How Do Oceans Influence Climate Change?

The ocean has a massive influence on global warming. We explain how it acts as a sponge, soaking up more than 90 percent of the heat produced by global warming. We also look at the three carbon cycle pathways in the ocean that help to mitigate climate change: the solubility pump, the biological pump and the ocean-atmosphere exchange of carbon dioxide (CO2).
Coral Reef with Fish
As the ocean warms, more poisonous stonefish are appearing in Sydney, Australia. Photo: Jayne Jenkins/Coral Reef Bank

Oceans influence climate change by absorbing 90 percent of all the heat and around 30 percent of all the CO2 produced by global warming.

In other words, because of the ocean, we have experienced less than 10 percent of the heat caused by global warming, and only 60 percent of its CO2 emissions.

As far as climate change mitigation is concerned, the ocean and its thousands of marine biomes and ecosystems are absolutely critical. The reason for this, is that the oceans store about 39,000 billion tonnes of carbon – carbon that would otherwise escape into the atmosphere as carbon dioxide (CO2) and raise Earth’s temperature even further. This is 67 times greater than the amount of carbon held in all the world’s plants and forests, and 25 times greater than the carbon held in all the soil. 1 2 3 4

Why Are Oceans Important?

The ocean provides life, biodiversity, a vast number of essential marine species and habitats, and an abundant stock of healthy foods. Because of its huge size it plays a critical role in Earth’s climate system, absorbing excess heat, acting as a buffer against global warming and keeping Earth’s temperature as stable as possible.

In addition to its ‘thermal inertia’ (see panel below), the ocean plays an important role in the carbon cycle, which reduces the impact of climate change even further. It soaks up carbon dioxide (CO2) from the atmosphere and either recycles it reasonably quickly, or else holds onto it for centuries or even millennia.

The oceans influence climate by absorbing our steadily increasing CO2 emissions since the beginning of the Industrial Revolution. If it wasn’t for the ocean, Earth’s climate would be in much more serious trouble. 5 Unfortunately, as global warming continues to intensify, the chemistry of the oceans is changing and, with it, its capacity to protect humanity from rising temperatures across the globe.

Indeed, scientists are concerned that the slowness of the ocean to respond to all the heat it has absorbed, could hide the fact that we may have reached one or more climate tipping points in the ocean without realizing it. As a result, we may be experiencing a runaway process that is only slowly gaining momentum.

How Are Oceans Good For The Environment?

The oceans are one of Planet Earth’s greatest environmental treasures. Believed to be the original source of all planetary life 6 they are estimated to contain between 300 to 500 million species of organisms, representing roughly one fifth of all life forms on Earth.

The oceans are also one of humanity’s largest and most valuable food sources. Seafood, for example, represents one-fifth of animal protein and 5 percent of the total protein in the human diet. Fish provides 1.5 billion people with about 20 percent of their average per capita intake of animal protein, and 3 billion people with roughly 15 percent of animal protein. 7 8

Most of the ocean’s known biological wealth and biodiversity is packed into a relatively narrow strip formed by continental shelves, coastal margins, coral reefs and river estuaries. These contain the largest fishing grounds, which account for more than three quarters of the world’s fishing catch. They also contain some of the world’s most biodiverse habitats and ecosystems. They include coral reefs (upon which 25 percent of the ocean’s fish are dependent at some point in their life cycle, according to NOAA), seagrasses, kelp forests and seaweed beds, as well as mangroves, salt marshes, mudflats and other wetlands that filter and cleanse the water flowing from rivers and streams into the ocean.

Mangroves, salt marshes and seagrass beds are also significant “blue carbon” sinks, noted for capturing more CO2 per acre than continental forests. Oceans influence climate by keeping all that CO2 locked up. 9

What exactly is thermal inertia?

Thermal inertia describes an object’s resistance to a change in temperature. In the case of the oceans, their vast area and massive water content can absorb extremely large amounts of heat, without significant warming. This creates thermal inertia in the Earth’s climate system. So, there’s a time lag between when the Earth receives heat and when the climate fully responds to it. Some people say this buys us time to replace fossil fuels with renewables, thus dealing with the causes of global warming. Climate deniers say that thermal inertia proves the Earth is resistant to climate change. Thermal inertia is another example of how oceans influence climate.

Oceans are a Source of Power

The ocean also happens to be a huge source of energy, in the form of tidal power and wave power. Although still in their infancy, these marine technologies could produce the sustainable energy of the future.

Evaporation and condensation of seawater is already an important source of renewable energy, which is harnessed in rivers in the form of hydropower (hydroelectricity).

Oceans, Heat Absorption And Thermal Inertia

Oceans cover approximately 71 percent of Earth’s surface – an area of 360 million square kilometers (139.5 million sq mi) – and have an average depth of 3.6 kms (2.3 mi). They hold about 1,300,000,000 cubic km (328,000,000 cu mi) of water, roughly 97.5 percent of the world’s total. Because of their size and extent, they have an extraordinary capacity to absorb and store very large amounts of solar heat without an accompanying rise in temperature. This thermal inertia alone, guarantees the ocean a central role in stabilizing Earth’s climate and temperature. 10 Yet another example of how oceans influence climate.

According to the “Special Report on the Ocean and Cryosphere in a Changing Climate” (SROCC), published in September 2019 by the Intergovernmental Panel on Climate Change, it is “virtually certain” that Earth’s oceans have been continuously warming since 1970 and it is “likely” that the rate of warming has doubled since 1993.

The IPCC’s Fifth Assessment Report stated that more than 90 percent of global warming over the past 50 years has occurred in the ocean. Ocean warming in the upper layer down to 700 meters in depth, accounts for just over 60 percent of the increase in stored heat in the climate system (from 1971 to 2010), while warming in the lower ocean adds another 30 percent. 11 According to the latest research, the ocean absorbs 93 percent of the heat of climate change. Thus you can see how oceans influence climate. 12 For more on the impact of rising temperatures on the hydrosphere, please see: Effects of Global Warming on the Oceans.

Not surprisingly, oceanographers are now becoming concerned that the hydrosphere will not be able to continue in its role as “climate buffer” indefinitely. For one thing, heat already stored in the ocean will eventually find its way out into the atmosphere, committing Earth to extra warming in the future. After all, heat is energy and energy doesn’t disappear it simply changes from one form to another.

When solar heat energy enters the ocean, it’s dispersed by wind and currents, that work incessantly to move heat from warmer to cooler latitudes. But eventually the dispersed heat will make its presence felt, either by melting Arctic sea ice, evaporating water as it moves towards the poles, warming the ocean depths or directly warming the atmosphere. In this way, heat entering the ocean can heat the planet for decades afterwards.

How is ocean temperature measured?
To record sea surface temperature (SST) scientists deploy temperature sensors on satellites as well as a wide variety of buoys and other devices, like conductivity-temperature-depth instruments (CTDs) and expendable bathythermographs (XBTs).

Several integrated systems are also used, such as the U.S. Integrated Ocean Observing System (IOOS), NOAA’s Center for Satellite Applications and Research (STAR), and the Global Ocean Observing System. Also used are more than 3,000 robotic floats that measure ocean temperature at different depths around the world.

Called “Argo floats”, these smart sensors drift through the ocean for a pre-set period, before rising to the surface, measuring temperature (and salinity) as they rise. On reaching the surface, they transmit their data and location to researchers, via satellite, and then return to their drifting depth. Researchers process the data and calculate an average ocean temperature every three months.

Sea surface temperatures exert a major influence over regional weather cycles like the El Niño-Southern Oscillation in the equatorial Pacific Ocean, the Indian Ocean Dipole (IOD) between Africa and SE Asia, and the travelling Madden-Julian Oscillation (MJO) which moves over the Indian and Pacific Oceans..

The 3 Pathways Of the Oceanic Carbon Cycle

The ocean plays a vital role in Earth’s carbon cycle, sequestering colossal amounts of carbon dioxide that would otherwise boost the greenhouse effect and raise global temperatures. When carbon dioxide (CO2) is emitted into the atmosphere from the burning of fossil fuels, about half remains in the atmosphere, one quarter is absorbed by plants and trees, while the remaining quarter is absorbed into the ocean. In 2017, for instance, the ocean absorbed a net 2.6 billion tonnes of carbon from fossil fuel emissions – an increase of over 36 percent on the average annual take-up of 1.9 billion tonnes, during the period 2005-2015. 13 14

Before the Industrial Revolution, the ocean released more CO2 than it absorbed so as to offset the absorption of CO2 by terrestrial plants and trees. Today, as a result of man-made emissions of carbon dioxide, this trend has been reversed: oceans now absorb more CO2 than they release.

Once dissolved into the surface waters of the ocean, carbon dioxide follows one of three pathways of the oceanic carbon cycle, whose journey times vary from a few years to several millennia. (1) It may evaporate, as part of the constant gaseous exchange at the ocean surface. (2) It may be downwelled and transported around the depths of the ocean by the so-called ‘solubility pump’. (3) It may be photosynthesized and recycled into the depths by the ‘biological pump’.

Interesting Note: The oceans influence climate by storing more heat in the uppermost 3 meters (10 feet) of water than the entire atmosphere. The key to understanding anthropogenic global warming is inextricably linked to the ocean. 

1. Ocean-Atmosphere Exchange Of CO2

The ocean’s continuous ocean-atmosphere carbon exchange is an ongoing transfer of CO2 between the ocean surface and the air above it, which allows many CO2 molecules that diffuse into surface waters to diffuse back to the atmosphere over a relatively short time scale. The CO2 can flow in either direction, depending on the relative balance or concentration of CO2 in the sea, compared to the air above it. CO2 concentration depends a lot on the temperature. It is low in cold water and high in warm water. So, a natural cycle results in which CO2 is absorbed from the atmosphere in colder oceans and released back to the atmosphere in tropical waters.

How The Ocean Carbon Cycle Works

This back-and-forth transfer of CO2 between the ocean surface and the atmosphere is an extremely important process in Earth’s carbon cycle, since the oceans are such a huge carbon reservoir with the capacity to store (and/or release) very large quantities of CO2. For various reasons – such as differences in salinity and temperature between the various ocean layers – there is comparatively little vertical mixing of ocean water. At present, mixing of the upper layers of the ocean with the next set of layers takes many years, and movement of surface layers to the deep ocean takes centuries. But if this were to change because of global warming, the ocean surface might become so saturated with CO2 that it may be unable to absorb any more from the atmosphere.

What happens when CO2 comes into contact with seawater?

When CO2 enters the ocean, it can become dissolved in the water where it reacts with other chemicals to form a number of products, as follows:

CO2 (dissolved gas) + H2O (ocean water) <=> H2CO3 (carbonic acid)
H2CO3 (carbonic acid) <=> H+ + HCO3- (hydrogen ion + bicarbonate)
HCO3 (bicarbonate) <=> H+ + CO3-2 (hydrogen ion + carbonate)

Bicarbonate (HCO3-) is the predominant form of inorganic carbon. Carbonate (CO32) and dissolved CO2 are important, but secondary. For example, seawater with a pH of 8.2 contains around 0.5 percent CO2, 10.5 percent carbonate and 89 percent bicarbonate. The main point of this, is that carbon chemistry converts some of the dissolved CO2 gas into bicarbonate and carbonate, so the concentration of dissolved CO2 does not increase as much as it would, if none of these reactions occurred. Which is why the oceans can absorb so much atmospheric CO2 without having their own CO2 levels rise very much.

2. The “Solubility Pump” – Recycling of CO2 Via Deep Ocean Currents

The term ‘solubility pump’ actually refers to thermohaline circulation, a system of deep-water ocean currents driven by the principle of water density. Namely, that denser water is heavier than less dense water, and will therefore sink. Sea water density is affected by salinity and temperature. Salinity, for example, is increased by sea ice and evaporation, while precipitation, land ice-melt and rivers reduce it. Temperature is influenced by heating/cooling that takes place at the ocean surface. Generally speaking, cold water with high salinity is the most dense; warm water with a lower salinity is least dense.

A good illustration of the solubility pump in action is the warm surface current known as the Gulf Stream. which belongs to a network of currents known as the Atlantic meridional overturning circulation (AMOC). The warm Gulf Stream carries tropical heat from the Gulf of Mexico northwards to the Arctic. As it moves north, driven by the wind, the warm water undergoes evaporative cooling. (Wind moves over the water which causes evaporation, which cools the water and increases its salt-content and density.) When it comes into proximity with the extremely cold and salty waters surrounding sea ice in the Arctic Circle, the water becomes colder, saltier, and so denser and heavier.

As the water becomes colder it absorbs more and more CO2 from the air above it, as per the cycle described in (1) above. Scientists calculate that the Atlantic Meridional Overturning Circulation (AMOC) sequesters about 700 million tonnes of CO2 from the atmosphere every year. 15

The Global Conveyor Belt

Global Map of Thermohaline Circulation - How Oceans Influence Climate
Map shows the pattern of thermohaline circulation also known as “meridional overturning circulation”. Photo: © NASA

Finally, in the Greenland-Norwegian Sea, and the Labrador Sea, this CO2-rich water becomes so heavy that it sinks down (downwells) through less salty and less dense water, taking the CO2 with it. These downwellings form a body of dense water, known as the North Atlantic Deep Water (NADW), which connects with a global circuit of ocean currents known as the Global Conveyor Belt. This circuit carries water along the sea bottom from the Arctic to the Antarctic. Here, local wind conditions and Ekman transport cause it to upwell in the Southern Ocean, after which it snakes around the Pacific and Indian Oceans, where it upwells and then returns (on the surface) to the Gulf of Mexico.

During its return passage on the surface, through the tropics, a proportion of the sequestered carbon dioxide will be emitted into the atmosphere due to the continuous ocean-atmosphere carbon exchange mechanism. However, a net amount is retained, adding to the 38,000 billion tons stored in the deep ocean. In comparison, only 1,000 billion tonnes of carbon are held in the upper ocean.

Oceanographers estimate that a cubic meter of water takes between 1,000 and 1,600 years to complete the journey along the global conveyor belt. In this way, heat and dissolved carbon dioxide may remain “buried” in the depths of the ocean for millennia, acting as a buffer against the initial effects of climate change. Unfortunately, they may come back to haunt us much later when they emerge from the depths. The oceans influence on climate may yet to be fully witnessed.

This may occur sooner than we think if recent surveys are correct, that the Atlantic Meridional Overturning Circulation (AMOC) is weakening due to climate change, especially since global temperature projections are on course to reach 3 degrees Celsius above pre-industrial levels. In April 2018 two scientific studies published in Nature concluded that the AMOC was at its weakest for 1,600 years. 16 17 If the AMOC were to weaken significantly, it would force a re-evaluation of most of our climate models. 18

3. The “Biological Pump” – Recycling Of Organic CO2 Via Geological Lithification

The so-called ‘biological pump’ describes a distribution method that transports about 12 billion tonnes of inorganic carbon every year from the atmosphere to the deep ocean. By sequestering significant amounts of CO2 over geological time spans, it keeps atmospheric levels of CO2 about 200 ppm (parts per million) lower that they would otherwise be. 19

The term biological pump actually refers to a highly complicated series of interconnected metabolic cycles and food webs that play out in the near surface waters of the ocean, especially in the tropics. So the following summary is a gross oversimplification of what happens. See, for example, our article on the microbial loop, and viral shunt: Marine Microbes Drive the Aquatic Food Web.

Key organisms in the biological pump include:

(a) Phytoplankton – the photosynthesizing primary producers at the base of the marine food web, such as diatoms, coccolithophores, and cyanobacteria (blue-green algae).

(b) Zooplankton – the tiny marine animals that feed on the phytoplankton. They include larval krill, radiolarians, foraminiferans, and dinoflagellates, cnidarians, crustaceans, chordates, and mollusks.

(c) Decomposers that breakdown and remineralize the remains of others, recycling nutrients as they work. The main decomposers include bacteria, fungi, marine worms, echinoderms, crustaceans and mollusks. In colder oceans, only bacteria and fungi are able to survive the extreme temperatures. The tiniest include femtoplankton (viruses) (up to 0.2 micron; 1 micron = one millionth of a meter), picoplankton (0.2-2 micron), nanoplankton (2-20 micron) and microplankton.

These marine organisms form part of the “microbial loop”, the highly active, interconnected base of the food web, that creates organic matter from sunlight thus providing the nutritional foundation for all creatures in the marine food chain.

The process starts when CO2 from the atmosphere dissolves in seawater and is absorbed during photosynthesis (in the sunlit shallows) by phytoplankton. The phytoplankton also take in calcium (derived from the chemical weathering of carbonate or calcium silicate rocks) and bicarbonate (from CO2 and hydrogen) from the surrounding sea, to produce calcium carbonate (CaCO3) – the main component of their shells, and the building blocks for the skeletons and shells of many other marine organisms. For example, creatures such as coral, lobsters, oysters and sea urchins use calcium carbonate to build their shells, plates and inner skeletons.

The chemical equation for this calcification process is as follows:

Ca2 (calcium) + 2HCO3 (bicarbonate) -> CaCO3 (calcium carbonate) + H2O (water) + CO2 (released into air)

Once the phytoplankton have converted the atmospheric carbon into organic tissue and shells, they are either eaten by zooplankton or other sea creatures – in which case the carbon passes up the food chain – or else they die, whereupon their body parts sink into the depths.

On the way down, most of the carbon either dissolves into the surrounding water or is consumed by fish and other creatures, whose carbon remains are themselves dissolved or recycled. The dissolved carbon is eventually returned to the surface via ocean currents – although it may take thousands of years to arrive.

However, a small but significant percentage of the sinking organic material does find its way to the bottom of the ocean, where it becomes incorporated into the thick mud-like layer of sediment that covers the ocean floor. 20 This thick layer of sediment grows about six meters every million years. 21 According to figures compiled by the Deep Carbon Observatory, ocean sediments contain about 3,000 billion tonnes of carbon. 22

In due course, after millions of years, a combination of heat and pressure turns this layer of shells and sediment into sedimentary rock, in a process known as lithification. The carbon is recycled from the lithosphere back into the atmosphere through a process of out-gassing from hotspot volcanoes, midocean ridges, and subduction-related volcanic arcs. Scientists calculate that these out-gassings to the atmosphere and oceans account for between 280 and 360 million tonnes per annum. 22

Interesting Note: As the oceans influence climate and life on planet Earth in so many ways, we must try to safeguard this vital resource. This means conserving coastal carbon ecosystems under the reducing emissions from deforestation and forest degradation (REDD+) mechanism – as well as ensuring all countries stick to their agreed targets under the Paris Climate Agreement.

Conclusion: Oceans Exert A Moderating Influence on Climate and Global Warming

(A) Oceans absorb more than 90 percent of the excess heat from the atmosphere. (B) They also downwell considerable amounts of carbon dioxide into their depths, via the thermohaline circulation, ensuring that it remains out of the atmosphere for centuries. (C) The ocean also facilitates the lithification of sediments containing the calcified remains of crustaceans, made from the carbon dioxide ingested by photosynthsizing phytoplankton. By these three actions alone, the oceans exert a considerable moderating influence on climate and global warming across the planet.

References

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  20. Exactly how much carbon makes it to the ocean floor is unclear. For example, compare the article text with this sentence from the American Chemical Society (ACS). “Like inorganic carbon, most organic carbon ultimately ends up in sediments on the ocean floor.” Ocean Chemistry. ACS Climate Science Toolkit. American Chemical Society. acs.org []
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