The oceans, guarantors of the global climatic balance, are under serious threat from global warming. The UNESCO 2024 report reveals some worrying data: in twenty years, the rate at which the oceans are warming has doubled. Associated phenomena are affecting marine ecosystems, exacerbating the loss of biodiversity and threatening coastal populations. Urgent, coordinated action is needed to protect these vital resources in the face of the climate crisis.
by Laurie Henry
UNESCO’s 2024 report on the state of the oceans, the fruit of the work of more than 100 scientists from 28 countries, confirms an alarming situation. Ocean temperatures are rising at an unprecedented rate, threatening not only marine ecosystems but also the global climate balance. Rising sea levels, increasing acidification and decreasing dissolved oxygen are affecting marine species and human populations, and hampering conservation efforts. All these factors call for urgent action.
Accelerated warming of the oceans
Over the last two decades, the temperature of the oceans has increased at an accelerated rate, doubling compared to previous years. This doubling does not mean that the temperature of the oceans has doubled, but that the rise in temperature is occurring twice as fast. In practical terms, this means that the ocean is absorbing and retaining the excess heat generated by human activities at an increasingly rapid rate.
According to the report, the oceans have absorbed almost 90% of the excess heat. This figure is the result of the gradual accumulation of heat from the industrial revolution, around 150 years ago, when greenhouse gas emissions began to increase significantly due to the burning of fossil fuels. The ocean continues to absorb this excess heat year after year.
Global average heat content of the oceans from 1960 to 2023 for the first 2,000 m of depth © UNESCO
Warming water expands, resulting in an increase in ocean volume. This thermal expansion is responsible for around 40% of the global rise in sea levels, a figure that corresponds to an increase of 9 cm in thirty years. This figure may seem modest, but this rise has also occurred at twice the rate previously observed, with increased risks for coastal regions and their populations.
Some ocean areas are particularly hard hit. The waters of the tropical Atlantic, the Mediterranean and the Southern Ocean are experiencing temperature rises in excess of 2°C above pre-industrial levels. This situation is all the more worrying in that marine ecosystems, already weakened, are struggling to adapt to these rapid changes. Warming waters are not the only consequence of rising sea levels, as biodiversity in these regions is under threat and extreme weather events are becoming more frequent. For Vidar Helgesen, Executive Secretary of UNESCO’s Oceanographic Commission, ‘ the oceanic crisis is progressing faster than our knowledge ’, underlining the urgent need to speed up research to better understand these phenomena and anticipate their impact.
Acidification and oxygen loss, two major risks for coastal species
Another issue highlighted by the report is ocean acidification. This poses a growing threat to marine ecosystems, particularly coastal species. By absorbing between 25% and 30% of fossilCO2 emissions, the acidity of the oceans is increasing at an alarming rate. Since the beginning of the industrial era, this acidity has risen by 30%, a phenomenon that could reach a 170% increase by 2100 if current trends continue, according to the UNESCO report. These chemical changes particularly affect young marine species, whose fragile shells and skeletons are unable to form properly in acidic environments, leading to high mortality rates. Coastal waters, where these changes are most pronounced, are therefore particularly hard hit.
Variations in mean annual pH values from a series of representative sampling stations in open waters (top) and coastal waters (bottom) © UNESCO
At the same time, this situation is exacerbated by a loss of oxygen in the oceans, which has fallen by around 2% since the 1960s. This decline is mainly due to rising temperatures and pollution from human activities (such as agricultural waste and sewage). These phenomena are creating ‘dead zones’ – ocean regions where there is insufficient oxygen to sustain marine life. According to Professor Steve Widdicombe, an expert in ocean acidification from the University of Plymouth, ‘ these phenomena are all the more worrying given that it is in these coastal areas that the majority of oceanic biodiversity is found ’. He also points out that these regions remain crucial for the interaction between man and the ocean, making this crisis even more urgent to resolve.
The resilience of marine ecosystems in question
Despite the challenges they face, marine ecosystems play an essential role in mitigating global warming. Marine forests, such as mangroves, seagrass beds and salt marshes, are proving to be highly effective carbon sinks. They can store up to five times more carbon than terrestrial forests, by trapping it in their roots and surrounding sediments. This role is all the more vital as these ecosystems also protect coastlines against erosion and flooding.
However, despite their importance, almost 60% of countries do not include the restoration and conservation of these marine forests in their Nationally Determined Contributions (NDCs), which are vital if they are to meet the climate commitments made in the Paris Agreement.
Regional sea level trends (mm/year) from January 1993 to June 2023, based on satellite altimetry. The colour transition from green to yellow corresponds to the rise in global mean sea level © Copernicus Climate Change Service altimetry data
Marine protected areas (MPAs) are also essential to the resilience of marine ecosystems. According to UNESCO, 72% of endangered marine species, as identified by the IUCN Red List, are found in these protected areas. The stricter the level of protection within these MPAs, the more effective they are at restoring and preserving local biodiversity. However, the management of these areas remains uneven on a global scale. Although some MPAs are well managed, others lack sufficient resources or regulations to fully protect ecosystems. The report highlights the need to improve scientific knowledge and management capacity to ensure the sustainability of these ecosystems in the face of the growing pressures of climate change and human activities.
Act now
Finally, the report highlights the critical role of GEBCO’s Seabed 2030 project, which aims to map the entire seabed by 2030. Since the project was launched, some 90 million km² of bathymetric data have been collected, but 75% of the oceans have yet to be mapped. The report states that technological advances, such as the use of unmanned surface vehicles (USVs), will help to fill these gaps.
Mapping the ocean floor will enable us to understand the interactions between ocean currents and ice caps, which influence the rise in sea levels. This will improve our ability to predict the impact of global warming on marine ecosystems and help us to manage ocean resources. But despite some progress, the report stresses the need to speed up data collection efforts, particularly in the polar regions, which are still under-mapped.
The publication of the UNESCO 2024 report therefore calls for an immediate mobilisation of governments, industries and researchers to halt the rapid degradation of the oceans. ‘ We know enough to act ’, insists Vidar Helgesen, underlining the urgency of deploying scientific and political solutions.
The actions taken today will be decisive for the survival of marine ecosystems and for the future of generations to come. The ocean, as a climate regulator, is at the heart of the fight against climate change. More active protection, based on reliable and continuous data, is essential to ensure the resilience of this precious ecosystem.