The ocean is constantly exchanging with the atmosphere. It stores and distributes large amounts of heat around the globe via ocean currents. The ocean plays a key role for the global climate. It controls how heat, carbon, nutrients and dissolved gases are exchanged between the upper and lower layers of the ocean. However, this regulatory mechanism is presently disturbed by global warming, consequence of the greenhouse effect. It is important to raise awareness of tipping points in the Earth system under human-induced climate change.
Ocean heatwaves have led to mass coral bleaching and to the loss of half of the shallow-water corals on Australia’s Great Barrier Reef. A staggering 99% of tropical corals are projected to be lost if global average temperature rises by 2 °C, owing to interactions between warming, ocean acidification and pollution. As well as undermining our life-support system, biosphere tipping points can trigger abrupt carbon release back to the atmosphere.
The ocean is dynamic in nature, playing a crucial role as a planetary thermostat that buffer global warming. Global warming is causing massive amounts of fresh water to flush into the upper ocean from melting ice sheets and glaciers, lowering the salinity of the upper layer and further reducing its density. This increasing contrast between the density of the ocean layers makes mixing harder, so oxygen, heat and carbon are all less able to penetrate to the deep seas and disrupting the normal cycle that carries warm, salty surface water northwards from the equator and sends low-salinity deep water back southwards.
Ocean circulation can be conceptually divided into two main components: a fast and energetic wind-driven surface circulation, and a slow and large density-driven circulation which dominates the deep sea. The slow and deep circulation is largely driven by water density, and thus its temperature and salinity. It acts on the ocean as a whole and has a major influence on the abyssal properties where wind-driven circulation has no effect.
Ocean currents redistribute the absorbed solar energy. Ocean circulation is controlled by surface winds, by the rotation of the earth and by certain physical properties such as temperature and salinity. Warm water masses carry surface heat accumulated in the tropics near the poles, thus reducing latitudinal temperature differences. The collapse of the global ocean overturning circulation, or the rapid partial disintegration of the West Antarctic Ice Sheet exemplifies low-probability high-impact events with severe consequences for the Earth system.
The two main mechanisms transfer the carbon dioxide (CO2) from the atmosphere to the ocean. The most important phenomenon is physical: about nine-tenths of atmospheric carbon dioxide is transferred to the ocean by simple dissolution of the gas into surface seawater which is then transported by ocean currents to the deep layers of the ocean. Coldwater currents at great depths follow the opposite direction. This global conveyor belt circulation contributes, with constant exchanges to and from the atmosphere, to the redistribution of heat across the planet.
Circulation changes would cause alterations in Earth’s heat budget, while the ice sheet instability would cause a sea level rise of several meters.
The second mechanism, which represents 10% of the accumulation of carbon in the oceans, is biological: phytoplankton, suspended in the sunlit ocean surface layer, takes part in the carbon cycle by producing organic matter via photosynthesis. This plankton is considered as a lung for the planet, comparable to forests on land: it absorbs CO2 and produces oxygen (O2). Over geological timescales, photosynthesis has led to the oxygenation of our atmosphere. The role of plankton biodiversity in climate regulation therefore represents one of the major issues for the global climate.
In addition, Deep Sea ecosystems capture huge quantities of carbon. For instance, on the continental shelf, microorganisms play a major role in sustainable storage of carbon produced by phytoplankton but are also filters for methane formed by this fossilized matter. By using methane as energy, these microorganisms transform this greenhouse gas, which is much more powerful than CO2, into minerals. This process prevents greenhouse gases from resurfacing and accelerating climate change. The biological pump is sensitive to disturbances and relies on ecosystems’ good health. In the high seas the planktonic ecosystem is a major player. All organic materials that reach the bottom participate in the biological pump. The barrier layer separating the ocean surface and the deep layers had strengthened world-wide – measured by the contrast in density – at a much larger rate than previously thought. Winds strengthened by climate change had also acted to deepen the ocean surface layer by five to 10 metres per decade over the last half century. A significant number of marine animals live in this surface layer, with a food web that is reliant on phytoplankton. But as the winds increase, the phytoplankton are churned deeper, away from the light that helps them grow, potentially disrupting the broader food web.
Seawater density increases with depth too, because the sunlight that warms the ocean is absorbed at the surface, whereas the deep ocean is full of cold water. The change in density with depth is referred to by oceanographers as stability. The faster density increases with depth, the more stable the ocean is said to be. The surface mixed layer occupies the upper (roughly) 100 meters of the ocean and is where heat, freshwater, carbon and dissolved gases are exchanged with the atmosphere. Turbulence whipped up by the wind and waves at the sea surface mixes all the water together.
The lowest layer is called the abyss, which extends from a few hundred meters depth to the seafloor. It is cold and dark, with weak currents slowly circulating water around the planet that remains isolated from the surface for decades or even centuries.
Dividing the abyss and the surface mixed layer is something called the pycnocline. It is invisible and flexible, but it stops water moving through it. When this divide is ripped into shreds, which happens in the ocean when turbulence effectively pulls the pycnocline apart, water can leak through in both directions. But as global temperatures rise and the ocean’s surface layer absorbs more heat, the pycnocline is becoming more stable, making it harder for water at the ocean’s surface and in the abyss to mix.
If a stabler pycnocline traps more heat in the surface of the ocean, it could disrupt how effectively the ocean absorbs excess heat and pile pressure on sensitive shallow-water ecosystems like coral reefs and this increasing stability causes a nutrient drought; just as the ocean surface contains heat that must be mixed downwards, the abyss contains an enormous reservoir of nutrients that need to be mixed upwards. Eroded seabed rocks are providing an essential source of nutrition for drifting marine organisms at the base of the food chain. Iron — an essential nutrient for microscopic marine algae, or phytoplankton — is being released from sediments on the deep ocean floor. As mentioned the building blocks of most marine ecosystems are phytoplankton: the microscopic algae which use photosynthesis to make their own food and absorb vast quantities of CO2 from the atmosphere, as well as produce most of the world’s oxygen.
Phytoplankton can only grow when there is enough light and nutrients. During spring, sunshine, longer days and lighter winds allow a seasonal pycnocline to form near the surface. Any available nutrients trapped above this pycnocline are quickly used up by the phytoplankton as they grow in what is called the spring bloom.
For phytoplankton at the surface to keep growing, the nutrients from the abyss must cross the pycnocline. And so another problem emerges. If phytoplankton are starved of nutrients thanks to a strengthened pycnocline then there’s less food for the vast majority of ocean life, starting with the tiny microscopic animals which eat the algae and the small fish which eat them, and moving all the way up the food chain to sharks and whales. Just as the ocean is less effective at shifting heat into the deep sea and regulating the climate, it’s also worse at sustaining the vibrant food webs at the sunlit surface which society depends on for nourishment.
The ocean plays a major role in climate regulation as it acts as a carbon pump and source of oxygen thanks to plankton. Nonetheless this pump is increasingly affected by global climate change, which raises questions and concerns. The global ocean therefore plays a role in the regulation and control of the large natural planetary balances. How vulnerable is this sophisticated climate machine?
Human-caused environmental changes can materialize very rapidly, or abruptly, typically at rates much faster than sustained natural changes of the past. Such changes are already ongoing and documented for ocean warming, for acidification, and, to a certain degree, also for deoxygenation. Super imposed on fast changes in these ocean state variables are extreme events, such as heat waves, coastal hypoxia, and ocean acidification events linked, for example, to strong upwelling episodes. Since these developments are likely to aggravate over this century, they are important, as are the extraordinary abrupt singular events.
The intervention time left to prevent tipping points could already have shrunk towards zero, whereas the reaction time to achieve net zero emissions is 30 years at best. Hence we might already have lost control of whether tipping happens. The stability and resilience of our planet is in peril, should we be worried?