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

As seen from space, wildfires in Australia on January 4, 2020 Geopix/Alamy
The vast majority of carbon dioxide released in Australia's extreme wildfires in 2019-2020 has been captured by huge ocean algal blooms. These blooms were seeded with nutrient rich ash. A surprising new study suggests that this carbon capture may not last long.

Between November 2019 and January 2020, Australia was hit by the worst wildfires in its history. Over 70,000 kilometres of bushland, an area as large as the Republic of Ireland was destroyed.

The atmosphere was emitted 715 million tonnes of carbon dioxide each year as the vegetation burned. This is roughly the equivalent of Germany's entire annual emissions. There was concern that these fires could contribute to global warming.


New research indicates that around 80 percent of the carbon dioxide has been absorbed into the ocean by algal blooms. These blooms were created when iron-rich, ash from fires fell into the water.

According to Richard Matear, CSIRO (Australia's national science and research agency), ash contains iron which can encourage the growth of microscopic marine alga called phytoplankton. Photosynthesis is the process by which phytoplankton capture carbon dioxide from the air as they grow.

Matear and his team discovered that algal blooms, which are large phytoplankton colonies made up of phytoplankton colonies, were found in areas where wildfire ash had drifted out into the sea. The one to the south was in Australia, the other was located thousands of kilometres east at the Pacific Ocean.

Researchers were able estimate the amount of carbon dioxide that they released from the atmosphere based on the growth rate and time of the blooms for three months.

Richard Matear, CSIRO. Location of algal blooms due to wildfires Richard Matear

Together, the two blooms exceeded Australia's area. Matear says that they did not look like the thick algae mats that can grow in coastal areas and cause harm to fish and other animals because they were out in the open ocean. He says that the concentration of phytoplankton in the water is low due to its deep, cold, and well-mixed nature.

Matear says that phytoplankton is at the bottom marine food chain and may have helped other marine life. However, this has not been investigated.

Wildfires were once considered carbon neutral, as the carbon dioxide that they released was captured through photosynthesis after burned vegetation had grown back.

Scientists are concerned that wildfires will become more frequent and intense due to climate change. They fear that the amount of carbon emitted by wildfires may not be offset by vegetation regrowth.

According to Pep Canadell, CSIRO's researcher, marine algal blooms could be another tool nature uses to capture wildfire emissions. He says it shows a nice connection between land and ocean, and how the system attempts to balance them out.

Canadell says that it is important to consider how long the carbon capture will last. Research has shown that some carbon from algal blooms is lost to the deep seas, while others can be re-entered into the atmosphere. However, it is not clear what percentage. He says that we don't know whether this is 50% or 20%, and that longer-term research is needed to determine.

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

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