Rising temperatures, acidification, deoxygenation… the effects of climate change are putting ocean biodiversity at risk.
For decades, the ocean has absorbed most of the excess heat that the Earth has accumulated due to the increase in greenhouse gas emissions, and nearly a third of the excess carbon dioxide (CO2) emitted by human activities. However, despite its vastness, this has not been without effect on its dynamics and the ecosystems it hosts. In 2019, the Intergovernmental Panel on Climate Change (IPCC) already warned about the impact of the current climate change on the ocean and the need to act immediately, as the consequences for coastlines, the climate itself, marine biodiversity, and the populations dependent on it could be drastic and long-lasting due to its inertia. Around the world, biologists, oceanologists, and ecologists are investigating how marine life is reacting to climate change. Sylvie Dufour and Marina Morini, in particular, study the temperature sensors of marine animals and their evolution, which is crucial for understanding how they respond to ocean warming.
The five oceans on current maps (Atlantic, Arctic, Southern, Pacific, Indian) actually form one single body of water due to their interconnection through the great thermohaline circulation. However, they host a diversity of habitats and species, from the poles to the tropics, from the coasts to the open sea, and from the surface to the deep waters. It is in the ocean that life first appeared. The oldest known fossils of marine origin date back about 3.7 billion years: these are stromatolites, structures built by cyanobacteria. These bacteria were also the first organisms capable of photosynthesis, meaning they could split carbon dioxide (CO2) and produce oxygen (O2). This gas accumulated in the ocean and atmosphere, leading to a change in the Earth’s atmosphere conducive to the great diversification of life.
It is still in the ocean that the great groups of life diversified for more than 3 billion years before the first life forms left the water to conquer the continents about 500 million years ago. Some groups remained tied to the marine world, such as echinoderms (sea urchins, starfish, sea cucumbers), while others returned to the sea, like certain dinosaurs, turtles, and mammals.
Today, the ocean hosts remarkable biodiversity, from extraordinary microorganisms like giant viruses to the largest animals in the world (the blue whale measures between 20 and 25 meters on average), or the oldest (the Greenland shark can live for 400 years). This wealth remains largely unknown, especially in the deep ocean. Since prehistoric times, marine ecosystems have provided numerous benefits to human societies. However, multiple pressures are now being placed on the ocean due to the intensification of human activities in recent decades. Overfishing, unsustainable aquaculture, coastal urbanization, the intensification of maritime trade, and various forms of pollution are destroying marine ecosystems. Equally worrying, although less tangible than on land, the current climate change, caused by the drastic increase in greenhouse gas emissions, particularly CO2, from human activities, poses a threat to many marine species, the extent of which is only now becoming apparent.
Ectothermic Species
To date, the ocean has absorbed nearly 90% of the excess heat induced by greenhouse gases. This has largely protected us by limiting atmospheric warming. However, this has consequences for its dynamics and the biodiversity it supports. The ocean has warmed at an average rate of 0.11°C per decade in its surface waters, but also at greater depths. Unlike homeothermic species that regulate their temperature, and except for some that have returned to the sea, such as marine birds and mammals, marine animals are ectothermic: they do not regulate their temperature. Therefore, rising temperatures directly affect them, whether they are corals, mollusks, crustaceans, fish, or turtles. Temperature also impacts phytoplankton—the microalgae at the base of marine food webs, i.e., the entire network of oceanic food chains. Temperature affects the functioning of these organisms at all levels, from molecular and cellular activities to regulating life cycles—development, growth, reproduction. These disturbances affect populations and species interactions, disrupting food webs and ecosystems.
A dramatic and widely publicized consequence of warming is coral bleaching, which reflects the loss of their symbiotic algae, essential to their metabolism, leading to their death. It is estimated that half of the Great Barrier Reef along Australia’s coast has been lost in the past 30 years. Given the vital role of coral reefs as habitats for a wide variety of species, including tropical fish that feed local societies, the cascading consequences for ecosystems and the societies that depend on them are significant. Coral reefs also provide protection against strong waves and even tsunamis, which is increasingly important as extreme weather events intensify.
Warming also affects cold-water species, as seen in the striking example of the snow crab. This iconic animal of the Bering Sea was the subject of significant fishing activity. However, a collapse in its population has been observed, with more than 10 billion individuals disappearing since 2018. In 2023, researchers from the Alaska Fisheries Science Center showed that this collapse was linked to a severe heatwave that struck the eastern Bering Sea in 2018 and 2019.
The warming of the water also changes the distribution of species. Just as on land, where butterfly or bird populations are gradually migrating north, marine species are moving toward colder waters, either further north or deeper. For example, in Brittany, species like the royal bream, previously found mainly in the Mediterranean and on the Atlantic coast, are now seen, as well as an increasing number of tropical species in the Mediterranean, coming from the Red Sea via the Suez Canal or from the Atlantic via the Strait of Gibraltar, like barracuda and lionfish.
While the arrival of new species in the waters of the English Channel and the Baltic Sea represents a new resource for fisheries in northern countries, the decline of biodiversity in the tropics is one of the multiple threats posed by global changes to societies in the « Global South, » whose food security, economic activities, and cultural practices depend heavily on this biodiversity. For example, the migration of West African fish species, such as the round sardinella, a major source of animal protein and Senegal’s national dish, is now a rare sight on Senegal’s coasts.
Current studies on numerous species indicate that marine species are migrating toward the poles much faster than terrestrial species. In 2013, a vast meta-analysis conducted by an international team estimated the average speed at which marine organisms are moving toward colder waters at around 50 kilometers per decade, with significant disparities depending on the species, with phytoplankton moving faster than pelagic fish, which in turn are faster than benthic fauna (marine bottom dwellers). These disparities themselves cause disturbances in food webs that amplify the cascading consequences of ocean warming.
More Acidic, Less Oxygenated
Furthermore, the impact of current climate change on the ocean has other effects on its biodiversity. In addition to excess heat, the ocean absorbs more than 30% of the excess CO2 emitted. This phenomenon is not trivial, as it captures CO2 in two ways: first, through planktonic photosynthesis, the foundation of oceanic food webs, and second, through the direct dissolution of CO2, which leads to acidification. This acidification alters the formation of carbonate skeletons in species like corals and mollusks (abalone, oysters…), and also disrupts various physiological functions such as ionic regulation in fish, which increases their energy expenditure, digestion in sea urchin larvae, and even the sense of smell in sharks.
Global warming, along with pollution, is also a cause of ocean deoxygenation. It is estimated that the ocean lost 2% of its oxygen between 1960 and 2010, a vital threat to marine biodiversity that contributes to changes in species distribution and the mortality of certain species, such as fish, while favoring others, such as jellyfish, which are less sensitive, as seen in the Mediterranean.
Finally, a major consequence of climate change is the rise in sea levels, which has increased by more than 20 centimeters on average over the past century, due to thermal expansion and the melting of glaciers. The effects are significant on coastal ecosystems and human societies in coastal areas, from small Pacific islands already facing submersion, like Tuvalu, to coastal megacities across all continents, with the striking example of Indonesia, which has begun relocating its capital, Jakarta, to the island of Borneo.
Source: Pour La Science
