Por Víctor Quintanilla y Natalia Oviedo*

Although the ocean is essential for stabilizing the planet's climate, it is rarely the focus of attention when we talk about the global climate crisis.

The ocean is our best ally in the face of the climate emergency because it absorbs much of the greenhouse gases that humanity emits and that are the source of the problem.

At the same time, the ocean is a victim of the climate crisis, whose impacts are pushing it to the limit with acidification of its waters, rising sea levels, and loss of oxygen, processes that seriously affect marine life.

Despite its importance, the relationship between the ocean and climate has not been fully included in international negotiations in which governments seek agreements and policies to address the climate crisis.

Faced with this gap and in a historic breakthrough, the International Tribunal for the Law of the Sea issued a ruling in 2024 clarifying the obligations of States to protect the marine environment from the climate crisis.

Below, we present five keys to understanding the link between the ocean and climate.

1. The role of the ocean in the climate crisis.

According to the Intergovernmental Panel on Climate Change, the ocean is “a fundamental climate regulator on timescales ranging from seasonal to millennial.” Since 1955, it has absorbed 90 percent of the excess heat caused by global warming, along with a quarter of the carbon dioxide released by human activities.

Ocean currents  transport warm water from the tropics to the poles, sending colder water back. This balances the temperature and makes much of the Earth habitable. The ocean influences climate variations on land by being the main source of rain, which feeds rivers and other vital freshwater systems.

The ocean is known as a lung for the planet because, through microscopic organisms known as phytoplankton, it is responsible for generating approximately half of the world's oxygen supply. In turn, coastal ecosystems such as mangroves, marshes, and seagrass beds absorb enormous amounts of carbon from the atmosphere, mitigating the climate crisis. 

2. The impact of the climate crisis on the ocean.

The climate crisis alters the physical and chemical properties of the ocean, affecting its ability to regulate the climate. One of the irreversible impacts of climate change is ocean warming, whose rate and heat absorption has more than doubled since 1993. As its waters warm, they begin to release carbon dioxide back into the atmosphere.

In addition, rising ocean temperatures are expected to reduce the amount of oxygen available, altering nutrient cycles and thereby affecting fish distribution and abundance. Another consequence is sea level rise, which is due to thermal expansion of the ocean and loss of land ice.

Finally, the absorption of increasing amounts of carbon dioxide has resulted in ocean acidification, understood as a decrease in pH. This reduces calcium levels, a substance necessary for the shells and external skeletons of various species of marine fauna and ecosystems such as coral reefs.

3. The inclusion of the ocean in international climate negotiations.

Although the link between the ocean and climate change has been recognized since the beginning of negotiations under the United Nations Framework Convention on Climate Change (UNFCCC)—by including the ocean in the definition of “climate system”—its presence in these negotiations has been gradual. A decisive milestone occurred at the 25th UN Climate Change Conference (COP25) in 2019, where the first official dialogue on the subject was called for, resulting in recommendations to align climate and ocean action, while also promoting the mobilization of financing to protect marine ecosystems.  

Since then, the ocean has earned a permanent place on the climate agenda. At subsequent conferences, the inclusion of the ocean was further expanded. The Glasgow Climate Pact (COP26, 2021) recognized the ocean as an ally in carbon absorption, called for the integration of ocean-based action into UNFCCC work plans, and mandated an annual dialogue on the topic. At COP27 (2022), countries were encouraged to include ocean-based action in their nationally determined contributions (NDCs). And at COP28 (2023), the ocean was included in the first global stocktake of the Paris Agreement.

Ahead of this year's COP30 in Brazil, which will have a specific agenda on “Forests, Oceans, and Biodiversity,” the challenge is to move from political recognition to the implementation of concrete actions to protect the ocean.

4. States' obligations to protect the ocean in the face of the climate crisis.

The health and resilience of the ocean is essential not only for addressing the climate crisis, but also for the exercise of fundamental human rights such as life, health, culture, food, access to water, and the right to a healthy environment. This highlights the interdependence between the ocean, climate change, and human rights.

Recognizing this link, on May 21, 2024, the International Tribunal for the Law of the Sea issued an important decision clarifying the obligations of States to preserve the ocean in the face of the climate crisis. These duties include adopting concrete mitigation measures to minimize the release of toxic substances into the marine environment, as well as exercising strict due diligence to ensure that non-state actors effectively comply with such measures.

The tribunal emphasized the obligations of states to prevent climate change-related pollution that affects other states and the marine environment outside national jurisdiction. In relation to the right to a healthy environment, the ruling emphasizes the use of precautionary and ecosystem approaches in the context of states' obligations to conduct environmental and socioeconomic impact assessments of any activity that may cause marine pollution related to climate change. This includes that, in the face of the possibility of serious or irreversible damage to the marine environment, the lack of full scientific certainty should not be used as an excuse to delay protective measures.

5. Some key actions to protect our ocean and, with it, the climate.

Effective protection of our ocean requires the commitment of governments, which must act at the national and international levels to prioritize its health. These include:

• Prioritizing concrete measures that integrate the ocean into climate mitigation and adaptation actions. Among the most effective are the protection and restoration of ecosystems, especially those that, in addition to capturing and storing enormous amounts of carbon, protect coastlines and maintain vital ecosystem services (mangroves, marshes, seagrass beds, coastal wetlands, and coral reefs, among others).
• Ensuring the protection and sustainable use of biodiversity in the ocean area beyond national jurisdiction. This involves the effective implementation of the High Seas Treaty, which will enter into force on January 17, 2026, as well as its ratification by countries that have not yet done so, to ensure fair, equitable, and sustainable governance.
• Defending the deep sea from mining. This requires imposing moratoriums on deep-sea mining activities on the grounds that there is insufficient technical and scientific information to prevent, control, and mitigate the potential impacts on the biological diversity of unknown ecosystems in deep waters and on the seabed.
• Protect the rights of coastal and island communities, which depend on fishing and local tourism for their livelihoods. These populations face increasing impacts from the climate crisis and multiple environmental pressures. Governments have a responsibility to ensure their resilience and well-being by safeguarding the marine and coastal biodiversity that sustains their ways of life and culture.

Acknowledging the link between the ocean and climate, and translating it into concrete and effective measures, is essential to protect and maintain the balance of both life systems.

This is what will guarantee the health of marine and coastal biodiversity, our food security, and, ultimately, a future for the planet.

* Víctor Quintanilla is AIDA's Content Coordinator; Natalia Oviedo is a Costa Rican international lawyer and former intern at the organization.