Ocean guardians: The role of whale sharks in carbon capture

Our world is rapidly changing due to human emissions causing the Earth to warm up, leading to extreme weather events, water shortages, glaciers melting, rising sea levels and more. One of the main gases responsible for these changes is carbon dioxide (CO₂), which is released into the atmosphere through our daily activities—mainly by burning fossil fuels for energy and transportation.

According to the National Oceanic and Atmospheric Administration (NOAA) and the Stanford Doerr School of Sustainability, humans produce approximately 40 billion metric tonnes of carbon annually. Although natural sources such as ocean-atmosphere exchange, decomposition, plant respiration and volcanoes produce around 366 billion metric tonnes of carbon annually—almost ten times that produced by humans—it is the extra carbon dioxide from human activity that is disrupting the balance of nature’s carbon sequestration (the process of capturing and storing atmospheric carbon dioxide), leading to climate change.

Can we still halt the severe impacts these emissions are causing now and the havoc they will wreak on our planet in the near future? I’d like to believe so—and science agrees!

However, this will require changes in behaviour from all of us: changes in energy production methods and greater protection and growth of our remarkable natural resources to absorb the carbon we emit into the atmosphere.

Natural carbon sinks

From a young age, we learn that trees take in carbon dioxide to produce energy and grow through a process called photosynthesis. Plants also release oxygen back into the atmosphere which humans need to breathe. This makes forests one of the planet’s great carbon sinks, balancing out naturally produced carbon dioxide and now also having to sequester the excess produced by humans.

Yet forests are not the only ecosystems involved in absorbing carbon dioxide. Scientists have recently discovered the power of marine and coastal ecosystems in absorbing carbon from the atmosphere, which is known as blue carbon.

The ocean hosts an impressive array of ecosystems, all playing a role in carbon sequestration. Just as large forests on land absorb carbon, so do coastal wetlands, such as mangrove forests, marshes and seagrass meadows, as well as kelp forests and microscopic algae known as phytoplankton, each in distinct ways.

A study by Hao et al. (2024) mentions that coastal wetlands collectively store 210 grams of carbon per square metre per year, compared to 4.0-5.1 grams of carbon per square metre stored by terrestrial ecosystems (tropical, boreal and temperate forests). Their marine counterpart is sequestering carbon at rates dozens of times higher than terrestrial ecosystems, making them extremely valuable for climate change mitigation.

The role of marine megafauna in carbon capture

But that is not all. Recently scientists are learning that aquatic plants and algae aren’t the only players in blue carbon. While the science is still new and ongoing, we are uncovering evidence that large marine organisms also play a vital role in carbon sequestration. Innovative research is demonstrating how whales, krill and fish support the movement of carbon from the ocean surface to deep sea, where it can be stored long term. This system is known as the biological pump.

A study published by Chami et al. (2019) in the International Monetary Fund’s report ‘Nature’s Solution to Climate Change’ revealed how whales contribute to carbon storage via the biological pump through their feeding behaviours, movements across the ocean and carbon storage over long lifetimes. By protecting them and helping their populations recover, we can reduce the impact of greenhouse gases and combat global warming.

The study notes that this ecological role applies not only to whales but also to other large marine megafauna, particularly highly migratory species. Although no research has focused on whale sharks specifically up until now, I present whale sharks as a new, plausible component in the carbon sequestration system based on the mechanisms identified in similar large marine species.

Whale sharks migrate long distances in the ocean, both horizontally—travelling approximately 40–100km daily—and vertically, diving into mesopelagic and bathypelagic waters, with the deepest recorded dive being 1,928m. Whale sharks are voracious feeders, ingesting large amounts of prey biomass daily. For example, a juvenile whale shark (6.6m in length and weighing 1,716kg) was estimated to consume 28,121KJ of plankton per day. They contribute to nutrient transportation by travelling from coastal feeding areas to open ocean regions, where they defecate and enrich otherwise nutrient-deficient waters. Additionally, by physically mixing water during their dives, they help move nitrogen—a key nutrient—from the ocean depths back to the surface, stimulating phytoplankton growth.

This is critical because phytoplankton sequesters between 30 to 50 billion metric tonnes of carbon annually, representing about 40% of all plant-based carbon sequestration. Phytoplankton also produces around 50% of the oxygen on Earth.

However, whale sharks don’t only contribute to carbon sequestration indirectly by promoting phytoplankton growth. They also do so due to their sheer size. Whale sharks can grow anywhere between 12m- 20m, with the largest recorded whale sharks at 18.8m and 20m and weighing up to 34 tonnes. Every living being is made up of carbon, and the larger and longer-lived the organism, the more carbon it stores as biomass.

In the Chami et al. study, the authors estimated that each great whale (over ten metres in length) sequesters around 33 metric tonnes of CO₂ on average. By extension, adult whale sharks would likely sequester a similar amount of carbon via their biomass. When whales and whale sharks die, they sink to the ocean floor, taking their carbon with them, where it can remain buried for centuries, far from the atmosphere and gradually transferred to deep-sea organisms.

Overfishing and climate change

However, a study by Mariani et al. (2020) has shown that ocean fisheries are reverting this benefit. When the large marine organisms are fished and eaten, rather than left to sink to the bottom of the ocean, the carbon stored in their bodies is released into the atmosphere instead, contributing to additional CO₂ emissions. The study suggests that reducing blue carbon extraction by fisheries, especially in less profitable areas, could lower CO₂ emissions by decreasing fuel use and reactivating a natural carbon pump.

The exact amount of carbon sequestered by whale sharks through phytoplankton promotion and biomass is difficult to quantify. More studies are needed to understand their migratory routes, habitat use, diving behaviour and longevity to calculate more precise numbers on their food intake, nutrient transport across the ocean, nitrogen cycling and carbon accumulation over their lifetimes. Nonetheless, the fact that whale sharks and other marine megafauna play a role in the blue carbon system is now clear and undeniable. Consequently, researchers Martin, Scheffold, and O’Leary (2023) recommend including the carbon functions of fish in marine politics, sustainable fisheries and maximum sustainable yield calculations and the management of carbon stocks in coastal and marine ecosystems.

Although marine vertebrates store only a fraction of the total carbon in marine ecosystems, recovering populations of fish, whales and other large marine animals such as whale sharks could have an impact on carbon sequestration comparable to current carbon capture projects. The more of these animals we have in the ocean, the stronger our ocean carbon sink becomes.

It’s crucial to remember that alongside the protection of our carbon sinks and large marine megafauna, each of us plays a significant role in carbon emissions. Behavioural and societal changes must accompany ocean protection to reduce disruptions to the Earth’s natural cycles.