Metafact Review: Oceans
Our planet is a contradiction: mostly covered in water, yet we call it Earth. Our lives and livelihoods depend on the oceans, which make up 97% of the water on our planet. The uppermost layer of the ocean, known as the photic zone, produces oxygen through photosynthesis. Oceans also regulate our climate and serve as carbon sinks. Over three billion people rely on the oceans for their livelihoods and nearly 40% of the world's population lives near the coast.
The oceans are critical for our planet and for ourselves, so we asked experts all about them. How much of the ocean remains unmapped? A documentary claimed the ocean will be empty of fish by 2048 - is that true? Is sea level rising globally? Will they soak up nearly all of our carbon pollution and become acidic? Here’s what we found..
1,386,000,000: Volume of water on, in and above the Earth in cubic kilometres.
35: Grams of salt per litre of sea water.
1,634,701: Coastline kilometres in the world.
59%: Percentage of whales examined with plastic in their bodies.
50-75 trillion: Pieces of plastic in the ocean in 2022.
20-30%: Percentage of human-induced CO2 absorbed by the ocean since the 80s.
90%: Percentage of global trade enabled by the oceanic commercial routes.
1.5 trillion: Dollars contributed to the world’s economy by ocean-related activities.
Living in the ocean depths
A 2011 study estimated that oceans are home to around 2.2 million species. However, only 9% of them have been actually discovered. You might not be shocked by that, considering the average depth of the oceans is about 4 km and half of the world's oceans are over 3,000 metres deep.
Ocean areas situated 1,000 to 4,000 metres below the surface are called bathypelagic zones. Unsurprisingly, life there is rather extreme. The lack of sunlight, low water temperatures, high atmospheric pressures and absence of vegetation force these organisms to adapt in very specific manners.
Such extreme conditions make it hard for us when it comes to studying these creatures as they are hostile to exploration projects. Yet, there is a bizarre, almost alien-looking fish you have probably come across before - the so-called horned lantern fish or prickly seadevil (Centrophryne spinulosa).
Only 25 specimens of this fish have been captured, mostly around 2,000 metres below the surface. You would think these fish go about their life happily unaware of what happens up in the surface. Unfortunately, that is not quite true. The climate pattern known as El Niño, which triggers high water temperature fluctuations, largely affects their ecosystem and results in their death. In addition, despite living in the ocean depths, human activities do affect them. Horned lantern fish are particularly sensitive to water pH, so acidification of the ocean may also endanger them. Just how much, we are yet to see.
Is most of the ocean unmapped and unexplored?
Many of us have heard that, despite the oceans covering such a large area, we know far less of what’s under the surface than we do about what’s on the highest mountain peaks. Historically, the oceans were largely seen as a means to get to new places to explore, and (with a few exceptions) few looked below the water. With today’s technologies, satellite information means a fairly accurate map of mount Everest is just a few clicks away. And yet, the ocean remains as a big blue blob. Is most of it still waiting to be mapped?
“We can have a pretty precise representation of the ocean surface via satelital images and remote sensing (LIDAR for instance),” writes Rodrigo Garcia del Campo from Spain’s National Biotechnology Centre. However, he admits that the deeper layers of the ocean are much harder to map. Megan Cook, from the Ocean Exploration Trust, agrees that little has been visually surveyed. According to the GEBCO Seabed 2030 project, just a fifth of oceans deeper than 200 metres are mapped in high-resolution to date. For Cook, this is largely to do with scale as “the average ocean depth is around 3.9 km or almost 13,000 feet.”
Just how much of the ocean has not been mapped? Up to 80%, according to figures from the National Oceanic and Atmospheric Administration (NOAA). “This is explained by the challenging environmental conditions (particularly high pressures at depth) and tremendous costs associated with the exploration of the deeper oceanic realm,” explains Dr Martine Lizotte from Université Laval.
Progress is being made, however. Modern technologies are helping us to map the seafloor. These include sonar, remotely operated or autonomous vehicles, says Dr Lizotte. The international ARGO program has also contributed. It consists “of a fleet of robotic instruments that drift with the ocean currents and move up and down between the surface and the first 2000 m of the water column,” she explains. “Oceans represent the great frontier for the next generation of explorers and researchers, where vast opportunities for inquiry and investigation remain,” concludes Dr Lizotte.
Is world-wide sea level rise occurring?
Sea level rise is one of the events most commonly linked to the climate crisis. The increase in temperatures will melt glaciers and ice sheets, and water will start to take over areas not previously submerged. Is sea level rise already occurring, however? And if so, just how dangerous can it be?
All our experts agree: sea rising is happening. Prof Marco Marani from Duke University refers to The Intergovernmental Panel on Climate Change, which is a“panel of hundreds of scientists put together by the United Nations and the World Meteorological Organisation to which about 195 countries are contributing.” Their analysis in this report from 2013 already showed with “high confidence” that sea levels are rising. Dr Mark Schuerch from University of Lincoln mentions the existence of “a range of different global datasets that evidence global sea level rise.” Prof Neil Saintilan from Macquarie University dissects the evidence for us as he explains that “there are two sources of evidence which independently concur that sea-level has been rising.” One of them is tide gauges, which, “in tectonically stable coastlines (like Australia) [...] show a consistent increase in sea-level.” The other one is satellite altimeters, “which continuously assess the distance between the ocean surface and the satellite [...] and provide a global picture of sea-level rise.”
There’s more. Our experts warn that this process is speeding up. Indeed, Prof Frank Pattyn from Université Libre de Bruxelles explains that “sea level has been rising continuously at a rate of approximately 2.2 mm/year during the 20th century,” but this rate has “accelerated to 3.4 mm/year during the last decades.” According to Prof Pattyn, this rise was previously caused mostly by thermal expansion of the oceans, whereby “ocean waters getting less dense due to surface warming, hence increasing in volume.” More recently, however, sea level rise is more influenced “by the loss of glacier ice, melting of the Greenland ice sheet and mass loss from the Antarctic ice sheet,” he explains.
If you were wondering if this is dangerous, the answer is again a unanimous yes. Dr Martine Lizotte reminds us that “230 million people currently live by the coast, less than 1 m above the sea.” For these communities, sea rise will increase their vulnerability to “flooding, storm surges, and contamination by salty waters of underground stores of fresh water for drinking and agricultural crops,” she warns.
Will the oceans absorb 80% of all human carbon dioxide emissions from the atmosphere?
Trees function as carbon sinks by absorbing atmospheric CO2. But there is another natural carbon sink - you guessed right - oceans. About one-third of the carbon pollution from humans is absorbed by the oceans every year. We asked our experts to what extent we could count on this over the longer-term. Is it realistic to expect oceans to mop up 80% of all the CO2 we produce, as claimed by some studies?
Although there is no discussion among experts that oceans do absorb CO2, the volume they absorb and the timescale of it all remain uncertain. Prof Katsumi Matsumoto from the University of Minnesota clarifies that, eventually, most CO2 “will end up in the ocean, because it is by far the largest carbon reservoir.” Indeed, he explains “the ocean holds 50-60 times more carbon than the atmosphere.” Experts leave no room for complacency, however. Prof Matsumoto warns that “it takes time for the oceans to mop up the excess carbon humans are emitting.” A similar view is shared by Dr Martine Lizotte, who claims that “50-80% of the cumulative anthropogenic CO2 emissions” will eventually “reach deeper layers of the ocean over several hundreds of years.”
This slow timescale worries experts, as oceans will struggle to absorb CO2 at the rate we are pumping it into the atmosphere. “The rest of the CO2 would remain in the atmosphere generating a very serious greenhouse effect problem (worse than the one we already have...),” warns Rodrigo Garcia del Campo.
In this line, Dr Lizotte points out that the natural equilibrium where oceans will be able to absorb most of the CO2 in the atmosphere will only be achieved if human-derived emissions cease. Otherwise, she explains that absorption of CO2 by the oceans - which have already removed around 40% of the CO2 in the atmosphere - may get impaired as global warming increases ocean temperature. She goes on to explain that this will be due to “warmer waters decreasing the solubility of CO2“, and to a slowing down of “the transfer of CO2 from the ocean surface deeper into the ocean depths.” Finally, Dr Lizotte reminds us that ocean warming also alters “the transport of nutrients from deeper waters to the surface.” This affects the “photosynthetic organisms that participate in the capture of carbon,” so it could not just have knock-on effects on the ecosystem but also further impair the ability of marine microorganisms to absorb CO2.
Will the oceans become acidic in the future?
You have probably heard that the climate crisis will mean three things for the oceans - sea level rise (we just spoke about this), warmer water, and acidification. The first two are relatively intuitive, but acidification may be less so. Are our oceans becoming acidic, to what extent and should we worry?
There's an apparent split among experts, but the science is clear. The debate seems to come down to vocabulary, instead. “The oceans are definitely acidifying currently and will continue to do so into the future for decades to centuries,” writes Dr Robert Letscher from University of California, Irvine. Yet, “this acidification will not push the ocean pH below a value of 7 which is needed for a solution to be classified as acidic,” he argues.
“Today the surface oceans average pH is ~8.1, making them alkaline,” according to our very own Dr Ben McNeil, a climate expert from UNSW Sydney. However, CO2 does lower the pH when dissolved in water. “CO2 combined with water becomes an acid, carbonic acid. So long as CO2 will continue to be emitted, the oceans will become more acidic” writes Dr Katsumi Matsumoto from the University of Minnesota. This is due to the exchange of gases between the air and the sea, he explains.
“It is not as if the ocean is going to turn into a churning vat of acid,” in the words of Prof Steven Campana from the University of Iceland. However, “it will become acidic enough to disrupt the calcification of crustaceans and shellfish. And that by itself could cause huge problems in the marine ecosystem,” he explains. “The pH of the ocean surface has decreased by about 0.1 over the past century and will continue into the future,” says Dr McNeil. The problem is, even small changes in pH increase seawater acidity. “In our body even slight changes (~0.1) to our blood pH can cause serious health problems. For the oceans, these changes (0.1-0.4) in pH are likely to cause problems for some marine organisms and coral reefs,” he predicts.
Will the oceans be empty of fish by 2048? Overfishing is a serious issue, but no, oceans will not be empty by the year 2048. This claim, echoed by the famous documentary Seaspiracy, traces back to a 2006 study, but many experts have since criticised that work for its methodological flaws, its misleading benchmarks and other incorrect premises.
Are current pharmaceuticals levels in natural waterways dangerous? Drugs have been detected in aquatic environments. They are at small concentrations so they are harmless for human beings and unlikely to kill aquatic animals, but they do affect the ecosystem. Even once watered down, drugs may impact the ecosystem’s food supply, change animal behaviours or disrupt processes like photosynthesis.
Is half of the world's oxygen produced via marine phytoplankton? Yes. These results were presented by a study in 1998, which used satellite measurements to work out that 45% of our oxygen is made by plankton. Technically, all oxygen on Earth came from plankton at some point, as marine single-cell organisms were the first ones to ever make oxygen.
Is there evidence that climate change is impacting the Great Barrier Reef? Absolutely. Rising temperatures affect corals, including Australia's Great Barrier Reef. Warmer water stresses corals as the symbiosis of different organisms upon which they rely on starts to break down. Bleaching events in recent years have wiped out most of the corals globally.
Could the frequency of hurricanes double or triple in the coming decades from climate warming? Not exactly. The overall frequency is unlikely to change, say our experts. Still, hurricanes and tropical cyclones are predicted to become more intense (and a higher fraction of them will be intense). So if you count just events that are stronger than a certain intensity or duration, then yes, those will probably get more frequent.
And do hurricanes affect sea life? Yes. Large hurricanes create currents that can affect marine life almost 100 metres under the surface. These currents can also change the water’s salinity and temperature, which also has knock-on effects on marine life. Closer to the coast, these currents make the water go muddy, which prevents sunlight from reaching photosynthetic marine creatures.
Can human activity create dead zones? Dead zones are marine areas that have too little oxygen to support marine life (<2 mg/L, if you’re curious). Although this can occur naturally, dumping of wastewater treatment products and spillover crop fertilisers trigger overgrowth of algae. The decomposition of these algae by microbes consumes oxygen, creating dead zones where marine life can no longer thrive. On top of this, warming of the ocean also contributes to this process by lowering the oxygen levels in the water.
Are off-shore wind farms dangerous to marine wildlife?
Humans see offshore wind farms as tall structures reaching skyward with slowly-spinning blades. It is these moving blades that have the potential to harm seabirds flying through the area, since the birds may misjudge the speed at which the blades move, and thus can be accidentally struck.
In most cases, the number of birds so affected is quite small, but if a wind farm were placed on a route normally transited by seabirds as they forage, then the affected numbers would increase. But for underwater marine life, the situation is very different.
Fish and other aquatic organisms see only the underwater supporting structure, which is embedded in the seabed and stretches to the water surface in a frame-like structure. This supporting structure does not move, and thus is easily avoided. Perhaps more importantly, the structure provides new surfaces and habitats for colonizing organisms, both for sessile invertebrates like barnacles and corals, and for crevice-dwelling fishes and motile invertebrates.
Habitat availability is often the limiting factor for marine organisms requiring a crevice or surface upon which to attach, so increased habitat surfaces (like the undersea wind farm structure) increase the local carrying capacity of the region. Thus the underwater structure is not only very unlikely to harm underwater marine life, it actually tends to increase local diversity and abundance of many marine creatures. This principle is well established in marine biology, and is the basis for artificial reefs used to increase diversity and fish abundance, such as intentionally-sunken decommissioned ships and oil rigs.
Prof Steven Campana, University of Iceland
Oceans are largely unknown to us, although recent technologies are helping us get a better grasp of what’s going on well below the surface.
The sea is rising. It has been for decades, but the rise is getting faster in recent decades.
Oceans absorb lots of CO2 and will likely soak up our pollution over thousands of years. Problem is, we are emitting too much, too fast for the oceans.
As they absorb CO2, oceans are acidifying. They won’t get technically ‘acidic’ anytime soon, but their acidification is already impacting the ecosystem - coral reefs are a dramatic example of it.
Hurricanes are likely getting more intense, which could affect human communities and ocean wildlife alike.
Human residues can make areas in the ocean go hypoxic - these are called dead zones.
Off-shore wind farms may not significantly impact the marine ecosystem, and may even support biodiversity.