Ningaloo Reef, part of the World Heritage-listed Nyinggulu or Ningaloo Coast, in the northwest of the state. (The word Ningaloo comes from the Aboriginal name Nyinggulu, which means a promontory or headland).

For coral, sustained heat stress is measured using degree heating weeks (DHW). At 4 DHW, coral bleaching is likely. At 8 DHW, many corals are at risk of bleaching and dying. At 20 DHW 80% mortality is predicted. At Ningaloo Reef last summer, more than 20 DHW were recorded.

In October, we returned and found that two in every three corals in the shallow lagoonal areas of the northern Nyinggulu Reef, including popular tourist sites such as Turquoise Bay, had died.

In March, in the middle of the bleaching period, our surveys of corals in the northern Ningaloo Reef lagoon showed up to 90% of the coral had bleached. Bleaching doesn’t automatically mean death, but in a heatwave........

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