Diver Surveys Coral Bleaching, Great Barrier Reef
Heron Island, Great Barrier Reef. A diver surveys coral bleaching. Photo: © The Ocean Agency/Richard

Effects Of Global Warming On Oceans

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This article looks at seven of the most serious effects of global warming on the oceans, involving deep waters, blue carbon coastal shallows, coral reefs, ocean degradation, marine heatwaves and more.

Planet Earth may be feeling the effects of climate change, but it’s the ocean that’s taking the brunt of it. According to recent studies, the ocean absorbs 93 percent of the heat of global warming 1 and around 31 percent of all man-made carbon dioxide emissions, even as CO2 concentrations jump to their highest levels for over 800,000 years. 2 This means the oceans and seas are now absorbing well over 2,000 billion tonnes of carbon a year, making them a key component in the carbon cycle and by far the largest carbon sink on the surface of the planet.

At this point, no one is suggesting that the ocean is about to stop absorbing mankind’s greenhouse gas emissions and allow Earth’s climate system to collapse, but many scientists are asking how long can the ocean continue to shield us from the effects of global warming before it suffers irreparable damage to its chemistry and ecosystems. In this article, we look at the damage that’s already happening and what lies ahead.

1. Rising Sea Levels

One of the most sensitive areas in the ocean is the cryosphere, which includes ice sheets in both Antarctica and Greenland. It’s a sensitive area because global warming is melting the ice much faster than expected. This of course raises sea levels, which were already rising due to the effect of ocean warming – a process known as thermal expansion. Note that thermal expansion is the biggest cause of sea level rise (SLR), accounting for 30-55 percent of the increase. 3

When Arctic sea ice melts does it raise sea levels?

No. Melting Arctic sea ice doesn’t cause sea level to rise, because it’s already in the water. The volume of water it adds to the ocean when it melts is the same as the volume of water it displaces as ice.

The IPCC’s Fifth Assessment Report (2014) forecast a rise of up to 82 cm (nearly 3 feet) by 2100. 4 This compares to a report by NASA which says sea levels will rise up to 4 feet. 5 Meantime, other studies conducted by researchers from the National Oceanic and Atmospheric Administration suggest that a rise of 200-270 cm (6.5 to 9 feet) this century, is plausible.

In all cases, it is the glaciers on the Greenland ice sheet and the Antarctica ice sheet (notably in West Antarctica) that are are viewed with most concern. 6 Other research confirms that global sea levels could rise up to 1.5 meters (5 feet) by 2100, if emissions are not brought under control. 7 8

New Science Of Ice-Cliff Collapse

Some of these studies predict sea level rises much greater than those outlined in the IPCC’s Fifth Assessment Report. This is partly due to the time taken by the IPCC to compile its reports. In areas of rapid scientific change, this can leave the IPCC slightly behind the curve, as its more conservative estimates of sea level rise show. In the case of polar ice melt, new research has uncovered potential weaknesses in the structure of ice sheets in West Antarctica, due to the ice-cliff collapse mechanism – especially when accompanied by hydrofracturing, due to surface melt draining into crevasses, and atmospheric warming.

So what exactly is ice-cliff collapse? Well, ice sheets are typically always moving down to the sea. And this flow of ice from ice sheets to the ocean is often resisted by the buttressing effect of floating ice shelves (that is shelves that form where a glacier or ice sheet flows down to and onto the ocean surface.) These shelves face possible collapse as temperatures rise, exposing taller cliff faces to their rear. Some scientists suggest that ice cliffs higher than 90 m (300 feet) could collapse under their own weight, exposing even taller cliffs in the interior of a thickening ice sheet, leading to an irreversible ice-sheet retreat. 9

The ice-cliff collapse mechanism was seen at first hand during the abrupt collapse of Antarctica’s Larsen B ice shelf (2002), and later during the calving of icebergs from Greenland’s Jakobshavn and Helheim glaciers. According to one study, if carbon dioxide emissions continue along a worst-case scenario, the full 11 feet of ice locked in West Antarctica might be released. 9

Just as alarming is the news that East Antarctica’s ice sheet – previously thought to be reassuringly stable – is also melting. A new study led by Eric Rignot, using an improved climate model, has shown that East Antarctica has actually been losing ice for the last four decades. 10

IPCC Special Report On The Ocean & Cryosphere (2019)

Undeterred by these findings, the IPCC’s 2019 Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) maintains a conservative approach, stating that – by 2100 – global mean sea level is expected to rise by 0.43 meters (1.4 feet) under a low emissions scenario, and 0.84 meters (2.8 feet) under a high emissions scenario. However, it raised its worst-case maximum (under RCP8.5) by about 10 cm, to 110 cm. It also conceded that the rate of sea level rise is “unprecedented” over the past century and that, if fossil fuel emissions continue to increase strongly, then a rise of 2 meters (6.5 feet) by 2100 “cannot be ruled out.”

Regarding the polar ice sheets, the report acknowledges that they hold about 66 metres of potential global sea level rise, but states that the loss of a large fraction of it will not happen for millennia. As to the theoretical studies identifying the potential dangers of ice-cliff collapse in West Antarctica, the IPCC admits to having “low confidence” in the theoretical likelihood of irreversible retreat caused by ice sheet instability, which in any case is likely to take “a few centuries” to happen. The issue, it says, is an example of “deep uncertainty”, where climate experts are unable to agree on how to model the key driving forces of cliff-collapse, or the probabilities and uncertainty that attach to different outcomes. 11

In any event, the report warns, the accelerating ice loss now being seen – together with the more rapid sea level rise it engenders – will continue to gather pace over the course of the 21st century, regardless of whether or how fast greenhouse gas emissions are reduced.

Effect Of Sea Level Rise On Coastal Regions And Small Island States

The IPCC’s special report also highlights the consequences of global warming-induced sea level rise on low-lying islands and coastlines. Roughly 680 million people – around 10 percent of the world’s population – inhabit coastal zones less than 10m (35 feet) above sea level. Indeed, flooding and coastal erosion already pose serious problems for 4 out of 5 small island residents who live near the coast. 

The risks to these areas and their inhabitants are expected to get “disproportionately higher” later this century, as sea levels rise, boosted by ocean warming and melting permafrost. Some island nations, the report says, are likely to become uninhabitable under high emission scenarios, although such possibilities are hard to assess.

Oceans Are Getting Hotter

Oceans Are Getting Hotter - Graph
Oceans are getting warmer. In fact the world’s oceans are now heating at the same rate as if five Hiroshima atomic bombs were dropped into the water every second. Source: “Record-Setting Ocean Warmth Continued in 2019” Lijing Cheng 12

2. Marine Heatwaves

The latest research shows that the ocean absorbs 93 percent of the heat of climate change. 13 By the end of the century, under a comparatively low-emissions scenario, the ocean is likely be two to four times hotter than it was in 1970. Under a high-emissions scenario, the ocean is likely to be five to seven times hotter.

One of the most damaging effects of this ocean warming has been the growing frequency of marine heatwaves (MHWs) – sudden, intense episodes of warming. These are extreme weather events, where abnormally high ocean heat is sustained for days or months. These MHWs have significant adverse impacts on marine animals, habitats and ecosystems.

Ocean Warming is Affecting the Marine Food Web
The marine food chain has two critical actors: phytoplankton who create food energy out of sunlight, and krill who eat phytoplankton and in turn serve as a food source for nearly all other marine creatures, including blue whales. Krill are the keystone species in the Southern Ocean around Antarctica, and their numbers are dropping due to a lack of phytoplankton and habitat (sea ice). Both these problems are caused by ocean warming.

Marine Heatwaves Will Increase In Frequency

According to the IPCC’s Special Report on the Oceans 14, marine heatwaves have “very likely doubled in frequency since 1982”, and have become “longer-lasting, more intense and more extensive”. It also states that it is “very likely” that 84-90 percent of the marine heatwaves which occurred during the period 2006-2015 were caused by anthropogenic activities.

The report also states that although lowering greenhouse gas emissions would significantly reduce the increase in adverse effects, “there is high confidence that marine heatwaves will increase in frequency, duration, and intensity in all ocean basins”. In a low emissions scenario, marine heatwaves will become 20 times more frequent by the end of the century, relative to the pre-industrial period. If emissions remain high, heatwaves could become 50 times more frequent. Global warming is also likely to intensify existing hazards, including the El Nino southern oscillation (ENSO), which is projected to occur about twice as often under both RCP2.6 and RCP8.5, when compared to the 20th century.

What exactly is a marine heatwave?

A marine heatwave occurs when the surface temperature of seawater is warmer than 90 percent of past measurements, for at least 5 consecutive days. Similar successive measurements with gaps of 2 days or less are deemed to be part of the same event. Marine heatwaves can occur in summer or winter – they are defined simply in relation to normal temperatures for the location and time of year. 15 They can produce some of the most distressing effects of global warming on oceans that we are likely to see.

Examples Of Recent Marine Heatwaves

  • During 2003, sea areas off the coasts of France and part of Italy in the Mediterranean and Adriatic, experienced a 30-day marine heatwave with temperatures 4°C higher than normal. It caused large scale mortality among marine life in rocky reefs. 16
  • In 2011, seawater along the entire western coast of Western Australia experienced a 60-day marine heatwave involving temperatures of 3°C above the norm. As well as destruction of habitats and increased mortality, it triggered a southwards migration of seaweed, fish and sharks.
  • In 2012, in the Northwest Atlantic, off the Canada-USA coast up to Newfoundland, a 56-day marine heatwave saw temperatures rise 2.5°C above normal. Consequences included premature lobster maturity, which caused economic tensions between Canada and America.
  • During the period 2013-2015, a 226-day marine heatwave hit the Pacific Northwest off the USA and Canada. Nicknamed “The Blob”, it saw temperatures rise to 3°C above normal and remains the longest heatwave on record. It had incalculable effects on marine and kelp forest ecosystems. It also created toxic algae blooms, killed sea lions and endangered whales by compelling them to look for food closer to shore.
  • Between March and November 2016, a marine heatwave hit the Great Barrier Reef between 2016 and 2017, causing an estimated loss of 30-50 percent of all the corals. The northern section of the reef, roughly 700km in length, was worst affected, with more than half the coral in the reef’s shallowest areas dying within eight months.
  • In September 2019, a marine heatwave occurred in the north-east Pacific off the western coast of North America, when temperatures rose 3°C above normal.

What Causes Marine Heatwaves?

MHWs can be caused by a wide range of factors, local as well as regional or global. The most common drivers include ocean currents which can pile up areas of warm water and air-sea heat flux, warm winds and terrestrial heatwaves. Climate phenomena like El Nino can also increase the frequency and intensity of these events.

Effects Of Marine Heatwaves

MHWs can lead to severe impacts on marine ecosystems from the base of the food chain — plankton and zooplankton, for example, upon which everything in the ocean depends, are smaller and contain less fat and calories – to the highest trophic levels. Additional consequences include mass mortalities, toxic algal blooms, reduction in kelp beds, and substantial loss of biodiversity.

Mangrove Die-Off: Gulf of Carpentaria, Australia
Aerial view of disastrous coastal mangrove die-off, caused by marine heatwaves in Australia’s Gulf of Carpentaria. The die-off, which coincided with the Great Barrier Reef’s worst-ever coral bleaching, impacted 1,000km (600 mi) of coastline between the Roper river in the Northern Territory and Karumba in Queensland. Photo: © Professor Norm Duke/James Cook University

Marine heatwaves impact on the habitats, geographic distribution and behaviors of numerous marine species. Temperate foundation (habitat-forming) species, are particularly affected 17, especially by warmer water temperatures and by scarcity of nutrients due to thermal stratification or to a reduction in coastal upwelling 18.

MHWs can also alter the habitat ranges of species: an example is the spiny sea urchin off the SE coast of Australia which has been pushing southwards into Tasmania at the expense of kelp forests upon which it feeds. Looking ahead, prolonged marine heatwaves are likely to trigger increased poleward dispersal of species.

Biodiversity can be drastically affected by marine heatwaves. In 2015-2016, heatwaves in the ocean north and east of Australia led to severe bleaching of the Great Barrier Reef and a major mangrove die-back in the Gulf of Carpentaria. In 2019, scientists discovered an even larger mangrove die-off along the exact same coast, affecting about 8,000 hectares (20,000 acres) of mangrove forest. A combination of heatwaves, rising sea levels, and back-to-back tropical cyclones left a 400 km trail of dead and badly damaged mangrove trees. 19

MHWs are affecting coastal biomes around the world 20 21 as well as the human populations that depend on them, through impacts on fisheries and aquaculture. For example, in 2011 in Western Australia, a marine heatwave severely affected abalone fisheries in the north of the state and in 2015/16 a marine heatwave off SE Australia caused high levels of abalone mortality in Tasmania. MHWs have also been associated with outbreaks of Pacific oyster mortality syndrome and impacts on Atlantic salmon aquaculture. In 2015, the Blob caused a huge algal bloom to appear along the coast of California, which shut down the commercial crab harvest in Oregon and Washington for a period of months.

3. Ocean Acidification

Significant changes are already underway in the water chemistry of the world’s oceans, due to growing levels of carbon dioxide in the atmosphere, caused by the burning of fossil fuels in factories and power plants. The problem is, as much as 30 percent of these CO2 emissions is soaked up by the ocean.

Ocean Carbon Circulation: Infographic
Circulation of Carbon Through the Oceans. Image: © International Union for Conservation of Nature and Natural Resources

One particularly harmful consequence of this, is the growing acidification of the ocean, which is another of the most damaging effects of global warming on oceans. What happens is this. When CO2 dissolves in sea water, it forms carbonic acid, decreasing the ocean’s pH and making it more acidic. In a nutshell, ocean acidity change is a direct consequence of increased man-made emissions, whose reduction is now more urgent than ever. To avoid serious harm, atmospheric levels of CO2 need to fall to around 320-350 ppm, compared to present-day levels of about 410 ppm. 22

Why is ocean acidification so serious? Because it’s happening ten times faster than at any time over the last 300 million years. Which is much too fast for Earth’s climate system to handle, as it has in the past. What’s more, when combined with a reduction in ocean oxygen levels and increased warming – ocean acidity has the potential to destroy marine ecosystems around the world, especially those in coastal areas. And once it occurs, a lot of this damage to the marine environment is irreversible. 22

How Acidic Is Seawater Now?

The oceans have suffered a drop in pH of about 0.1 units since preindustrial times, which may sound small but it’s not. Due to the exponential nature of the pH scale a 0.1 drop actually represents an increase in acidity of about 28 percent. (A lower pH means the ocean is more acidic.) Ocean climate models indicate that by the year 2100 increasing ocean acidity could lower surface pH by between 0.4 and 0.7 pH units. Since a difference of one pH unit is equivalent to a tenfold difference, a 0.7 reduction in pH would mean that seawater would be seven times more acidic than before the Industrial Revolution.

Ocean acidification diagram
Over the past 200 years, the level of carbon dioxide (CO2) in the atmosphere has increased due mainly to the burning of fossil fuels, like coal, petroleum and natural gas. The ocean absorbs around 30 percent of this atmospheric CO2. This absorption leads to chemical reactions that cause the seawater to become more acidic, and also reduces the amount of carbonate ions vital for the building of structures, such as sea shells and coral skeletons, used by clams, oysters, corals, and certain plankton. Image: © Plymouth Marine Laboratory

The largest increases in ocean acidity are projected to occur at high latitudes, with smaller increases in the tropics. Initially, deeper waters will experience less of an increase, with models forecasting a decline in pH of between 0.2 and 0.5 at a depth of about 1000 meters, depending on the emissions scenario and location. 23

Ocean Acidity Affects Formation Of Calcium Carbonate Shells

Compelling evidence of the adverse effects of increased sea water acidity on the physiology of marine organisms, can be seen in its impact on the formation and strength of calcium carbonate shells and skeletons in a range of marine species, such as molluscs such as clams, mussels, oysters and scallops, crustaceans like crab, crayfish, lobster, prawns and shrimp, as well as many phytoplankton and zooplankton that form the base of the marine food web in all the oceans.

The delicate shells and skeletons of all these creatures are weakened by even slight variations in the ocean’s acid balance, in much the same way that acid rain corrodes stone statues. It’s no wonder the oceanic weakening of these structures is sometimes called the “osteoporosis of the sea.”

Effect of Climate Change on Microbial Population
Scientists are coming to realize that microscopic organisms like bacteria and viruses, play a huge part in the marine food web and the carbon cycle. However, the sheer numbers and multiplicity of species makes it almost impossible to determine the effect of global warming on their activities and numbers. For more about this fascinating topic, including an explanation of the microbial loop and viral shunt, please see: Marine Microbes Drive the Aquatic Food Web.

Corals are also severely impacted. Acidification makes it harder for corals to build their skeletons. On top of this, ocean warming and, in particular, the increasing frequency of marine heatwaves, is causing a number of drastic coral bleaching events, where corals expel the symbiotic algae living inside them, and as a consequence turn completely white.

Ocean acidity also slows down the molting of crabs and other crustaceans, it confuses fish by interfering with their sense of smell. It even weakens sound absorption in the water, making the marine environment slightly noisier.

By 2050, oceanographers project that more than 85 percent of the world’s oceans will be warmer and more acidic than anything in modern history.

4. Ocean Deoxygenation

One of the most pervasive effects of global warming is ocean deoxygenation which affects marine animals and ecosystems around the globe.

Deoxygenation of the ocean, diagram
Oxygen loss in the open ocean, referred to as “deoxygenation”, is a major effect of climate change on the hydrosphere and is worsened by other human activities. An average global loss of oxygen of 2% or more, has been recorded in the open ocean over the past 50-100 years, although the decline has been greater in intermediate waters (100-600 meters or 400-2,000 feet) of the Northern and Eastern Pacific, tropical waters, and the Southern Ocean. Image: © Lisa A Levin. 24

Before mankind started burning enormous amounts of fossil fuels and using large quantities of nitrogen and phosphorus on their farmland and gardens, oxygen existed in the ocean from two sources: (a) From atmospheric oxygen dissolving in surface waters, via the continuous air-sea gas exchange mechanism. (b) From photosynthesis, performed in near-surface waters by marine plants, (phytoplankton). In this process, oxygen is produced as a by-product. Deeper waters could not receive oxygen directly from the atmosphere or from photosynthesizing phytoplankton, so they relied upon the downwelling and upwelling of the thermohaline currents to distribute oxygen from the ocean surface to the depths.

What Causes Ocean Deoxygenation?

Since the Industrial Revolution, the increasing impact of man-made global warming has warmed the oceans, causing a reduction in the amount of oxygen in the water. How come? Because among other things, warmer water holds less oxygen. Also, ocean warming affects the circulation patterns that carry oxygen-rich water to the deeper ocean. As a result, oxygen absorbed at the near-surface doesn’t reach deeper water. And even the surface oxygen gets used up more quickly because fish and other marine life use more oxygen when temperatures are warmer. 25

In addition, the increasing use of agricultural nitrogen-rich and phosphorus-rich fertilizers, as well as their domestic equivalents, has led via surface run-off to a gradual process of eutrophication in oceans and other water bodies. This occurs when water contains excessive amounts of minerals and nutrients, which leads to excessive growth of algae. When the algae and other plants die, they decay, and this decay leads to low levels of dissolved oxygen in the water, which leads to the migration, weakening, or death of many marine and aquatic species.

The threat to oceans from nitrogen and phosphorus used on farms and in industry, has long been known to lower oxygen levels of oxygen in seawater, and still remains the primary cause. However, in recent decades the threat from climate change has significantly increased.

How Much Oxygen Has The Ocean Lost?

Scientists calculate that between 1960 and 2010, the amount of oxygen dissolved in the ocean declined by 2 percent. The loss of oxygen has actually quadrupled during this period. In some tropical locations the loss can range up to 50 percent. 26 As a result, about 700 ocean sites now suffer from low levels of oxygen, compared with 45 in the 1960s. Unless emissions are brought under control, the ocean is expected to lose 3-4 percent of its oxygen by the end of the century, with the worst effects occurring in the tropical regions of the world. Most of the loss is expected in the top 1,000 meters (3,500 feet), which is richest in biodiversity. 27

What Are The Effects Of Ocean Deoxygenation?

Oxygen depletion is most threatening for larger species fast-swimming species like tuna, marlin and sharks. Big fish have greater energy needs and will inevitably migrate to shallower surface layers that contain more oxygen. However, this change of habitat renders them more vulnerable to over-fishing. 

Eutrophication and the formation of hypoxic or dead zones
Diagram shows processes involved in eutrophication and the formation of hypoxic or dead zones, a phenomenon that affects mainly coastal areas. It starts with excess nutrients (from fertilizers, soil erosion, sewage residues) that are carried to the ocean by rivers. Being less dense than colder salty seawater, this freshwater forms the surface layer. And because it’s nutrient-dense it causes algae populations to skyrocket. Unfortunately when algae die and decompose, oxygen is consumed in the process, depleting the surrounding water of its oxygen content. This kills marine life and damages marine ecosystems. To make matters worse, the stratification of the ocean (warm freshwater layer on top of colder salty layer) stops the mixing of oxygen-poor water on the bottom of the ocean with the oxygen-richer surface water, thus also keeping the bottom water short of oxygen. Image: © United States Environmental Protection Agency

A decline in oxygen levels also has severe consequences for zooplankton (an important link in the marine food chain) many of whom inhabit oxygen minimum zones (OMZs) – large regions of very low oxygen located below the surface and above the seafloor. Zooplankton are specially adapted to live in these zones where other organisms – notably predators – cannot. But even slight changes to the low oxygen levels can push zooplankton beyond their physiological limits. 28

Low levels of oxygen in the ocean also affect biogeochemical cycles that redistribute elements crucial for life on Earth, including nitrogen and phosphorus. It will also alter the energy and biochemical cycling in the oceans although the effects of this are not yet understood.

Low Oxygen Zones

In various locations in the oceans, there are areas that contain little or no oxygen – including sites in the tropical oceans off California, Peru and Namibia and the subsurface waters of the Arabian Sea. These so-called “oxygen minimum zones” are a natural phenomenon, resulting from a combination of weak ocean currents and the decomposition of organic matter from sites of ocean upwelling. The low oxygen levels in these areas can be lethal to most marine life. These zones can also release nitrous oxide – a potent greenhouse gas – into the atmosphere. The latest evidence indicates that these low-oxygen regions are expanding and become more numerous.

Oxygen levels in the waters off Texas, Louisiana and Mississippi
Image showing oxygen in the waters off Texas, Louisiana and Mississippi. Red shows lowest level. According to 2019 estimates by NOAA oceanographers, the Gulf of Mexico’s hypoxic or ‘dead’ zone – the area of water that has little or no oxygen and so is unable to support fish and other marine life – will cover roughly 12,500 sq km (7,829 sq mi). The forecast is close to the record of 14,000 sq km (8,776 sq mi) set in 2017, but significantly larger than the 5-year average of 9,230 sq km (5,770 sq mi). Image: © NOAA

What’s The Solution To Ocean Deoxygenation?

“Ocean oxygen depletion is menacing marine ecosystems already under stress from ocean warming and acidification,” says Dan Laffoley, co-editor of “Ocean deoxygenation: everyone’s problem” a major study into the causes and impacts of ocean deoxygenation, as well as possible solutions. “To stop the worrying expansion of oxygen-poor areas, we need to decisively curb greenhouse gas emissions as well as nutrient pollution from agriculture and other sources.” 27

5. Weakening Of Deep-Water Currents

Deep water ocean currents, known as the thermohaline circulation, play a critical role in the hydrosphere. Unlike surface currents that driven by wind, deep water currents are driven largely by their density (the result of being very cold and salty), which makes them heavy and enables them to sink to the sea bottom. The main function of these currents is to distribute heat (warm water) from the tropics to the poles, and cold water from the poles to the tropics, thus helping the climate system to maintain even temperatures around the world. It’s a perfect example of how oceans influence climate on a regular basis, week in week out.

At the same time, they absorb large amounts of atmospheric carbon dioxide at the poles, thus evenly dispersing the chemical and ecological risk around the depths. For example, the Atlantic Meridional Overturning Circulation (AMOC) – the most important section of the thermohaline circulation – is now the largest carbon sink in the Northern Hemisphere, sequestering an estimated 700 million tonnes of CO2 from the atmosphere every year. 29

Operating like huge underwater rivers, they are in constant but slow motion, overturning and refreshing the ocean as they mix cold fresh water from the ocean floor with warmer surface water. Without this turnover of water, rising temperatures are likely to create a warm top layer of low salinity which never mixes with the colder, saltier layers in the depths. This would be a disaster for several reasons, not least the fact that warmer water with a lower salt content tends to release CO2 rather than absorb it. Any CO2 it did absorb would remain on or near the surface (rather than being dispersed throughout the ocean), thus discouraging (rather than enabling) further CO2 pulldown.

Warm surface waters are also revitalized by the upwelling of nutrient-dense waters from the ocean floor, that support the growth of seaweed, algae and other phytoplankton that are so important in the food chain.

Does Global Warming Affect Deep Ocean Currents?

Yes. Scientists believe that climate change is affecting the thermohaline circulation in two ways. First, it’s warming the surface of the Arctic seas. Second, it’s melting more Arctic sea ice, thus adding more freshwater to the ocean and lowering its salinity. Either of these actions reduces the density of high-latitude surface waters and inhibits deep water formation.

So, Is The Atlantic Meridional Overturning Circulation Weakening?

Yes, studies indicate that the AMOC is weakening, but there is no consensus as to how much.

A 2015 study suggested that the AMOC has declined by 15-20 percent in 200 years and that melting glaciers in Greenland may be responsible. 30 The weakening of the AMOC was confirmed in another study which was published in 2018. 31

The IPCC’s Special Report on the Ocean stated that the AMOC – which it described as a key tipping point of the Earth’s climate system – was very likely to weaken over the 21st century, but was not likely to collapse – although it stated  that even this was “physically-plausible” under certain circumstances, albeit none that were likely. 32

6. Loss Of Coral Reefs

One of the worst effects of global warming on oceans is the damage done to sea corals. These marine creatures are being attacked on two sides by human-induced global warming.

To begin with, when water gets too warm, corals become stressed and respond by evicting a special type of algae (zooxanthellae) which lives inside them and provides them with 90 percent of their energy. After expelling the algae, the corals lose all their colour and become completely white – an outcome known as “coral bleaching”. But it’s not their colour but their loss of energy that does the damage. Without the algae they become stressed and many starve. 33

Marine heatwaves are particularly deadly. While coral bleaching is essentially a starvation process that unfolds over 1-2 months, a marine heatwave can cause rapid death in a process akin to cooking. In a recent survey of 3,863 individual reefs off Australia’s north-east coast, researchers discovered that 29 percent of communities were affected. In some cases, 90 percent of coral had perished. 34 35

The second attack comes from ocean acidification, which causes the skeletons of the corals to weaken, or even dissolve. 36 This is because ocean acidity prevents the up-take of calcium carbonate that corals need for their skeletons, thus stopping them from calcifying properly. 37

Together, these two processes, warming and acidification, are proving fatal for coral reefs, one of the first marine ecosystems to be destroyed by human-induced climate change. It’s almost unthinkable that a huge natural structure like the Great Barrier Reef, which was built by millions of corals over a period of 20 million years, could disappear completely with a few decades.

The truth is, reef-building corals are vital elements in coastal ecosystems. Like terrestrial rainforests, they form the most biodiverse habitats in the oceans, acting as important nursery grounds for fish and other marine life, and providing sanctuary for all manner of sea creatures. Scientists estimate that 25 percent of all marine life depends on coral reefs in one way or another, giving them a reach far beyond their perimeter.

Coral reefs offer several other practical benefits. For example, to help deter or deal with predators, many reef-based organisms have developed potent chemical defences, which is why pharmaceutical corporations are bioprospecting among reefs for active ingredients to treat cancer, Alzheimer’s disease, arthritis, heart disease, and bacterial infections. 38

According to the World Wildlife Fund and the Smithsonian Institute, coral reefs around the world are worth at least $1 trillion, as they generate an annual income of $300-400 billion from tourism, fisheries, aquaculture and medicines.

7. Damage To Blue Carbon Ocean Coastal Ecosystems

In 2018, a number of serving government leaders joined forces to form the “High-level Panel for a Sustainable Ocean Economy”, in order to work towards sustainable use of the oceans. The panel’s first report, published in September 2019 by the World Resources Institute 39, stated that ocean-based climate action could deliver more than one fifth of the emissions reductions needed by 2050 to limit global temperature rise to 1.5°C, thus avoiding some of the worst impacts of climate change.

Blue Carbon Ocean Coastal Ecosystems
Carbon content of coastal habitats compared to rainforest, in tons of CO2 equivalent per hectare (2.5 acres). Source: IUCN. Roughly 83 percent of the carbon in the global carbon cycle passes through the world’s oceans. Coastal habitats occupy around 2 percent of the ocean area, but account for about 50 percent of the total carbon stored in ocean sediments. Image: © International Union for Conservation of Nature (IUCN)

Among other things, it recommended the restoration and protection of “blue carbon” ecosystems. Fine. But what exactly is blue carbon? 

Blue Carbon is a term coined in 2009 by Christian Nellemann, to draw attention to the destruction and degradation of coastal ecosystems, and the necessity to protect them in order to mitigate climate change but also for the other environmental benefits they provide. Nellemann described blue carbon as the carbon which is absorbed and stored by coastal ecosystems, mostly mangrove swamps, salt marshes, kelp forests and seagrasses. 40

“Out of all the biological carbon (or green carbon) captured in the world, over half (55 percent) is captured by marine living organisms – not on land – hence it is called Blue Carbon.” Christian Nellemann PhD. Climate scientist.

Historically, when it comes to carbon sinks, most attention has been devoted to the deep ocean, as well as tropical rainforest ecosystems. Now, growing amounts of data on the role of vegetated coastal ecosystems and coastal wetlands has highlighted their efficiency as carbon reservoirs.

Seagrasses, mangroves and salt marshes capture and store CO2 from the troposphere by sequestering it in their underlying sediments, in their underground biomass, and in dead biomass. 41

Amazingly, even though the ocean’s vegetated habitats account for less than 0.5 percent of the seabed, they are responsible for more than 50 percent and potentially up to 70 percent, of all carbon storage in ocean sediments. 40 And let’s not forget – according to the latest figures, compiled by the U.S. Deep Carbon Observatory, ocean sediments contain about 3,000 billion tonnes of carbon.

Loss Rate Is The Highest Of Any Ecosystem

Unfortunately, the rate of loss of these vital blue carbon ecosystems is much higher than almost any other ecosystem on the planet: higher even than rainforests. Current studies estimate a loss of 2-7 percent per year, which represents a considerable amount of lost carbon sequestration. For example, as described above, marine heatwaves which swept across the Gulf of Carpentaria in 2015 and 2019, decimated mangoves along the coastlines of Australia’s Northern Territory and Queensland. 19 In addition, man-made interventions involving coastal development, aquatic cultures and commercial fisheries are also taking their toll.

In their Special Report on the Ocean (2019), the IPCC states that the degradation of ocean coastal ecosystems is exacerbated by “pervasive human coastal disturbances”, which cause additional stress and limit their ability to adapt.

The report also emphasises the importance of conserving and restoring the main “blue carbon” ecosystems: mangroves, salt marshes and seagrass meadows.

“Some 151 countries around the world contain at least one of these coastal blue carbon ecosystems and 71 countries contain all three. Below-ground carbon storage in vegetated marine habitats can be up to 1000 tonnes of carbon per hectare, much higher than most terrestrial ecosystems.”

References

  1.  “2018 Continues Record Global Ocean Warming.” Lijing Cheng, Jiang Zhu, John Abraham, Kevin E. Trenberth, John T. Fasullo, Bin Zhang, Fujiang Yu, Liying Wan, Xingrong Chen, Xiangzhou Song. Advances in Atmospheric Sciences, March 2019, Volume 36, Issue 3, pp 249–252. First published online 15 Jan, 2019. []
  2. The oceanic sink for anthropogenic CO2 from 1994 to 2007.” Nicolas Gruber, et al;. Science Vol. 363, Issue 6432, pp. 1193-1199. 15 Mar 2019. []
  3. “What the new IPCC report says about sea level rise.” Freya Roberts. Carbon Brief. October 3. 2013. []
  4. “Sea Level Change”. In Stocker, T.F.; et al. (eds.). Climate Change 2014: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. Church, J.A.; Clark, P.U. (2013). []
  5. Sea Level Will Rise 1-4 feet by 2100.” NASA. []
  6. Global and Regional Sea Level Rise Scenarios for the United States. NOAA Technical Report NOS CO-OPS 083. National Oceanic and Atmospheric Administration. January 2017. https://tidesandcurrents.noaa.gov/publications/techrpt83_Global_and_Regional_SLR_Scenarios_for_the_US_final.pdf []
  7. Evolving Understanding of Antarctic Ice-Sheet Physics and Ambiguity in Probabilistic Sea-Level Projections.” Robert E. Kopp, Robert M. DeConto, Daniel A. Bader, Carling C. Hay, Radley M. Horton, Scott Kulp, Michael Oppenheimer, David Pollard, Benjamin H. Strauss. Earth’s Future. 13 December 2017. []
  8. Ice sheet contributions to future sea-level rise from structured expert judgment.” Jonathan L. Bamber, Michael Oppenheimer, Robert E. Kopp, Willy P. Aspinall, and Roger M. Cooke. PNAS June 4, 2019 116 (23) 11195-11200. []
  9. Contribution of Antarctica to past and future sea-level rise.” Robert M. DeConto & David Pollard. Nature. Vol 531, p.591. March 2016. [][]
  10.  “Four decades of Antarctic Ice Sheet mass balance from 1979–2017.” Eric Rignot, Jeremie Mouginot, Bernd Scheuch, Michiel van den Broeke, Melchior J. van Wessem, Mathieu Morlighem. PNAS January 22, 2019 116 (4) 1095-103. []
  11. See for example: “A Speed Limit on Ice Shelf Collapse Through Hydrofracture.” Geophysical Research Letters, 46, 21, (12092-12100), (2019). Alexander A. Robel and Alison F. Banwell. []
  12. Study []
  13.  “2018 Continues Record Global Ocean Warming.” Lijing Cheng, Jiang Zhu, John Abraham, Kevin E. Trenberth, John T. Fasullo, Bin Zhang, Fujiang Yu, Liying Wan, Xingrong Chen, Xiangzhou Song. Advances in Atmospheric Sciences, March 2019, Volume 36, Issue 3, pp 249–252. First published online 15 Jan, 2019. []
  14. Chapter 6. IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) []
  15.  Hobday, A. J. et al. (2016), A hierarchical approach to defining marine heatwaves, Prog. Ocean., 141, pp. 227-238. []
  16. This and other examples are taken from: “Marine Heatwaves occur everywhere in the ocean.” marineheatwaves.org []
  17. “Status and Trends for the World’s Kelp Forests. In World Seas: An Environmental Evaluation.” Amsterdam: Elsevier. Wernberg, T., Krumhans, K., Filbee-Dexter, K., and Pedersen, M. F. (2019). []
  18. Schiel, D. R., and Foster, M. S. (2015). The Biology and Ecology of Giant Kelp Forests. Oakland, CA: Univ of California Press. []
  19. “Shocked scientists find 400km of dead and damaged mangroves in Gulf of Carpentaria.” Graham Readfearn. BBC News. Oct 3, 2019. [][]
  20. Mass mortality in northwestern mediterranean rocky benthic communities: effects of the 2003 heat wave.” Glob. Change Biol. 15, 1090–1103. Garrabou, J., Coma, R., Bensoussan, N., Bally, M., Chevaldonné, P., Cigliano, M., et al. (2009). []
  21. Marine heatwaves threaten global biodiversity and the provision of ecosystem services.” Nat. Clim. Change 9, 306–312. Smale, D. A., Wernberg, T., Oliver, E. C., Thomsen, M., Harvey, B. P., Straub, S. C., et al. (2019). []
  22. Ocean acidification.” International Union for Conservation of Nature (IUCN) [][]
  23.  “How will ocean acidification affect ocean sound levels?” University of Rhode Island. dosits.org/ []
  24. “Manifestation, Drivers, and Emergence of Open Ocean Deoxygenation” Lisa A Levin. Annual Review of Marine Science, Vol. 10:229-260 (January 2018) []
  25. Drivers and mechanisms of ocean deoxygenation.” Andreas Oschlies, Peter Brandt, Lothar Stramma & Sunke Schmidtko. Nature Geoscience volume 11, pages 467–473 (2018) []
  26. Expanding Oxygen-Minimum Zones in the Tropical Oceans.” Lothar Stramma, Gregory C. Johnson, Janet Sprintall, Volker Mohrholz. Science 02 May 2008: Vol. 320, Issue 5876, pp. 655-658. []
  27. “Ocean deoxygenation: everyone’s problem.” D. Laffoley, J. M. Baxter. IUCN, Global Marine and Polar Programme. Dec 2019. [][]
  28. Ocean deoxygenation and zooplankton: Very small oxygen differences matter.” K. F. Wishner, B. A. Seibel, C. Roman, C. Deutsch, D. Outram, C. T. Shaw, M. A. Birk, K. A. S. Mislan, T. J. Adams, D. Moore, S. Riley. Science Advances, Dec 2018; 4 (12) []
  29. Interannual variability in the North Atlantic Ocean carbon sink.” Science 298.5602 (2002): 2374–2378. Gruber, Nicolas, Charles D. Keeling, and Nicholas R. Bates. []
  30. “Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation.” (PDF) Rahmstorf, Stefan; Box, Jason E.; Feulner, Georg; Mann, Michael E.; Robinson, Alexander; Rutherford, Scott; Schaffernicht, Erik J. (2015). Nature Climate Change. 5 (5): 475–480. See also: “The Atlantic Meridional Overturning Circulation and Abrupt Climate Change.” Lynch-Stieglitz, Jean. Annual Review of Marine Science, vol. 9, p.83-104. January 2017. []
  31. Observed fingerprint of a weakening Atlantic Ocean overturning circulation”. Nature. 556 (7700): 191–196. Caesar, L.; Rahmstorf, S.; Robinson, A.; Feulner, G.; Saba, V. (11 April 2018). []
  32. Chapter 6: Extremes, Abrupt Changes and Managing Risks (PDF). IPCC (Report). Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). September 25, 2019. []
  33. “Coral Reef Bleaching: Ecological Perspectives” Earth and Environmental Science”. Coral Reefs. 12 (1): 1–17. Glynn, P.W. (March 1993). []
  34. Global warming transforms coral reef assemblages.” Terry P. Hughes, James T. Kerry, Andrew H. Baird, Sean R. Connolly, Andreas Dietzel, C. Mark Eakin, Scott F. Heron, Andrew S. Hoey, Mia O. Hoogenboom, Gang Liu, Michael J. McWilliam, Rachel J. Pears, Morgan S. Pratchett, William J. Skirving, Jessica S. Stella & Gergely Torda. Nature volume 556, pages 492–496 (2018) []
  35. For more about the effects of global warming on oceans, see: “Rapid Coral Decay Is Associated with Marine Heatwave Mortality Events on Reefs.” William P. Leggat, Emma F. Camp, David J. Suggett, Unnikrishnan Kuzhiumparambil, C. Mark Eakin, Tracy D. Ainsworth. Current Biology. Vol 29, Issue 16, pp 2723-2730. Published Aug 8, 2019. []
  36.  “Effect of calcium carbonate saturation of seawater on coral calcification”. Gattuso, J.-P.; Frankignoulle, M.; Bourge, I.; Romaine, S. & Buddemeier, R. W. (1998). Glob. Planet. Change. 18 (1–2): 37–46. []
  37. Carbon dioxide addition to coral reef waters suppresses net community calcification.” Rebecca Albright, Yuichiro Takeshita, David A. Koweek, Aaron Ninokawa, Kennedy Wolfe, Tanya Rivlin, Yana Nebuchina, Jordan Young & Ken Caldeira. Nature volume 555, pp 516–519. 22 March 2018. []
  38. NOAA []
  39. “The Ocean as a Solution for Climate Change: 5 Opportunities for Action.” High Level Panel for a Sustainable Ocean Economy. World Resources Institute. Summary for Decision-makers. []
  40. “Blue Carbon. The role of healthy oceans in binding carbon. A rapid response assessment.” Page 6. Arendal, Norway: United Nations Environment Programme, GRID-Arendal. Nellemann Christian, Corcoran E, Duarte CM, Valdes L, DeYoung C, Fonseca L, Grimsditch G (eds). (2009) [][]
  41. A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2.” Elizabeth Mcleod, Gail L Chmura, Steven Bouillon, Rodney Salm, Mats Bjork, Carlos M Duarte, Catherine E Lovelock, William H Schlesinger, and Brian R Silliman. Frontiers in Ecology and the Environment 2011. []
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