A new study claims an unprecedented ocean heatwave responsible for wiping out a range of species in West Australian waters seven years ago released huge amounts of carbon dioxide into the atmosphere — equivalent of up to the yearly output of two coal-fired power plants.
During a four-month period in the summer of 2010-11, an extraordinary La Nina event and a particularly strong Leeuwin Current combined to drive a 2-4C rise in mean sea surface temperatures in Shark Bay .
The paper, published in the journal Nature Climate Change, found that seagrass cover had reduced by 20 per cent, or about 1000 square kilometres, when compared to records from 2002.
The effect on marine life in the World Heritage-listed area was total devastation, with mass deaths of sea animals, coral bleaching and the loss of huge swathes of seagrass meadows.
Edith Cowan University (ECU) scientist Oscar Serrano said the 8,000-year-old seagrass meadows acted as a huge store for carbon dioxide, but when the heatwave struck these areas, an estimated 2 to 9 million metric tonnes of carbon dioxide was released into the atmosphere within three years.
"The seagrass meadows are known as 'blue carbon' ecosystems, because they store carbon dioxide in their soils," Dr Serrano said.
"So when you have an event such as the losses at Shark Bay, you not only lose the seagrass as a way of removing CO2, but the sequestered gas is released back into the atmosphere during seagrass matter decomposition.
Dr Serrano said the worst case emissions from the seagrass loss was equivalent to the annual CO2 output of around 800,000 homes, two coal fired power plants or 1,600,000 cars.
Seagrass recovery 'will take decades'
Dr Serrano said the 2010-11 heatwave event could eventually result in up to 21 million metric tonnes of carbon dioxide being released into the atmosphere over 40 years, assuming none of the seagrass recovers.
"Seagrass is an organism that grows very slowly and it takes ages to recover, so this can't happen in a couple of years … it will take decades," he said.
"Recently we went back there to assess the quality of the seagrass meadows and basically it's all gone.
"You can only see the roots left after the erosion of the banks they grow in."
More heatwave pain predicted
Dr Serrano said it was also likely another ocean heatwave would occur before the seagrass could return to full health.
"The future is unknown always, but climate change is causing an increase heatwaves and other extreme events … so it's most likely we will have another heatwave like this," he said.
"And it's likely it will have a severe impacts again, particularly in areas like Shark Bay, which are in the fringe zone between temperate and tropical areas.
"This is a very difficult problem to manage, because climate change is happening on a global level."
Dr Serrano said transplanting more resilient species of seagrass was one measure that could be considered.
"It's sort of controversial and not much is known, but one possibility could be to transplant more tropical species in these temperate climates," he said.
"Because they are better adapted to cope with heatwaves and warm waters."
But Dr Serrano said ultimately, global action on climate change was key to ensuring the survival of the seagrass meadows, namely through the reduction of greenhouse emissions.
