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Most CO2 from Australia’s megafires has been offset by algal blooms

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Wildfires in Australia as seen from space on 4 January 2020

Geopix / Alamy

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Most of the carbon dioxide released by Australia’s extreme wildfires of 2019-2020 has already been sucked out of the atmosphere by giant ocean algal blooms that were seeded by the nutrient-rich ash, a surprising new study suggests – though it is unclear how long this carbon capture will last.

Australia experienced its worst wildfires on record between November 2019 and January 2020. More than 70,000 square kilometres of bushland – an area the size of the Republic of Ireland – burned to the ground.

As the vegetation combusted, about 715 million tonnes of carbon dioxide were released into the atmosphere – roughly equivalent to the entire annual emissions of Germany. This led to fears that the fires would be a major contributor to global warming.

However, new research suggests that approximately 80 per cent of this carbon dioxide has been absorbed by ocean algal blooms that began growing when iron-rich ash from the fires rained down into the water.

Ash contains iron that can promote growth of microscopic marine algae called phytoplankton, says study author Richard Matear at CSIRO, Australia’s national science research body. As phytoplankton grow, they capture carbon dioxide from the atmosphere through the process of photosynthesis.

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While analysing data from satellites and floating measurement stations, Matear and his colleagues found that two large phytoplankton colonies – known as algal blooms – grew in regions where ash from the wildfires drifted out to sea. One was to the south of Australia and the other was thousands of kilometres east in the Pacific Ocean. 

Based on the rate of growth of the algal blooms and the length of time they existed – about three months – the researchers were able to estimate how much carbon dioxide they removed from the atmosphere.

Map of algal blooms

Location of algal blooms caused by wildfires

Richard Matear, CSIRO

The two blooms together exceeded the area of Australia. But because they were in the open ocean, they didn’t look like the thick carpets of algae that can grow in coastal regions and harm fish and other creatures, says Matear. “The concentration of phytoplankton is relatively low because the water is deep and cold and well-mixed,” he says.

Since phytoplankton sit at the bottom of the marine food chain, their rapid growth may have boosted other marine life in these areas, but this hasn’t yet been studied, says Matear.

Wildfires used to be considered carbon neutral because the carbon dioxide they released was recaptured through photosynthesis when burnt vegetation grew back.

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But as climate change increases the frequency and intensity of wildfires, scientists are worried that vegetation regrowth won’t be enough to offset the carbon emissions of wildfires.

The latest study suggests that marine algal blooms may be another tool that nature can use to capture wildfire emissions, says Pep Canadell at CSIRO, who wasn’t involved in the research. “It shows a very nice connection between the land and the ocean and how the system tries to balance things out,” he says.

However, one important consideration is how long this carbon capture is likely to last, says Canadell. Research shows that when algal blooms die, some carbon is transported to the deep ocean, but the rest can re-enter the atmosphere, and what proportion this happens to is unclear. “We don’t know if this is 50 per cent or 20 per cent or what so we need longer term research to find out,” he says.

Journal reference: Nature, DOI: 10.1038/s41586-021-03805-8

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