“It is much more effective to restore or conserve one hectare of seagrass than Amazonian forest in terms of [carbon] mitigation potential – 30 to 50 times more," Dr Serrano told Fairfax Media.

Unfortunately, when seagrass is dies, it has the potential to release huge amounts of carbon-dioxide back to the atmosphere - potentially increasing the likelihood of further heatwaves by fuelling global warming.

The researchers - ranging from Australia, Spain, Malaysia, the United States and the Kingdom of Saudi Arabia - estimated the loss from the heatwave event released as much as 9 million tonnes of CO2, or the equivalent annual emissions of 800,000 homes or 1,600,000 cars.

The estimates were based on modelling releases based in-situ studies from 50 sites.

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Rob Coles, a seagrass expert at the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said all natural systems that store carbon were "at some level fragile", noting the recent bushfires in Victoria and NSW.

“If you're relying on carbon storage in the marine world, then you also need to need to think about the probability of that being mobilised at some stage in the future, and how you might try to minimise that risk,” Professor Coles said.

“You need to realise these risks can accumulate very quickly if you’re not careful - we could be in for a very different world.”

While the Nature paper recommended supporting seed dispersal or debris removal to help the seagrass recover, Professor Coles said the costs would likely prove prohibitive.

Dr Serrano said the seagrass species most affected inclined below-ground biomass of Amphibolis and to a lesser extent Posidonia -  the only two species forming large continuous beds.

While there had been "a bit of recovery", the process of full recolonisation would take "decades or hundreds of years”, he said.

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“When you lose the canopy of the seagrass, you lose this sequestration capacity of the meadows, and it results in CO2 emissions from the soil carbon," he said.

Professor Coles said the seagrass in the Great Barrier Reef was of more tropical species and had not suffered similar heat impacts as in Shark Bay - although water quality and cyclones had taken their tolls.

While tropical species might be faster growing, they typically store less carbon than the more temperate variety found off WA.