Oceans play a pivotal role in the Earth’s climate system, regulating the climate by absorbing and distributing heat globally, elevating atmospheric humidity, and acting as a significant carbon sink.

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.

What’s happening? UK-based start-up Seafields plans to establish farms in the Atlantic Ocean, growing sargassum seaweed that can be used to capture and sequester CO2. The algae will absorb CO2 during its growth, after which the plant can be harvested, formed into bales and sunk to the bottom of the sea where a portion of the CO2 will be retained. Sargassum’s vigorous growth rate has led to its invasion of large areas of the Caribbean coast. To address this, Seafields aims to build dedicated farms in areas where it would not ordinarily survive to prevent it from becoming an invasive species elsewhere.

Why does this matter? Interest in exploring ocean-based carbon sequestration is growing as an alternative to land-based offsetting projects – such as reforestation – which have historically been more widely recognised in the context of nature-based carbon solutions. The role of the ocean as a natural carbon sink is immense, given it covers around 70% of the Earth’s surface. Some ocean ecosystems have also been proven to be more efficient in carbon sequestration compared to terrestrial forests, with seagrass, salt marshes and coastal mangrove habitats able to store up to 10 timesmore CO2, while large marine organisms also represent as yet unquantified carbon stores. There is also a huge carbon accounting gap on the broader seabed itself.

The High-Level Panel for a Sustainable Ocean Economy states using ocean-based solutions for climate mitigation could reduce the emissions gap – the difference between current emissions pledges and those consistent with the 1.5C temperature trajectory – by up to 21%. Alongside private sector interest, ocean-based climate action is also gaining recognition and high-level action from governments and becoming a feature of international agreements.

Utilising seaweed to sequester carbon could have significant potential in long-term carbon storage. Sargassum requires minimal nutrients, is resilient once established and is an unattractive food source for marine life. Adding seaweed to the ocean can offer other benefits, such as improving fish populations and encouraging marine biodiversity. Elsewhere, seaweed is being explored for other applications including mixed-use offshore systems and cultivation for use in low-emission livestock feed.

Seafields is aiming to sequester 1 gigatonne of CO2 from the atmosphere each year by 2025. The aquafarms will be irrigated using underwater pipes to upwell nutrients, taken from deeper waters to sustain and control production. The firm’s process of shredding and baling the seaweed is designed to reduce the likelihood of remineralisation on the seabed, enabling it to sequester carbon for longer periods of time. Once established, Seafields plans to sell carbon credits in the voluntary carbon market to fund its work. However there are multiple uncertainties to be resolved, including the impacts of baled sargassum on marine ecosystems and habitats when it sinks to the ocean floor. Additionally, the company will have to quantify and monitor how much carbon is sequestered in the sargassum in order to verify its blue carbon-capture technology.

Utilising seaweed for carbon sequestration is being explored by other companies, including Carbon Kapture, which plans to establish a network of aquaculture farms to cultivate kelp across south Wales, Ireland and the UK’s south coast which will sequester CO2 30 times faster in comparison to land-based reforestation.

The agglomeration of sargassum is invasive and has been exacerbated by warmer ocean temperatures linked to climate change, creating problems for the environment and local communities such as Mexico and the Caribbean coast. Seafields’ farms will be located in areas of the south Atlantic, where sargassum does not naturally survive due to inhospitable conditions including water temperature and depth, reducing the risk of it becoming an invasive problem. Additionally, researchers are exploring commercial ways to mitigate the presence of sargassum in areas struggling with excessive natural build-ups, such as its potential as an energy source, including producing biogas, bioethanol and biodiesel.

Source:

iNews

Aquaculture conversion of Indonesian mangrove forests has the largest impact on the ecosystems’ carbon sequestration abilities, removing 60% of soil carbon stocks and 85% of biomass carbon, compared to forest harvesting practices which removed most biomass carbon with minimal impacts on soil stocks. The study, published in Global Change Biology, analysed land-use and land-cover change (LULCC) over a 25-year period and compared carbon stocks between undisturbed forest and areas that had undergone LULCC.

Mangroves left to regenerate recovered previous carbon levels after 25 years. LULCC characteristics need to be incorporated in emission-reduction strategies involving mangrove restoration, the authors said.

Mangroves act as nurseries and feeding grounds for many aquatic and terrestrial species and stabilise shorelines against flooding and extreme weather. They are also excellent carbon sinks, particularly within their soils, and have been found to have greater carbon sequestration potential than terrestrial rainforest per hectare.

Mangroves, therefore, represent an excellent nature-based solution for tackling climate change and offsetting emissions.

Carbon sequestration in mangroves involves many uncertainties, such as variability between species or the impact of methane emissions on net carbon stocks, and a limited number of studies have fully considered the different factors involved. This makes it difficult to accurately valorise mangrove restoration initiatives.

This study adds new information regarding the impact of LULCC on blue carbon stocks in mangrove forests; the location of carbon-rich mangroves within blue mangrove forests; the level of carbon losses from different economic activities; and a timeframe for mangrove regeneration. All of this can inform mangrove conservation decision-making.

Corporates should be mindful that offsetting does not fully consider the complex carbon flows within marine ecosystems and the potential for LULCC to erase carbon savings. Mangrove restoration is not a “quick win” but something that requires long-term attention to achieve consolidated results.

Payments-for-ecosystems and carbon resilience payments, whereby coastal communities are incentivised to conserve mangrove forests, may represent a more sustainable form of corporate action, delegating mangrove guardianship to local stakeholders through a multi-factor approach.

Source: Global Change Biology

https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.15056