What’s happening? The ocean’s carbon storage capacity was underestimated by 20% in the latest IPCC report, according to a study published in Nature.

Plankton plays a crucial role in transporting carbon from surface waters to the seabed by converting CO₂ into organic tissue. As plankton die, they form “marine snow”, sinking to the seabed, storing carbon, and providing nutrients for deep-sea organisms.

Analysing decades of oceanographic data, the study estimates the ocean’s carbon storage at 15 gigatonnes per year, a 20% increase from previous IPCC estimates. While insufficient to offset current CO₂ emissions, this discovery underscores the ocean’s vital role in long-term climate regulation.

Why does this matter? The oceans play a pivotal role in the Earth’s climate system, serving as a primary force in maintaining the planet’s climatic equilibrium for thousands of years. Their regulation involves absorbing and distributing heat globally, elevating atmospheric humidity, and acting as a significant carbon sink.

The ecosystem services provided by the world’s oceans render Earth a habitable planet. For example, oceanic biogeochemical processes contribute to providing at least half of the Earth’s atmospheric oxygen.

The ocean’s role in global temperature control cannot be overstated. Researchers estimate that if the ocean stopped absorbing atmospheric heat, the average global temperature would sit at 50C, up from 15˚C today. In some areas, temperatures would exceed 100˚C.

The ocean is the largest carbon sink on Earth, holding approximately 38,000 billion tonnes of carbon – over 28 times more than land vegetation and the atmosphere combined. The ocean’s ability to absorb, store and release carbon has shaped the Earth for millions of years.

In geological timescales, this mechanism has both caused and ended ice ages. In more recent times, since the onset of the Industrial Revolution in the mid-19th century, the oceans have absorbed almost one-third of human-induced CO₂ emissions.

How does it work? The ocean stores carbon via two pathways, known as the biological carbon pump and the solubility carbon pump.  The biological pump is responsible for the vast majority of the carbon stored within the ocean, with an estimated 15 gigatonnes of carbon sequestered via this mechanism each year, according to the aforementioned study.

In the sunlit layer of the ocean, tiny plants called phytoplankton use sunlight to perform photosynthesis, extracting CO₂ from the atmosphere and converting it into organic matter.

This process, known as primary production, forms the foundation of food chains in the ocean. Despite the small size of these plants, they generate large amounts of dissolved organic carbon (DOC) to sustain marine food chains.

A portion of the produced organic material sinks to the ocean depths before being stored in sediments for millennia. An estimated 0.1%-1% of these sediments go on to become fossil fuels.

The second, less productive carbon pathway is powered by physico-chemical processes, known as the solubility pump. Here, surface waters in cold regions absorb atmospheric CO₂ and become dense, sinking to vast depths and effectively storing carbon.

These processes are crucial for removing CO₂ from the carbon cycle over long periods. However, the rapid release of carbon into the atmosphere by human activities contrasts with the slower rate of CO₂ sequestration by photosynthesis and CO₂ absorption.

Concerningly, a growing body of literature suggests that climate change may impact both the biological and solubility pumps. For example, warmer waters lower the capacity of the solubility pump.

The behaviour of the biological pump is harder to predict, however, as temperature, light, inorganic nutrients, and pH, among a variety of other variables, will have a significant impact on phytoplankton’s productivity.

As the climate crisis worsens, calls to utilise geoengineering solutions in the ocean have intensified. Suggestions in the past have included ocean fertilisation, where iron is used to simulate plankton growth. However, a study conducted by MIT suggested that ocean fertilisation may have no impact on the global scale.

Seaweed farming is a rapidly advancing ocean carbon removal method. Numerous startups and research initiatives are exploring the feasibility of cultivating seaweed, and then submerging it in the deep sea as an efficient means of capturing and storing carbon.

The US start-up Running Tide is initiating field trials near Iceland, deploying thousands of wooden buoys as small floating kelp farms. Once the buoys dissolve, the seaweed they foster sinks into the deep ocean.