According to Ocean Climate, the oceans absorb a quarter of the CO₂ from human activities. The ocean is a carbon sink that helps moderate global warming.
By absorbing a large amount of CO₂, ocean acidification results in a change in water chemistry. When CO₂ is absorbed by the ocean, it is dissolved and causes a decrease in pH (the lower is the pH, the higher is the acidity) and also a decrease in carbonate ions (CO32-) which are necessary for the proper growth of marine ecosystems. Indeed, carbonate ions allow marine animals to build their skeletons, shells and other calcareous structures.
What are the impacts on marine organisms and humans?
The absorption of CO₂ by the oceans induces an increase in protons (H+), carbonic acid (H₂CO₃) and the decrease in carbonate ions (CO32-). Knowing that carbonate ions allow marine organisms (molluscs, corals, oysters...) to build their shells or calcareous skeletons, these organisms are threatened by this phenomenon. It is certain that the more the oceans acidify, the more difficult it will be for these organisms to build their skeletons and shells, because the acidity of the water has a corrosive effect on the limestone.
Laboratory research has been conducted and it has been stated that some organisms will not survive an increase in ocean acidity. In addition, adverse effects have been observed on different species such as calcareous algae which are affected by ocean acidity and lower CO₂ absorption power.
Ocean acidification has an impact on marine organisms and on the many industries that depend on them. Indeed, these different sectors are affected: fishing, aquaculture and tourism. For example, as a food resource, marine animals caught and consumed by humans, such as fish and shellfish, are threatened by ocean acidification and are not resilient.
How to limit ocean acidification?
Even if humans stopped emitting CO₂, ocean chemistry would remain altered for hundreds of years. However, it is possible to limit the progression of ocean acidification and mitigate its impacts through geoengineering techniques. In particular, with negative emission technologies, NETs.
Several methods are proposed by Ocean Nets: ocean fertilisation, artificial upwelling, ocean alkalinisation and blue carbon management.
Fertilizing the oceans
Ocean fertilisation is a geoengineering technique that aims to increase marine primary production so that fertilized areas allow marine organisms to absorb more carbon dioxide (CO₂). This project aims to seed the surface of biomass-poor oceans with nitrogenous nutrients, phosphorus or iron, to stimulate phytoplankton development and photosynthesis. Phytoplankton fix CO₂ in their biomass and increase carbon uptake. When phytoplankton die and their biomass sinks to the depths of the ocean, the fixed carbon can be stored deep in the ocean for hundreds of years. However, this technique is contested because it upsets the marine balance and poses risks to marine biodiversity.
This technique aims to artificially pump up deep waters to provide nutrients using pipes or wave pumps. This process has a fertilizing effect by stimulating phytoplankton and can make the waters of ocean deserts more productive. In a similar way to ocean fertilisation, phytoplankton growth fixes more CO₂ and increases carbon uptake in the upper ocean.
Blue carbon management
Blue carbon ecosystems, including seagrass beds, mangroves, salt marshes, seagrass meadows or even kelp forests, are considered a powerful carbon sink in the biosphere and play an important role in climate change mitigation. Indeed, these ecosystems have a high capacity to absorb CO₂ through photosynthesis and to store carbon in the soil. Blue carbon management aims to restore these ecosystems and have the following benefits: reduced coastal erosion and flooding, improved water quality, increased habitat for marine life, reversal of acidification and hypoxia.
Alkalinisation of the oceans
Alkalinity is the ability to neutralize acids in a solution. For the ocean, Ocean Nets proposes that this is achieved by adding minerals or solutions containing carbonate which then react with CO₂ and water to form bicarbonate (HCO3-) and carbonate (CO32-) ions. Ocean Nets offers:
- The addition of alkaline rocks such as limestone to have lime and added it directly to the ocean in powder form.
- Interacting seawater with alkaline and silicate minerals by electrolysis in a complete cell before releasing the solution into the oceans.
Alkalising the oceans would increase the rate of CO₂ uptake but also increase the pH to counteract the increase in acidity.
ClimateWorks Foundation supports the development of projects to reduce greenhouse gas emissions in the ocean through new scientific approaches. One of the objectives of this foundation is to help develop proposals for carbon removal technologies (CDR technology) aimed at boosting the ocean's natural pumps, such as large-scale algal cultivation to store CO2. The following video develops the different carbon removal technologies aimed at reducing and sequestering CO2 from the ocean: https://www.youtube.com/watch?v=brl4-xa9DTY&t=3s
The proposed CDR technologies relate to the management of ocean alkalinity to reduce ocean acidification. However, these proposals remain theoretical and have not been tested in the field. There is an urgent need to improve scientific understanding of the impacts associated with these proposals. Despite other solutions deployed on land to remove carbon, reducing ocean acidity is essential to restore a healthy balance.
Improving understanding and facilitating the development of carbon removal mechanisms in the oceans are important tools for climate action.
Geoengineering is a controversial discipline that involves manipulating the environment to address climate change, and it is developing solutions that are attracting particular technical, legal and geopolitical attention. Currently, research is still underway to estimate and quantify the viability of initiatives to reduce ocean acidification without developing adverse effects on marine biodiversity.
Initiatives such as Oceans 2015 aim to provide COP21 negotiators with essential information on the future of the oceans. Bringing together leading research laboratories, researchers are analyzing the future of the oceans and alerting decision-makers to take action. In 2022, it is time to act for the oceans, the environment, biodiversity and climate. For more information on ocean acidification, here is an explanatory video from Le Monde : https://www.dailymotion.com/video/x2tb7or