How Many Pathways We Have for the Marine Carbon Neutrality

Global warming is one of the major challenges that the international community is currently facing. The Intergovernmental Panel on Climate Change (IPCC) has indicated that if the concentration of greenhouse gases in the atmosphere continues to rise, global warming is expected to pose risks to future food security, as well as to human health and wealth in the 21st century. China became the world's largest emitter of greenhouse gases in 2006. In 2018, the global emission of greenhouse gases was 33.1 billion metric tons of CO₂, with China accounting for 28% of the total global emission, ranking first globally. According to estimates by the Chinese Academy of Engineering, China will reach its peak carbon dioxide emissions in 2027 at 12.2 billion metric tons of CO₂, making it imperative to achieve carbon neutrality and achieve harmony between humans and nature as soon as possible.

So what is carbon neutrality, and what can oceans do to help us achieve it? Carbon neutrality refers to the process of offsetting carbon emissions by natural and artificial carbon sinks during a specific period, so that the amount of emissions is balanced with the amount of absorption, thus achieving net-zero carbon dioxide emissions. Carbon sinks are essential for addressing climate change and achieving high-quality economic and social development, and they are also among the most effective means to achieve carbon neutrality.

The ocean is the largest carbon sink on Earth, absorbing about one-third of human-generated CO₂ emissions through two important carbon sink processes: dissolved pumps and biological pumps. These processes continuously transport human-induced CO₂ released into the atmosphere to deep ocean waters via changes in atmospheric CO₂ partial pressure and the marine carbonate buffer system. Although the dissolved pump process is significant and rapid in terms of CO₂ absorption, there are still many challenges to be addressed. For example, near-shore areas adjacent to human activities exhibit significant temperature fluctuations, which makes carbon pool assessment highly unstable. Moreover, while marine carbonate systems have relatively strong buffering properties, an essential challenge lies in their difficulty to be manipulated manually, which makes the dissolved pump process increasingly vulnerable in controlling future CO₂ emissions. Conversely, biological pumps, which are capable of adsorbing and sequestering CO₂ through biological processes and then sinking and discharging it into deep ocean waters, though having a relatively small volumetric value, exhibit a relatively high carbon sink efficiency and manipulability. As a result, researchers are increasingly favoring this process. China's marine resources have unique geographical advantages, with diverse and abundant marine and coastal ecosystems, implying enormous potential for carbon neutrality. There are approximately seven pathways for achieving carbon neutrality in oceans (Fig. 1), including:

Figure  1.  Pathways toward marine carbon neutrality. 1. Biological pump; 2. coastal wetlands carbon sink; 3. fishery carbon sink; 4. marine ecosystem carbon sink; 5. microbial carbon pump; 6. marine geological carbon sequestration; 7. land-sea coupling carbon sink.

DownLoad: Full-Size Img PowerPoint

(1) Biological pump for increasing carbon sinks. This process primarily involves marine organisms absorbing CO₂ through biological pump initiated in the euphotic zone through photosynthesis by phytoplankton, converting surface seawater CO₂ into particulate organic carbon that interacts with dissolved pumps. By analyzing the structure, processes, and mechanisms of marine biological pump in detail, we can enhance the carbon sequestration, storage, and efficiency of marine organisms (Sun et al., 2016). It is conservatively estimated that China's coastal marine biota could sequester an annual average of 53 billion tons of CO₂.

(2) Coastal wetlands carbon sink. Wetlands are considered one of the three largest global ecosystems due to their excellent ability to store and trap atmospheric CO₂. Although covering only 4%–6% of land area globally, they contain more than 30% of global carbon stocks and play a crucial role in global carbon cycles. Studies have shown that salt marshes, mangrove forests, and seagrass beds globally exhibit strong carbon sequestration capabilities.

(3) Fishery carbon sink. "Fishery carbon sink" mainly refers to the process of promoting aquatic organisms to absorb CO₂ from water bodies through fishing production activities and then harvesting aquatic organisms to remove CO₂ from these bodies. While developing marine aquaculture, efforts should be made to transform it into carbon-neutral aquaculture such as ocean ranching. Fishery carbon sink is an important practice in implementing the strategy of Blue Grain Rice Reserves.

(4) Marine ecosystem carbon sink. In the future, many artificial ecosystems will be established in the ocean, such as coastal reclamation projects, inner bay restoration, and artificial coral reefs. Through artificial ecological protection and restoration, we can achieve ecological health benefits by increasing carbon sinks in these artificial ecosystems while minimizing damage caused by human activities.

(5) Microbial carbon pump. It is estimated that about 5%–7% of dissolved organic matter in the ocean consists of inert dissolved organic matter that cannot be rapidly mineralized but rather accumulated over long periods, forming an inert dissolved organic matter pool in the ocean and realizing internal storage of CO₂ within the ocean (Jiao et al., 2011).

(6) Marine geological carbon sequestration. Like land carbon sequestration, submarine sedimentary deposits are ideal sites for storing carbon. Sedimentary reservoirs not only exhibit enormous potential for carbon sequestration but also possess superior physical, chemical, and hydrologic conditions compared to land reservoirs (Hurtado et al., 2016).

(7) Land-ocean co-management for reducing emissions and increasing carbon sink. Due to excessive nutrient inputs from land runoff into nearshore waters, nearshore environments may become overly eutrophic and suffer ecological disasters. Organic matter stored in nearshore waters is difficult to preserve, especially after being converted into CO₂ by riverine organic matter during their journey through estuaries and nearshore waters. Therefore, transforming nutrient inputs from land into treasure through waste recycling and turning pollutants into carbon sinks has become particularly important.

Ecological prosperity leads to civilization prosperity. The ocean is a cradle of life, a treasure house of resources, and also a strategic place for high-quality development. Achieving global carbon neutrality through oceanic carbon sinks is a massive undertaking that depends not only on concerted efforts from all stakeholders but also on supporting policies and regulations. Although coastal wetlands reduction and emission reduction are critical steps towards China's carbon neutrality goals, carbon neutrality can start with land-ocean co-management for reducing emissions and increasing carbon sinks by repairing eutrophication in nearshore waters as a starting point; shortening food chains for bottom-feeding organisms and fishery resources to increase ecosystem carbon sinks; relying more on artificial ecosystem operation; and looking forward to future advancements in biotechnology engineering that can fully unleash oceanic carbon sink values and potential while fulfilling China's commitment to achieve carbon neutrality goals, thereby demonstrating our responsibility as a major power.