Wildfires and Ocean Blooms

Image courtesy of Flickr.

From September 2019 to March 2020, Australia experienced one of the worst recorded wildfire seasons in its history. Spanning 18.6 million hectares, the wildfires were responsible for massive ecological and socioeconomic damage. Around three billion animals were estimated to have been displaced or killed, causing a severe loss of biodiversity that will prove difficult to recuperate from. Furthermore, 715 million tons of carbon dioxide and various aerosols were released into the atmosphere over the course of the bushfires, the effect of which was felt around the world.

Despite the undeniably catastrophic effects of these fires, their unprecedented nature provided researchers with a unique opportunity to study the global impacts of wildfires—including their previously under-studied consequences for marine ecosystems. The researchers hope that further research on this topic can help us better predict and respond to wildfires and their effects in the future.

Scientists Weiyi Tang and Joan Llort, helming an international team of researchers under biogeochemist Nicholas Cassar and climatologist Richard Matear, were intrigued by the relationship between two known phenomena: wildfires drag nutrients like iron into the atmosphere, and iron deposition can trigger phytoplankton blooms in water given the right conditions. Further motivated by the limited research on the marine effects of wildfires, the scientists used data from the bushfires to determine their effects on phytoplankton blooms in the South Pacific Ocean. 

“[The bushfires] provided us with a unique opportunity to see how and if such wildfires could have an impact on ocean ecosystems downwind. Very often in science it’s very difficult to detect the signal from the noise, but here we had an unprecedented wildfire event, and so this was a unique opportunity,” Cassar said.

To measure aerosols produced by wildfires, the team used black carbon aerosol optical depth (AOD) data provided by the Copernicus Atmospheric Monitoring Service (CAMS). Black carbon AOD cannot actually be measured directly, but is rather estimated from overall AOD, which is in turn measured spectroscopically by satellites in the atmosphere. This data reflects the concentration of all aerosols in a given air column—not just black carbon aerosols produced by fires. The CAMS aerosol model uses meteorological data to separate total AOD into many subcategories, including black carbon, dust, sulfate, and salt. 

AOD values for these wildfires were abnormal, reportedly over three-hundred percent higher than average values since 2004. They indicated eastward drift of black carbon into the South Pacific Ocean, which was then confirmed through modeling of air trajectories from meteorological data.

To quantify phytoplankton growth in the South Pacific, the team took advantage of the fact that phytoplankton, as photosynthetic organisms, produce the green pigment chlorophyll a (Chla). Chla concentrations could then be estimated from publicly available satellite observations as a proxy for phytoplankton biomass. This satellite data was subsequently confirmed by marine floats deployed by Argo, an international program that collects ocean data with an array of below-the-surface floats that occasionally surface to transmit their data to satellites. 

Just six months after the wildfires started, Chla concentrations increased by more than 150 percent compared to historical concentrations in oceanic regions along the path of aerosol transport. Furthermore, these increases in concentration occurred just days to weeks following spikes in black carbon AOD, suggesting a connection between wildfires and phytoplankton blooms, and further revealing just how swiftly events of this scale can impact the globe.

The international team also found that aerosols collected at a station downwind of bushfires had iron concentrations over five times the median value of concentrations observed at the same station from 2016 to 2019, when smaller wildfire events occurred. The formation of blooms requires a variety of nutrients and environmental conditions—iron alone is not always sufficient. In this case, the South Pacific Ocean likely had all the sufficient conditions for phytoplankton growth except for iron during the wildfire season, meaning the iron deposits from migrating aerosols were likely sufficient to support the Chla concentration increase observed in oceans. Thus, the aerosols produced from the Australian bushfires may provide an explanation for the observed phytoplankton blooms in the South Pacific.

Blooms can have varying ecological effects depending on the type of phytoplankton and the characteristics of the body of water and environment. In some scenarios, the blooms may, in fact, benefit the climate. Through photosynthesis, phytoplankton sequester carbon dioxide from the atmosphere. However, the authors have not yet been able to determine if the carbon dioxide sequestration is short-term, with carbon dioxide quickly recycled back into the atmosphere, or long-term, with carbon dioxide exported to the deep ocean as plankton biomass. In future wildfire seasons, they hope to determine what fraction of the carbon is sequestered long-term by using sediment traps to capture plankton biomass exported into the ocean from blooms. 

Further investigating long-term sequestration effects will provide important data that can be used to create better climate models. Current climate models do not sufficiently account for the various and widespread effects of wildfires. “You’re expecting that the frequency, the intensity, the duration of some of these wildfires is going to increase,” Cassar said. “And so, if we’re going to project our climate in the future, we need to understand how these wildfires also impact ocean ecosystems because of their role in the carbon cycle.” With further testing at other wildfire sites and more detailed biogeochemical analysis, the team hopes they can develop a better understanding of the climate-related impacts of wildfires that would facilitate future climate predictions.

Ultimately, this project would have been impossible without international cooperation. The majority of the data, including black carbon AOD data and Chla concentration data, was sourced from organizations and projects producing publicly available atmosphere and ocean data. This collaborative data-sharing and type of cooperation is typical in the field of oceanography. “Because the oceans are global, and because no one owns them, there’s a real strong international collaboration culture in this space,” Matear said. Such a spirit of collaboration provides a model for future climate research—international cooperation is necessary to address this global issue. 

With this project, the research team revealed the dynamic, interconnected nature of our climate and ecology. In fact, Matear and other climatologists are changing the language they use to reflect this connectivity. “I’m going to use the word ‘earth system’ rather than ‘climate,’” Matear said. “That’s probably a big change that’s happening in the climate modeling space, just acknowledging that climate and carbon cycle processes are intimately linked and you probably don’t want to separate them.” 

Climatological trends that may seem small on paper, such as a global increase in temperature by just two degrees Celsius, can have far-reaching and catastrophic effects. As weather patterns and natural phenomena become more severe due to climate change, so too do the impacts they have on regions around the world, on species nearing extinction, and on delicate climate systems. 

Today, humans have become deeply entangled in this environmental web. “We’re such a successful species that we’re impacting our climate… We’re impacting the temperature of the atmosphere and the likelihood of droughts in some regions, so I think there’s a local and global impact of our species that we have to take into account,” Cassar said. 

It is vital to understand how local perturbations by humans can cause drastic changes on a global scale. This study reveals just how much we have yet to uncover about the connectivity of Earth’s natural and artificial systems. Cassar, Matear, and their team, for example, observed links between wildfires and phytoplankton populations thousands of miles away. Future research into this issue will require searching for more unexpected connections between climate-related phenomena.