As the world struggles to decarbonize, it is becoming increasingly clear that we need to rapidly reduce emissions and actively remove carbon dioxide (CO₂) from the atmosphere. The Intergovernmental Panel on Climate Change's most recent report considered 230 pathways to limit global warming to well below 1.5°C, all of which required the removal of CO₂.
Some of the most promising carbon dioxide removal technologies, receiving government funding in the US, UK and Australia, aim to boost the ocean's vast carbon storage capacity, which involves fertilising tiny plants or tweaking ocean chemistry.
Ocean-based approaches are growing in popularity because they have the potential to store carbon at one-tenth the cost of “direct air capture”, which involves sucking CO2 from the atmosphere with energy-hungry machines.
But the ocean carbon cycle is much harder to predict: before scientists can move forward with ocean-based CO2 removal, they need to unravel many complex natural processes that could alter its efficiency, effectiveness and safety.
Our new research highlights a surprisingly important mechanism that has been overlooked until now: if CO2 removal technologies change the appetites of small animals at the bottom of the food chain, they can dramatically change how much carbon is actually stored.
Plankton dominates the carbon pump
Tiny marine organisms called plankton play a major role in the ocean's carbon cycle: these microorganisms drift with ocean currents, transporting captured carbon throughout the ocean.
Like land plants, phytoplankton grow through photosynthesis, using sunlight and carbon dioxide.
Zooplankton, on the other hand, are tiny animals that mainly eat phytoplankton. They come in all different shapes and sizes, and when lined up, they look like they're from another planet.
Despite this diversity, each zooplankton has a very different appetite: the hungrier they are, the faster they will eat.
The waste of uneaten phytoplankton and zooplankton sinks to great depths and sequester carbon from the atmosphere for centuries, some of it even sinking to the ocean floor where it is eventually turned into fossil fuels.
The movement of carbon from the atmosphere to the oceans is known as the “biological pump”. This keeps hundreds of billions of tonnes of carbon in the oceans and removes it from the atmosphere, equivalent to about 400 ppm of CO₂ and a cooling of 5°C.
Zooplankton lineup: tiny marine animals that look like they're from another planet. Julian Uribe Palomino/IMOS-CSIRO
A person who has strong likes and dislikes
In our new study, we wanted to better understand how zooplankton appetite influences the biological pump.
First, we needed to determine how zooplankton appetites vary across the ocean.
We used a computer model to simulate the seasonal cycle of phytoplankton population growth, which is based on a balance between reproduction and death. The model simulates reproduction very accurately.
Zooplankton appetite is the main driver of mortality, but this model is not very suitable for simulating mortality because there is insufficient information on zooplankton appetite.
So we tested dozens of different appetites and compared the results with real-world data.
To observe the seasonal cycles of phytoplankton on a global scale without using a fleet of ships, we used satellite data – this is possible because phytoplankton are very tiny but have light-capturing pigments that are visible from space.
We ran the model at over 30,000 locations and found that zooplankton appetites vary widely — that is, different types of zooplankton are not evenly distributed throughout the ocean — and they seem to congregate around their favorite prey.
Our latest work shows how this diversity impacts the biological pump.
We compared two models: one with only two types of zooplankton and one with no restrictions on the number of zooplankton, each with different appetites and individually tailored to their unique environment.
We found that incorporating realistic zooplankton diversity reduces the strength of the biological pump by 1 billion tonnes of carbon each year, which is bad for humanity because most of the carbon that doesn't end up in the oceans ends up back in the atmosphere.
Not all of the carbon in the phytoplankton sank deep enough to be sequestered from the atmosphere, but even if only a quarter of it did, that amount would be equivalent to the annual CO2 emissions of the entire aviation industry.
In the ocean carbon cycle, the biological pump begins with phytoplankton capturing carbon dioxide from the atmosphere during photosynthesis. When the phytoplankton die, their carbon is stored deep in the ocean. However, the carbon dioxide is released back into the atmosphere as zooplankton feed. IAEA
The Ocean as a Sponge
Many ocean-based CO₂ removal technologies alter phytoplankton composition and abundance.
Marine biological carbon removal techniques such as “marine iron fertilization” aim to boost the growth of phytoplankton – similar to spreading fertilizer on a garden, but on a much larger scale, with fleets of ships spreading the iron across the oceans.
The goal is to remove carbon dioxide from the atmosphere and pump it into the deep ocean, but adding iron can change the composition of phytoplankton, as some phytoplankton have a higher iron requirement than others.
Alternatively, abiotic ocean-based CO₂ removal techniques such as “ocean alkalinity enhancement” change the chemical balance, allowing more CO₂ to dissolve in the water before chemical equilibrium is reached. However, the most readily available sources of alkalinity are minerals that contain nutrients that promote the growth of certain phytoplankton over others.
If these changes in phytoplankton favour different species of zooplankton with different appetites, they are likely to alter the strength of the biological pump, which could make ocean-based CO2 removal technologies less or more efficient.
The Surprisingly Important World of Phytoplankton (NASA Goddard)
Moving forward from a sea of uncertainty
Emerging private carbon removal companies will need certification from a credible carbon offset registry, meaning they must demonstrate that their technology:
It removes carbon for hundreds of years (permanence), avoids major environmental impacts (safety), and allows for accurate monitoring (verification).
In the face of oceanic uncertainty, it is time for oceanographers to establish necessary standards.
Our research shows that CO2 removal techniques that alter phytoplankton communities may also cause changes in carbon storage by altering zooplankton appetite. A better understanding of this is needed to accurately predict how well these techniques work and how they need to be monitored.
Overcoming the challenges of observing, modeling and predicting zooplankton dynamics will require significant effort, but the rewards are enormous: a more reliable regulatory framework could pave the way for a new trillion-dollar, morally imperative carbon removal industry.
