Updated by ClimaTalk editing team June 2025.
Marine ecosystems provide a wide range of benefits – known as ecosystem services – that directly support human wellbeing and help regulate the Earth’s climate. These benefits are typically grouped into three categories: provisioning services (harvestable goods such as fish and seaweed), cultural services (non-material benefits derived from nature, such as spiritual value) and regulating services (environmental benefits such as coastal protection and carbon sequestration) [20].
Underpinning these benefits are the ecosystems themselves – distinct aquatic environments defined by their unique physical and biological characteristics [21]. This article explores these marine ecosystems, with a particular focus on their role in climate mitigation and their implications for strengthening climate resilience.
It should be noted that this article focuses on nature-based solutions to climate change, rather than approaches that require deliberate intervention such as geoengineering. This is because nature-based solutions are generally more effective at incorporating socio-economic co-benefits, making them more locally appropriate and resource-efficient [3].
How Climate Change Threatens Marine Biodiversity
It is estimated that oceans absorb approximately one-third of the total atmospheric carbon, making them the world’s largest carbon sink ahead of the atmosphere and the soil. This has led to many proposals for carbon dioxide removal (CDR) projects, a form of marine geoengineering which aims to manipulate the natural ecosystem to remove atmospheric carbon dioxide (CO2), counteracting climate change [4].
However, the paradox is that the more CO2 is absorbed by the oceans, the lower its ability becomes to absorb more. This effectively reduces the climate change mitigation potential of the global oceans as emissions increase. Furthermore, over-reliance on the carbon storage capacity of the oceans threatens to compromise their ability to provide vital ecosystem services such as food provision and coastal protection, especially for vulnerable small island nations [5].
The impacts of climate change on marine biodiversity occur through two main channels:
Ocean acidification
Ocean acidification (OA) refers to the decreasing pH levels, or increasing acidity, of the oceans due to the seawater’s absorption of CO2. This is measured by a decrease in carbonate, which is considered a crucial building block of the oceans. Indeed, calcifying organisms rely on calcium and carbonate to form their shells, including marine plants and animals such as coral, some plankton species and shellfish.
There are also socio-economic consequences of ocean acidification, including loss of food and job security which is significant given that over three billion people (most of whom live in developing countries) depend on marine biodiversity for their survival [5].
Rising Temperatures
The rise in sea surface temperature is another critical climate change indicator and affects marine ecosystems in several ways. Firstly, temperature variations can determine the regional diversity of plants, animals and microbes, change migration and breeding patterns, and threaten coral life [7]. The rising temperature is also cause for concern because it decreases the adaptive capacity of marine organisms, i.e. their ability to respond to the changing environment to ensure survival. This process reduces biodiversity and ultimately lowers the ecosystem’s resilience [8].
A study from Stanford University found that historically, marine biodiversity suffered in times of extreme warming because the high temperatures make it difficult for cold-blooded marine life to breathe. Thus, increased temperatures prove even more consequential to disrupting marine biodiversity than ocean acidification [9].
Similarly to the adverse effects of ocean acidification, changes to nutrient supply caused by rising temperatures leads to declines in fish populations which has drastic consequences for the billions of people who rely on fish for their livelihoods [7].
The Ocean: The Largest Carbon Sink
While the adverse effects of climate change on marine biodiversity are more well-researched, less is known about the ocean’s capacity to mitigate the impact of climate change. One of the ways in which it does this is by capturing and storing carbon, in what are known as blue carbon ecosystems. The IPCC defines blue carbon as the “carbon captured by living organisms in coastal and marine ecosystems” [2].
As demonstrated in Figure 1, of the three largest carbon sinks – the ocean (intermediate and deep sea), forests (vegetation) and soils – oceans store twelve times more carbon than forests and soils do combined; this difference is partly due to the effect of marine biodiversity. In light of the evidence, the EU Member States have outlined a plan of action in the EU Biodiversity Strategy 2030 to actively restore ecosystems [11].
Blue carbon ecosystems aren’t only necessary because of their carbon dioxide removal (CDR) potential. When these carbon stocks are degraded, they become sources of carbon dioxide and methane, the latter being an even more harmful greenhouse gas than CO2. An added benefit of carbon sequestration in seawater is that it can stimulate photosynthesis and, in turn, remove carbon, thereby reducing ocean acidification [12].
The caveat of the benefits of the blue carbon ecosystem is that the maximum mitigation potential is not more than 2% of present-day emissions. This is therefore – as with all mitigation methods – not a replacement for the reduction of greenhouse gas emissions [13].
Coastal Ecosystem Management
As previously mentioned, coastal blue carbon ecosystems include phytoplankton, coral reefs, calcifying organisms, and the coastal wetlands: mangrove forests, saltmarshes and seagrass. Of these, coastal wetlands are the most viable long-term carbon storage option [14].
Nonetheless, there are still limited strategies to take advantage of the carbon storage potential of coastal wetlands due to a lack of research and difficulties with monitoring and implementation. Despite these limitations, conservation of coastal ecosystems through Marine Protected Areas (MPAs) and implementing other regulatory policies are essential in protecting marine habitats and ensuring food and job security [15].
Phytoplankton also have carbon storage potential, as they absorb CO2 during photosynthesis and, when they die, sink to the ocean floor carrying the CO2 with them [19]. However, declining oxygenation levels in the ocean threaten phytoplankton populations, limiting the effectiveness of this natural carbon sink. Moreover, their capacity for sequestration cannot be manipulated without geoengineering which has raised significant concerns within the scientific community [14].
The Implications of Oceans for Climate Mitigation
As with all of the Earth’s carbon sinks, they can be used as a tool for climate mitigation rather than a source of CO2 emissions. As suggested in an article published for the World Bank, a broader blue economy approach should be adopted that prioritises the sustainable use of the ocean in a way that simultaneously allows for economic growth and coastal ecosystem conservation [17].
As part of this sustainable resource management approach, marine spatial planning provides an avenue to address coastal ecosystem degradation by restoring biodiversity in aquatic habitats, such as seagrass and mangrove forests, and implementing nature-based solutions that include establishing protected fishing areas [17].
Overall, such restoration and conservation measures will also indirectly aid in climate mitigation by reducing ocean warming and, in turn, acidification and habitat and biodiversity loss.
As this article has suggested, the health of our oceans also has a direct impact on human health and well-being. Coastal habitats provide a wide range of ecosystem services: food security, employment, clean water and protection against extreme weather events [18]. According to research from the World Economic Forum, the ocean economy contributes overs $2.5 trillion to global GDP annually and supports nearly 350 million jobs worldwide [20].
Yet, we still remain a long way from effective marine and coastal restoration. Ultimately, regulatory action is needed and the United Nations Oceans Conference (UNOC25) presents a critical opportunity to mobilise government support and action to conserve and sustainably use the ocean.
