The terrible bush fires in Australia in the summer of 2019/2020 destroyed millions of hectares of forest and bushland. Thousands of kilometers away, however, the smoke and suspended matter from these fires turned out to be life-carriers: They promoted an unprecedented bloom of algae in the southern ocean, the area of which clearly exceeded that of Australia, as satellite data show. The reason for this is a fertilizing effect of the fire aerosols: The iron contained in the smoke is an important plant nutrient and is normally only available in sparse in the southern ocean. The iron fertilization caused by the fire compensated for this deficiency and enabled the algae bloom never seen before.
It is normal for bushfires to break out in Australia during the hot summer months. But in the summer of 2019/2020 the fires reached dramatic proportions. As early as October and thus before the start of the actual fire season, fires began to blaze, especially along the east coast of the continent. Encouraged by extreme heat, prolonged drought and strong winds, the fires spread in the months that followed. “Millions of hectares of vegetation were burned, with serious ecological, environmental and socio-economic consequences,” said Weiyi Tang of Duke University in Durham and his colleagues. It is estimated that almost three billion animals died or were displaced as a result of the fires and the smoke from the fires, which rose into the stratosphere, released 715 million tonnes of CO2 – significantly more than Australia’s annual anthropogenic CO2 emissions, which were around 537 million tonnes in 2018 CO2 lay.
Soot and smoke over thousands of kilometers
The smoke from the bushfires was spread over thousands of kilometers by the wind and covered large areas of the southern Pacific and the southern ocean. With these clouds of smoke, enormous amounts of soot, suspended matter and aerosols were also released into the atmosphere. This is where the study by Tang and his colleagues comes in. It was known from previous studies that such pyrogenic aerosols also contain many trace elements such as iron, which are important plant nutrients. Especially in the cool, nutrient-poor marine areas of the southern Pacific and southern oceans, it is usually the lack of iron that limits algae growth in the upper water layers. “There was already earlier the hypothesis that the sinking of fire aerosols over the sea can compensate for this nutrient deficiency and increase marine productivity – but so far there has been a lack of direct observational data on this,” the researchers write.
For their study, Tang and his team evaluated satellite measurement data on the spread of smoke and aerosols in the summer of 2019/2020 and determined the phytoplankton density in various parts of the southern ocean using additional satellite data. In addition, they used measurement data from buoys, which allowed conclusions to be drawn about the extent of the suspended matter falling on the sea surface. The evaluations showed: The bushfire emissions from the south and east of Australia were transported thousands of kilometers in a south-easterly direction by the prevailing wind within a few days and distributed over a large area between the 20th and 55th parallel south. In two sea areas immediately south of the Australian south coast and southeast of the continent, the soot and aerosol concentrations reached historically unprecedented levels, as the team reports. The soot concentrations there were at least 300 percent above normal climatological values.
Unprecedented algal bloom
This had consequences: an abnormally strong algal bloom developed in these marine areas, which began in October 2019 and then lasted for over four months, according to satellite-based chlorophyll measurements. “The surprising thing is that this phytoplankton increase occurred at a time in the Australian summer when there is usually a seasonal decrease in chlorophyll in these marine areas,” report Tang and his colleagues. Because at this time the iron reserves in the upper water layers of the southern ocean are mostly already used up by the algae. “The area on which the chlorophyll concentrations were well above the historical monthly maximum was more than ten trillion square meters and thus exceeded the area of Australia,” writes the team. “The extent of this phytoplankton bloom caused by the Australian fires is unprecedented in the satellite data.”
According to the researchers, these results provide strong evidence that the fumed aerosols, and in particular the iron contained in smoke, can act as iron fertilizers for surrounding oceans. In return, the algae growth promoted by this means that the phytoplankton absorbs and binds more CO2 from the air through photosynthesis than would normally be the case. According to the scientists’ calculations, the algal bloom promoted by the 2019/2020 bushfires could have resulted in an additional uptake of around 186 million tons of carbon in the form of CO2. “That corresponds to around 95 percent of the carbon emissions released by the bush fires of summer 2019/2020,” write Tang and his team. However, it is unclear how much of the carbon absorbed and stored in the algae tissues reached the deeper ocean layers and was thus withdrawn from the carbon cycle in the long term. “It is also possible that the additional carbon did not get into the deeper layers and the CO2 sequestration was therefore only short-lived,” they emphasize.
In view of climate change and its intensifying effects on forest and bush fires worldwide, the researchers see in their results an important indication that the consequences of large fires can be more far-reaching than previously considered in the models. “These fires show us an unexpected and previously underestimated impact of climate change on the marine environment – with potential feedback also to our global climate,” says Tang.
Source: Weiyi Tang (Duke University, Durham) et al., Nature, doi: 10.1038 / s41586-021-03805-8