Severe mangrove dieback was recorded at Limmen Bight in the Northern Territory and at Karumba in Queensland in 2016, raising fears of threats to shoreline stability and Gulf fisheries.

Now, James Cook University scientists say they have solved the mystery of the death of the 40 million mangrove trees, showing that it was a result of an unusually low sea level due to large-scale swings in El Nino - Southern Oscillation events.

Other research found that a 1982 mass dieback also coincided with an unusually extreme drop in sea level during another severe El Nino event, providing the evidence for the cause of both events.

Satellite imagery shows that the mangroves took at least 15 years to recover from the 1982 dieback, and Dr Norm Duke from JCU's TropWATER Research Centre said the mangroves in the latest episode have not recovered seven years on, making it an ongoing coastal catastrophe.

"The key factor responsible for the mass dieback appears to have been the sudden 40-centimetre drop in sea level that lasted for about six months, coinciding with no rainfall, killing vast areas of mangroves," he said. "Essentially, the trees died of thirst."

More than 76 square kilometres of mangrove trees died along 2000 kilometres of coastline, releasing nearly one million tonnes of carbon, and Dr Duke said recovery had been stymied by severe cyclones and flooding.

Dr Duke said mangroves were vital to the ecology and stability of tropical and sub-tropical coastlines and their protection was critical.

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