Triple crisis in the Anthropocene Ocean

By Ian Angus

November 19, 2020 — Links International Journal of Socialist Renewal reposted from Climate & Capitalism — It is impossible to overstate the importance of the ocean to life on Earth. Covering 71% of the planet’s surface, it contains 97% of the world’s surface water and is central to the great biogeochemical cycles that define the biosphere and make life possible. Marine plants generate half of the world’s breathable oxygen.

Millions of species of animals live in the ocean. Seafood is a primary source of protein for three billion people, and hundreds of millions work in the fishing industry.

The ocean’s metabolism — the constant flows and exchanges of energy and matter that have continued for hundreds of millions of years — is a vital part of the Earth System. As famed oceanographer Sylvia Earle writes, our fate and the ocean’s are inextricably intertwined.

“Our lives depend on the living ocean — not just the rocks and water, but stable, resilient, diverse living systems that hold the world on a steady course favorable to humankind.”[1]

“The living ocean drives planetary chemistry, governs climate and weather, and otherwise provides the cornerstone of the life-support system for all creatures on our planet, from deep-sea starfish to desert sagebrush. … If the sea is sick, we’ll feel it. If it dies, we die. Our future and the state of the oceans are one.”[2]

The living ocean is now being disrupted on a massive scale. It has changed before, but never, since an asteroid killed the dinosaurs, as rapidly as today. The changes are major elements of the planetary transition out of the conditions that have prevailed since the last ice age ended, towards a profoundly different biosphere — from the Holocene to the Anthropocene.

“We are entering an unknown territory of marine ecosystem change … the implications for the ocean, and thus for all humans, are huge.”[3]

Most popular accounts of the relationship between the ocean and climate change focus on melting ice and rising sea levels, and indeed those are critical issues. Greenland alone loses over 280 billion metric tons of ice a year, enough to cause measurable changes in the strength of the island’s gravity. At present rates, by 2100 the combination of global glacial melting and thermal water expansion will flood coastal areas where over 630 million people live today. Well over a billion people live in areas that will be hit by storm surges made bigger and more destructive by warmer seawater. Rapid action to slash greenhouse gas emissions would be fully justified even if rising seas were the only expected result of global warming.

Devastating as sea level rise will be, however, more serious long-term damage to the Earth System is being driven by what biogeochemist Nicolas Gruber calls a “triple whammy” of stresses on the oceans, caused by the growing rift in Earth’s carbon metabolism.

“In the coming decades and centuries, the ocean’s biogeochemical cycles and ecosystems will become increasingly stressed by at least three independent factors. Rising temperatures, ocean acidification and ocean deoxygenation will cause substantial changes in the physical, chemical and biological environment, which will then affect the ocean’s biogeochemical cycles and ecosystems in ways that we are only beginning to fathom. …

“Ocean warming, acidification and deoxygenation are virtually irreversible on the human time scale. This is because the primary driver for all three stressors, i.e. the emission of CO2 into the atmosphere, will cause global changes that will be with us for many hundreds, if not thousands, of years.”[4]

Other marine ecologists have described ocean warming, acidification and oxygen loss as a “deadly trio,” because when they have occurred together in the past, mass extinctions of animal and plant life have followed.[5]

We will consider the elements of the deadly trio separately, but it is important to bear in mind that they are closely related, have the same causes, and frequently reinforce each other.

Corrosive seas

“Ocean acidification … is a slow but accelerating impact that will overshadow all the oil spills that have ever occurred put together.” —Sylvia Earle[6]

Ocean acidification has been called global warming’s equally evil twin. Both are caused by the radical increase in atmospheric CO2, and both are undermining Earth’s life support systems.

There is always a constant interchange of gas molecules across the air-sea interface, between atmosphere and ocean. CO2 from the air dissolves in the water; CO2 from the water bubbles into the air. Until recently, the two flows were roughly balanced, but now, when atmospheric CO2 has risen 50%, more carbon dioxide is entering the sea than leaving it.

That’s been good news for the climate. The ocean has absorbed about 25% of anthropogenic CO2 emissions and over 90% of the additional solar heat, half of that since 1997. If it hadn’t done so, global warming would already have reached catastrophic levels. As Rachel Carson wrote years ago, “for the globe as a whole, the ocean is the great regulator, the great stabilizer of temperatures…. Without the ocean, our world would be visited by unthinkably harsh extremes of temperature.”[7]

But there is a price to be paid for that service. Adding CO2 is changing the ocean’s chemistry. The formula is very simple:

H2O + CO2 → H2CO3
Water plus carbon dioxide makes carbonic acid.

Adding CO2 makes seawater more acidic.

Over the past century, the ocean’s pH level has fallen from 8.2 to 8.1. That doesn’t sound like much, but the pH scale is logarithmic, so a drop of 0.1 means that the oceans are now about 30% more acidic than they used to be.[8] That’s a global average — the top 250 meters or so are generally more acidic than the deeps, and acidification is more severe in high latitudes, because CO2 dissolves more easily in colder water.

The present rate of acidification is a hundred times faster than any natural change in at least 55 million years. If it continues, ocean acidity will reach three times the pre-industrial level by the end of this century.


Surprisingly, given that scientific concern about CO2 emissions started in the 1950s, little attention was paid to ocean acidification until recently. It was first named and described in a brief article in Nature in September 2003, and first discussed in detail in a 2005 Royal Society report that concluded acidification would soon go “beyond the range of current natural variability and probably to a level not experienced for at least hundreds of thousands of years and possibly much longer.”[9]

Those wake-up calls triggered the launch of hundreds of research projects seeking to quantify acidification more precisely, and to determine its effects. While there are still big gaps in scientific knowledge, there is now no doubt that ocean acidification is a major threat to the stability of the Earth System, one that is pushing towards a sixth mass extinction of life on our planet.[10]

Though formally correct, the word “acidification” is misleading, since the oceans are actually slightly alkaline, and the shift now underway only makes them a little less so. Even in the most extreme scenario, a thousand liters of seawater would still contain less carbonic acid than a small glass of cola.

However, just as raising the atmospheric concentration of carbon dioxide to 0.041 percent is causing global climate change, so a small increase in the amount of CO2 in seawater poses major threats to the organisms that live in that water. Reduced pH has already significantly changed the habitats that marine plants and animals depend on: a further reduction could be deadly for many of them.

The most-studied casualties of ocean acidification are calcifiers, the many organisms that take carbonate from the surrounding water to build their shells and skeletons. In seawater, carbonic acid quickly combines with available carbonate, making it unavailable for shell and skeleton building. Water with less than a certain concentration of carbonate becomes corrosive, and existing shells and skeletons start to dissolve.

As marine conservation biologist Callum Roberts writes, lower pH is already weakening coral reefs, and the problem will get much worse if CO2 emissions aren’t radically reduced soon.

“The skeletons of corals on Australia’s Great Barrier Reef have weakened measurably in the last twenty-five years and now contain 14 percent less carbonate by volume than they did before…. Ocean acidification has been dubbed ‘osteoporosis for reefs’ because of this skeletal weakening. …

“If carbon dioxide in the atmosphere doubles from its current level, all of the world’s coral reefs will shift from a state of construction to erosion. They will literally begin to crumble and dissolve, as erosion and dissolution of carbonates outpaces deposition. What is most worrying is that this level of carbon dioxide will be reached by 2100 under a low-emission scenario of the Intergovernmental Panel on Climate Change.”[11]

About 25% of all fish depend on coral reefs for food and shelter from predators, so the shift that Roberts describes would be disastrous for marine biodiversity.

Other calcifiers weakened by ocean acidification include oysters, mussels, crabs, and starfish. Of particular concern are tiny shelled animals near the bottom of the food chain: if their numbers decline, many fish and marine mammals will starve. In particular:

  • Single-celled Foraminifera are abundant in all parts of the ocean, and are directly or indirectly consumed by a wide variety of animals. A recent study compared present day foraminifera with samples collected 150 years ago in the Pacific by the famous Challengerexpedition. The researchers found that “without exception, all modern foraminifera specimens had measurably thinner shells than their historical counterparts.” In some types of foraminifera, shell thickness is now 76% less than in the 1800s.[12]
  • Pea-sized Pteropods, sometimes called sea butterflies, live mainly in cold water. An article in the journal Nature Geoscience reports “severe levels of shell dissolution” in live pteropods captured in the ocean near Antarctica, resulting in “increased vulnerability to predation and infection.”[13] Since pteropods are food for just about every larger marine animal from krill to whales, “their loss would have tremendous consequences for polar marine ecosystems.”[14]

Interference with shell and skeleton formation may not be the most deadly effect of ocean acidification. The metabolic systems of all organisms function best when the pH level of their internal fluids stays within a narrow range. This is particularly problematic for marine animals, including fish, whose blood pH tends to match the surrounding water. For some species, even a small reduction in blood pH can cause severe health and reproduction problems, even death.[15] A growing body of research suggests that ocean acidification alone will decimate some species of fish in this century, causing the collapse of major fisheries.[16]

Only long-term studies can determine exactly how acidification will affect global fish populations, but waiting for certainty is dangerous, because once acidification occurs, we are stuck with it. A recent study confirmed that “once the ocean is severely affected by high CO2, it is virtually impossible to undo these alterations on a human-generation timescale.” Even if some unknown (and probably impossible) geoengineering system rapidly returns atmospheric CO2 to the pre-industrial level, “a substantial legacy of anthropogenic CO2 emissions would persist in the oceans far into the future.”[17]

Warnings ignored

In 2008, 155 scientists from 26 countries signed a declaration “based on irrefutable scientific findings” about “recent, rapid changes in ocean chemistry and their potential, within decades, to severely affect marine organisms, food webs, biodiversity, and fisheries.”

“To avoid severe and widespread damages, all of which are ultimately driven by increasing concentrations of atmospheric carbon dioxide (CO2), we call for policymakers to act quickly to incorporate these concerns into plans to stabilize atmospheric CO2 at a safe level to avoid not only dangerous climate change but also dangerous ocean acidification. …

“Policymakers need to realize that ocean acidification is not a peripheral issue. It is the other CO2 problem that must be grappled with alongside climate change. Reining in this double threat, caused by our dependence on fossil fuels, is the challenge of the century. …”[18]

In 2009, twenty-nine leading Earth System scientists identified the level of ocean acidification as one of nine Planetary Boundaries — “non-negotiable planetary preconditions that humanity needs to respect in order to avoid the risk of deleterious or even catastrophic environmental change at continental to global scales.”[19]

In 2013, the always-cautious Intergovernmental Panel on Climate Change (IPCC) expressed high confidence that absorption of carbon dioxide is “fundamentally changing ocean carbonate chemistry in all ocean sub-regions, particularly at high latitudes.”

“Warming temperatures, and declining pH and carbonate ion concentrations, represent risks to the productivity of fisheries and aquaculture, and the security of regional livelihoods given the direct and indirect effects of these variables on physiological processes (e.g., skeleton formation, gas exchange, reproduction, growth, and neural function) and ecosystem processes (e.g., primary productivity, reef building and erosion).”[20]

The IPCC’s Special Report on the Ocean and Cryosphere, published in 2019, concludes that “the ocean is continuing to acidify in response to ongoing ocean carbon uptake,” that “it is very likely that over 95% of the near surface open ocean has already been affected,” and that “the survival of some keystone ecosystems (e.g., coral reefs) are at risk.”[21]

Despite overwhelming scientific evidence that acidification is a major threat to the world’s largest ecosystem, the governments of the world’s richest countries remain silent. The word oceans only appeared once in their Paris Agreement and acidification wasn’t mentioned at all. It remains to be seen whether the next UN Climate Change Conference, which has been postponed to December 2021, will respond appropriately — if it responds at all.

Running low on oxygen

“Ocean deoxygenation is the 3rd but less-reported member of an evil climate change trinity, along with global warming and ocean acidification. It is not so much another shoe dropping out of our CO2 emissions as it is a large boot kicking ocean ecosystems, with significant knock-on impacts for hundreds of millions of people who depend on the oceans for a living, and with feedbacks on climate.” —Skeptical Science[22]

The ocean is losing its breath, with deadly effects on marine life and the biogeochemical cycles that shape the entire biosphere.

Since 1960, low-oxygen areas in the open ocean have expanded by 4.5 million square kilometers, an area the size of the European Union. Some regions have lost 40% of their oxygen, and the volume of water containing zero oxygen has more than quadrupled. The ocean is losing about a billion metric tons of oxygen every year. At present rates, the decline in life-giving ocean oxygen will triple by 2100. Add that to the rapidly growing number of coastal dead zones, and we have a life support emergency.

Overall, the ocean’s oxygen content has fallen just 2%, but the decline has occurred almost entirely in parts of the ocean where marine life is usually most abundant, so its impact is far greater than that percentage suggests.

Previous articles in this extended series on metabolic rifts have examined two ecological gluts created by capitalism’s inherent drive to expand at all costs: the nitrogen glut created by industrial agriculture’s dependence on synthetic chemical fertilizers, and the carbon dioxide glut created by capitalism’s dependence on fossil fuels. Both are disrupting biogeochemical cycles that have shaped the biosphere for hundreds of millions of years, causing unprecedented rifts in the Earth System’s metabolism.

The ocean oxygen crisis is driven by the nitrogen and carbon dioxide gluts, in different parts of the ocean.

In coastal areas and estuaries, millions of tons of synthetic nitrogen fertilizer carried by rivers are creating seasonal dead zones in coastal areas around the world. About 900 such zones have been identified, and there are undoubtedly hundreds more. Scientists have been studying coastal dead zones since the 1980s, and there is broad agreement about their causes and effects. I discussed them in Climate & Capitalism last month.[23]

This section focuses on a parallel development that has only been studied in the past 15 years or so — the growth of hypoxic (low-oxygen) and anoxic (zero-oxygen) areas in the open ocean, caused by global warming. They are not always physically separate from coastal dead zones — in the Baltic and Arabian Seas, for example, they overlap — but they develop and are expanding differently.

# # #

All of the oxygen dissolved in seawater, no matter how deep, originated at or near the surface, in one of two processes.

  • There is a constant exchange of oxygen molecules (O2) between the atmosphere and the ocean, at the air-sea interface. In simple terms, O2 from the air dissolves in the water and O2 from the water bubbles into the air.
  • Considerably more O2 is produced by plants, especially phytoplankton, that grow on and in the water. Photosynthesis requires sunlight, and even in very clear water, light penetrates less than 200 meters down. That euphotic zone (euphotic is Greek for well-lit) is the origin of nearly half of the world’s oxygen, and most of the ocean’s supply.

In most of the ocean, the upper 200 meters or so is called the surface or mixed layer. Waves, wind-driven currents and convection constantly stir its contents, making its temperature, salinity and dissolved gas content roughly uniform. Dissolved oxygen spreads through the mixed layer relatively quickly and evenly.

The mixed layer is warmed directly by sunlight and is constantly replenished by fresh water from rivers, rain and melting ice, so it is lighter (less dense) than the water below it, where a sudden temperature drop defines the thermocline,a colder and denser layer that separates the mixed layer from the cold and very slow moving deep layer that comprises about 90% of the ocean’s volume and mass. The thickness of the thermocline varies with seasons and latitude — in polar seas, it and the mixed layer scarcely exist — but in most of the ocean it extends from 200 to 1000 meters below the surface.

A variety of processes known collectively as ventilation move some of the mixed water, and the oxygen it contains, into the thermocline. The distribution of oxygen depends on local, regional and global currents, tides, local topography, unpredictable turbulence and other factors, so it is uneven. Most notably, in some parts of the thermocline a combination of weak ventilation and oxygen-consuming microorganisms results in pockets called Oxygen Minimum Zones. Most of the thermocline at that depth is teeming with fish, but life in an OMZ is largely limited to microbes that can survive with very little or no O2.

A different and much slower process plays a major role in distributing oxygen in the deep ocean. When water freezes in the North Atlantic, it leaves salt behind, creating a layer of dense brine that sinks to the bottom and slowly moves south, bringing along any oxygen it absorbed at the surface. Thus begins the Global Conveyor Belt, a slow deep-sea current that carries almost 20 million cubic meters of water per second —100 times more than the Amazon River — distributing oxygen and nutrients though the deep ocean. [24] A full circuit takes about 1000 years. By the time water from near Greenland reaches the North Pacific, most of the oxygen is gone: this contributes to the formation of an OMZ in the West Bering Sea and the Gulf of Alaska.

Climate change versus oxygen

Ocean acidification is a direct result of skyrocketing CO2 emissions. Ocean deoxygenation has the same cause: greenhouse gases are heating the world, and over 90% of that additional heat has been absorbed by the surface layer of the ocean, reducing the ocean’s total oxygen and expanding Oxygen Minimum Zones.

Research into the details of deoxygenation is ongoing, but it is clear that climate change is responsible for most oxygen depletion in the open ocean. Three temperature-dependent processes — solubility, stratification and circulation, and aerobic metabolisms — are stealing the ocean’s breath.

SolubilityBasic physics: when water gets warmer, it can hold less dissolved oxygen. A given volume of water in the Arctic can absorb more oxygen than the same volume at the equator. If the water’s temperature increases from 4ºC to 6ºC, the amount of oxygen it can hold decreases 5%.

For many millennia, the two-way gas transfer across the air-sea interface was balanced, so the amount of dissolved oxygen in the ocean remained roughly constant. At some time in the past half-century, that balance was broken: the warming ocean began releasing more oxygen than it absorbed. A recent study estimates that between 1975 and 2005 the net loss of oxygen from the ocean to the atmosphere averaged more than a billion tons a year. If warming continues, that outgassing could nearly triple by 2100.[25]

Stratification and circulation. As we’ve seen, the ocean is divided by temperature and salinity into three sharply defined layers, with the least dense layer on top. Climate change has further reduced the density of the top layer, by warming the water, increasing rainfall, and melting glaciers. That makes it still more difficult for oxygen-rich waters to move into the thermocline.

Reduced solubility means that there is less oxygen in total, and increased stratification reduces the portion of the oxygen that circulates below the mixed layer. Since 1960, Oxygen Minimum Zones in the thermocline have grown 20%, from just over 25 million square kilometers to 30.4 million — to 8% of the ocean’s total area, and 7% of its volume.[26]

Global warming is also weakening the Great Conveyor Belt: the north Atlantic portion now carries 15% less water than it did in 1960. So far, no effect has been measured on deep ocean oxygen levels; that may reflect the current’s slow speed, or limited sampling of deep sea water. It has been estimated that at present warming rates, deep sea circulation will fall as much as 45% by 2100.[27]

Metabolic ratesWhen temperature increases, almost all chemical processes speed up, including the complex biochemical reactions that maintain life in all organisms. Metabolic rates increase in proportion to temperature — organisms need more oxygen to maintain the same level of activity. The effect is barely noticeable in warm-blooded animals such as us, because our bodies always use a great deal of energy to maintain a stable condition. But the respiration rates of cold-blooded organisms, which includes almost all marine life, increase substantially when the water gets warmer.[28]

So — while lower solubility and stronger stratification are reducing the supply of dissolved oxygen in seawater, aerobic respiration is increasing the consumption.

It is difficult to quantify the relative impacts of each of the three process, but so far solubility and stratification seem to have caused greater oxygen reductions than increased respiration. That is likely to change as global temperatures rise, because heat’s impact on metabolic rates is exponential. According to a recent study, “for 2°C warming, there will be a 29% increase in ocean oxygen consumption rates, and for 3°C warming, a 50% increase, leading to large-scale ocean hypoxia.”[29]


“Oxygen is fundamental to life in the oceans. The decline in ocean oxygen ranks among the most serious effects of human activities on the Earth’s environment.”— Denise Breitburg, Senior Scientist, Smithsonian Environmental Research Center [30]

An Ocean Anoxic Event (OAE) is a period when the level of dissolved oxygen in a large part of the ocean plunges to (or near) zero. That has happened many times in Earth’s long history, most recently about 94 million years ago, when loss of oxygen wiped out a large proportion of marine life. As scientists associated with the Woods Hole Oceanographic Institution point out, ocean conditions today are similar to those that prevailed before that crisis, and are rapidly getting worse.

“More widespread perturbation seems realistic under currently projected carbon emissions. Without positive human intervention, ancient OAE studies are destined to become uncomfortably applicable in the not-so-distant future.”[31]

We are not yet in an OAE, but if oxygen loss continues to accelerate, large-scale extinctions of marine species are virtually certain. Even at present levels of deoxygenation, the damage is extensive.

“The vast body of evidence from temperate marine ecosystems and a few studies from tropical marine ecosystems demonstrate that hypoxia stress often results in catastrophic ecosystem simplification through loss of biodiversity, trophic complexity and fisheries.”[32]

In fact, any reduction in available oxygen, not just hypoxia, is problematic for most ocean life. Although some marine animals, jellyfish for example, are little affected by oxygen reduction, others go into decline when the level falls even slightly.[33] As a result, the population balance in areas where oxygen levels are falling tilts quickly towards hypoxia-tolerant species. Others flee or die.

As well as directly threatening the lives and habitats of marine organisms, oxygen depletion is disrupting the global nitrogen cycle. For hundreds of millions of years, naturally occurring Oxygen Minimum Zones have played a key role in the nitrogen cycle, because the bacteria that convert reactive nitrogen (Nr) compounds into inert nitrogen gas (N2) are triggered to do so only in the absence of oxygen. The expansion of OMZs means that growing numbers microbes are removing reactive nitrogen from the ocean, unbalancing the cycle and reducing the availability of essential nutrients for marine life.

What’s more, when bacteria convert Nr to N2 in the presence of small amounts of oxygen — that’s the case in most parts of OMZs — they also produce nitrous oxide (N2O), a greenhouse gas that is about 300 times more powerful than carbon dioxide and also depletes the ozone layer. Multiple studies have found large amounts of N2O rising from the ocean surface above OMZs. This is a classic positive feedback — global warming accelerates production of nitrous oxide, which in turn accelerates global warming.

Finally, it is important to bear in mind that oxygen depletion does not happen in isolation — for example, organisms that consume more oxygen also increase acidification by exhaling more carbon dioxide, and fish trying to escape oxygen-starved water find that alternative locations are too acidic. 

The heat of 3.6 billion atom bombs

“The world’s oceans (especially the upper 2000 m) in 2019 were the warmest in recorded human history…. The past five years are the top five warmest years in the ocean historically with modern instruments, and the past ten years are also the top ten years on record.”[34]

Until the 1970s, the constant flow of energy that Earth receives from the sun was offset by heat reflected back into space, so the planet’s overall energy level did not change very much over time. The amount of incoming solar energy has not changed, but rising concentrations of greenhouse gases are trapping ever more of the reflected heat, preventing it from leaving the atmosphere. Climate scientists call this Earth’s Energy Imbalance.

The excess energy is not distributed evenly through the Earth System. Although global warming is usually expressed as increased air temperatures, the ocean is actually much better at storing heat than the atmosphere — one degree of ocean warming stores over 1000 times as much heat energy as one degree of atmosphere warming — so it isn’t surprising that the ocean has taken up most of the excess solar energy. Just seven percent warms the air and land and melts snow and ice — 93 percent is absorbed by the ocean.[35]

Scientists measure the ocean’s heat content in joules — the amount of energy required to produce one watt of power for one second. In a commentary on the latest data, Lijing Cheng of China’s Institute of Atmospheric Physics calculates that the increase in ocean heat content over the past 25 years required the addition of 228 sextillion joules of heat — that’s 228 followed by 21 zeroes.

“That’s a lot of zeros indeed. To make it easier to understand, I did a calculation. The Hiroshima atom-bomb exploded with an energy of about 63,000,000,000,000 Joules. The amount of heat we have put in the world’s oceans in the past 25 years equals to 3.6 billion Hiroshima atom-bomb explosions.”[36]

That’s about five Hiroshima bombs a second — and the rate is accelerating.

Since 1987 the ocean has warmed 4.5 times as fast as in the previous three decades. The Intergovernmental Panel on Climate Change (IPCC) projects that even if emissions are substantially reduced, by 2100 the ocean will heat 2 to 4 times as much as it has since 1970 — and if emissions are not cut, it will heat 5 to 7 times as much.[37]

By absorbing and storing immense amounts of heat, the ocean delays the impact of Earth’s Energy Imbalance on the global climate system. In oceanographer Grant Bigg’s words, the ocean “acts as a giant flywheel to the climate system, moderating change but prolonging it once change commences.”[38] The price paid for that storage and delay is record-setting ocean heat that is disrupting the world’s largest ecosystem in a multitude of ways.

  • Since 2010, the Atlantic ocean has been hotter than at any time in the past 2900 years.
  • The Arctic is warming two to three times as fast as the rest of the world. Summer sea ice may disappear entirely by 2035.
  • Sea levels are rising, threatening coastal communities and destroying sensitive wetlands. Depending on emission levels, by 2100 the oceans will be from 0.5 to 2.0 meters higher than today.
  • Warmer water contains less oxygen, causing many fish species to shrink. A recent study found an average five percent reduction in maximum body size for each 1.0ºC increase in water temperature.
  • Animal migration towards the poles is happening much faster in the ocean than on land. Marine biodiversity in tropical areas is declining, and food webs in cooler areas are being disrupted by the entry of new species.
  • Populations of organisms that cannot migrate are shrinking. Half of the corals in Australia’s Great Barrier Reef are dead.
  • Hurricanes and tornadoes that form over warmer water tend to be stronger, wetter and more destructive. Climate models indicate that by 2100, the number of Category 5 storms will increase 85% globally and 136% in the Atlantic.

Permanent heatwaves

Most climate change forecasts emphasize long-term global average changes. Those are important metrics, but they can be misleading when the average conceals serious short-term or regional changes and events. For example, although climate negotiations focus on future global average temperatures, regional heatwaves with atmospheric temperatures much higher than historical averages are already increasing in intensity, frequency and duration.[39]

The same is happening in the ocean.

The very idea of marine heatwaves is new: the term itself first appeared in 2011, in a government report on “a major temperature anomaly” in which “water temperatures off the south-western coast of Western Australia rose to unprecedented levels.”[40] As recently as 2015 only five articles in English-language scientific journals had marine heatwave in the title, but in 2019 there were 92 — an increase that reflects what the journal Nature recently said is “the appearance an entirely new subdiscipline: the study of marine heatwaves (MHWs), discrete periods of unusually warm temperatures in the ocean.”[41]

The sudden growth of scientific interest in marine heatwaves is no accident. It reflects a real shift in the ocean’s climate in the past two decades: a radical increase in the frequency, intensity and duration of periods of when water temperatures are much higher than normal. Such extreme events can have devastating impacts on ocean ecosystems: organisms that have evolved to live within a limited temperature range must adapt, flee or die when that range is exceeded.

Marine heatwaves are usually defined as five or more consecutive days in which sea surface temperatures are in the top ten percent of the 30-year average for the region. Using an even stricter definition — temperatures in the top one percent — the IPCC recently concluded that since 1982, marine heatwaves “have doubled in frequency and have become longer lasting, more intense and more extensive,” and “the observed trend towards more frequent, intense and extensive MHWs … cannot be explained by natural climate variability.”[42] Climate scientists at the University of Bern, Switzerland, report that “the occurrence probabilities of the duration, intensity, and cumulative intensity of most documented, large, and impactful MHWs have increased more than 20-fold as a result of anthropogenic climate change.”[43]

The 21st century has seen particularly devastating marine heatwaves in the Mediterranean (2003), Bay of Bengal (2010), western Australia (2011), northwest Atlantic (2012), northeast Pacific (2013–2016), Tasman Sea (2016), and New Zealand (2016). All had profound and lasting impacts on plant and animal life. Off the coasts of western Australia and Tasmania, for example, high temperatures killed massive kelp forests, home to innumerable fish species, and invasive warm water sea urchins then took over the seafloor, preventing kelp and other plants from regrowing.

The 2013–2016 northeast Pacific heatwave was largest, longest and deadliest MHW to date. It was nicknamed The Blob after the 1958 science fiction movie, and, like its space monster namesake, it grew rapidly and destroyed much of the life it enveloped. After forming in the Gulf of Alaska in the autumn of 2013, in less than a year it expanded south to Mexico, ultimately covering about 10 million square kilometers and penetrating up to 200 meters down.

Food webs that have sustained life for millennia collapsed in unprecedented heat. Populations of phytoplankton, copepods, krill and other small heat-sensitive creatures plummeted, and animals that normally eat those creatures, including over 100 million cod and millions of seabirds, starved to death. So did thousands of sea lions when their prey disappeared. Hundreds of kilometers of kelp forests wilted and died. Heat killed 95% of Chinook salmon eggs in the Sacramento River. The largest toxic algae bloom ever seen released deadly neurotoxins, forcing the closure of clam and crab fisheries from Vancouver Island to California.

The Blob finally dissipated in 2016, but intense marine heatwaves continue to affect the northeast Pacific. The second and third largest marine heatwaves ever seen in that area occurred in 2020 and 2019, respectively. As I’m writing this, in October 2020, the latest iteration covers 6 million square kilometers, down from 9 million a month ago.

Until five years ago, no one imagined that a marine “temperature anomaly” might encompass an area as large as Canada and last over two years. Past research on ocean climate change has focused on the effects of long-term changes in average water temperatures but now, as eighteen leading specialists in the field write, “discrete extreme events are emerging as pivotal in shaping ecosystems, by driving sudden and dramatic shifts in ecological structure and functioning.” They warn that marine heatwaves “will probably intensify with anthropogenic climate change [and] are rapidly emerging as forceful agents of disturbance with the capacity to restructure entire ecosystems and disrupt the provision of ecological goods and services in coming decades.”[44]

A major study published in December 2019 projects that the size and frequency of marine heatwaves will increase so much that many parts of the ocean will reach “a near-permanent MHW state” by late in this century. The researchers project that even if greenhouse gas emissions start falling by mid-century, by 2100 about half of the global ocean will experience heatwaves 365 days a year. If emissions don’t decline, by 2100 there will be permanent heatwaves in 90% of the ocean, and over two-thirds of those will be Category IV, the most extreme level. (For comparison: the Blob, which disrupted ecosystems in 10 million square kilometers of the Pacific, killing millions of fish, birds and marine animals and displacing millions more, was only Category III.)

It may then be necessary to introduce new categories, “allowing for identification of increase in ‘extreme extremes,’ as Category V, Category VI, etc.” By 2080, if emissions remain high, the Earth System will be in a “time when the MHW climate has changed completely from the range that species have previously experienced, and represents a qualitatively different climate.”[45]

‘Misery on a global scale’

All by itself, ocean warming is a major threat to the stability of the world’s largest ecosystem — but ocean warming does not occur “all by itself.” The deadly trio of ocean warming, loss of oxygen and acidification are all consequences of disrupting the global carbon cycle. Burning massive amounts of long-buried carbon has changed the ocean’s chemistry, heated the water and driven out oxygen. Those processes take place simultaneously and reinforce each other, making the ocean increasingly inhospitable, even deadly, for living things from microbes to whales.

Worse, the deadly trio isn’t acting alone. Overfishing has wiped out many species, and it’s predicted that most wild fish populations will be 90% depleted by 2050. Pollutants, including tons of plastics that essentially last forever, are poisoning marine life from coastlines to the deepest trenches. Nitrogen fertilizer run-off has created a thousand or more dead zones in coastal waters and estuaries. Off-shore oil wells are leaking deadly hydrocarbons, and mining companies are preparing to dredge rare minerals from the deep sea floor, destroying some of the few remaining undamaged parts of Earth’s surface.

As environmental geologists Jan Zalasiewicz and Mark Williams write, “a wholesale refashioning of the marine ecosystem” is now underway. If business as usual continues, “pervasive changes in the physical, chemical and biological boundary conditions of the sea … [will] transform, irreversibly, and for the worse, the Earth and its oceans.”[46]

The effect of that transformation was summed up by Agence France-Presse, in its account of the IPCC’s 2019 report on the oceans: “The same oceans that nourished human evolution are poised to unleash misery on a global scale unless the carbon pollution destabilizing Earth’s marine environment is brought to heel.”[47]


[1] Sylvia A. Earle, The World Is Blue: How Our Fate and the Oceans Are One (Washington, DC: National Geographic, 2010), 20.

[2] Sylvia A. Earle, Sea Change: A Message of the Oceans (New York: Ballantine Books, 1995), xii.

[3] Jelle Bijma et al., “Summary of ‘Climate Change and the Oceans.’”

[4] Nicolas Gruber, “Warming Up, Turning Sour, Losing Breath: Ocean Biogeochemistry Under Global Change,” Philosophical Transactions of the Royal Society A, May 2011, 1980, 1992.

[5] Jelle D. Bijma et al., “Climate Change and the Oceans — What Does the Future Hold?” Marine Pollution Bulletin Sept., 2013.

[6] Interviewed in John Collins Rudolf, “Q. and A.: For Oceans, Another Big Headache.” New York Times, May 5,  2010. 

[7] Rachel L. Carson, The Sea Around Us (New York: Oxford University Press, 2018 [1950]), 163-4.

[8] More precisely, there are 30% more hydrogen (H+) ions.

[9] Ken Caldeira and Michael E. Wickett, “Anthropogenic Carbon and Ocean pH,” Nature, Sept. 25, 2003, 365; Royal Society, Ocean Acidification Due to Increasing Atmospheric Carbon Dioxide (London: Royal Society, 2005), 39.

[10] Some argue that a mass extinction has already begun.

[11] Callum Roberts, The Ocean of Life: The Fate of Man and the Sea (New York: Penguin, 2013), 108,110.

[12] Lyndsey Fox et al., “Quantifying the Effect of Anthropogenic Climate Change on Calcifying Plankton,” Scientific Reports, January 31, 2020.

[13] N. Bednaršek et al., “Extensive Dissolution of Live Pteropods in the Southern Ocean,” Nature Geoscience, (December 2012) 881, 883.

[14] Matthias Hofmann and Hans Joachim Schellnhuber, “Ocean Acidification: A Millennial Challenge,” Energy & Environmental Science (September 2010), 1888-89

[15] This is also true of humans. Our normal blood pH is 7.4: a drop of 0.2 can be fatal.

[16] See, for example, Martin C. Hänsel et al., “Ocean Warming and Acidification May Drag down the Commercial Arctic Cod Fishery by 2100,” PLOS ONE, April 22, 2020. For a summary of research on biological and other effects of ocean acidification, see An Updated Synthesis of the Impacts of Ocean Acidification on Marine Biodiversity, published by the Secretariat of the Convention on Biological Diversity. 

[17] Sabine Mathesius et al., “Long-term Response of Oceans to CO2 Removal from the Atmosphere,” Nature Climate Change, December 03, 2015, 1107-14.

[18] “Monaco Declaration,” proceedings of Second International Symposium on the Ocean in a High-CO2 World (Unesco, 2008).

[19] Johan Rockström et al., “Planetary Boundaries: Exploring the Safe Operating Space for Humanity,” Ecology and Society 14, no. 2 (2009)

[20] Ove Hoegh-Guldberg et al., “The Ocean,” in Climate Change 2014: Impacts, Adaptation, and Vulnerability. (Cambridge University Press, 2014), 1658.

[21] IPCC, Special Report on the Ocean and Cryosphere in a Changing Climate (2019), 59, 66.

[22] Howard Lee, “Ocean Oxygen – Another Climate Shoe Dropping,” Skeptical Science, May 18, 2016.

[23] Ian Angus, “Dead Zones: Industrial Agriculture versus Ocean Life,” Climate & Capitalism, August 12, 2020.

[24] Cold water also joins the conveyor near Antarctica, and the return path carries warm water from the tropics to the north. Any selected “beginning” on this vast metabolic cycle is arbitrary.

[25] Changyu Li et al., “Increasing Escape of Oxygen From Oceans Under Climate Change,” Geophysical Research Letters, June 2020.

[26] Scientific Committee on Oceanic Research, “How Oxygen Minimum Zones Form”; A. Paulmier and D. Ruiz-Pino, “Ocean Minimum Zones (OMZs) in the Modern Ocean,” Progress in Oceanography 80, no. 3-4 (2009), 113-128.

[27] L. Caesar et al., “Observed Fingerprint of a Weakening Atlantic Ocean Overturning Circulation,” Nature 556, April 12, 2018; S. Rahmstorf, “New Studies Confirm Weakening of the Gulf Stream Circulation (AMOC),” Real Climate, September 17, 2020.

[28] Historical sidelight: the formula for calculating the effect of increased heat on the speed of chemical reactions was discovered by Svante Arrhenius, the Swedish scientist who was the first to show, in 1896, that CO2 emissions from burning fossil fuels could cause global warming.

[29] John G. Shepherd et al., “Ocean Ventilation and Deoxygenation in a Warming World: Introduction and Overview,” Philosophical Transactions of the Royal Society A, September 07, 2017, 6.

[30] Quoted in “The Ocean Is Losing Its Breath. Here’s the Global Scope,” SERC news release, January 4, 2018.

[31] Chadlin M. Ostrander, Jeremy D. Owens, and Sune G. Nielsen, “Constraining the Rate of Oceanic Deoxygenation Leading Up to a Cretaceous Oceanic Anoxic Event (OAE-2: ~94 Ma),” Science Advances, August 9, 2017.

[32] David J. Hughes et al., “Coral Reef Survival Under Accelerating Ocean Deoxygenation,” Nature Climate Change, March 2020.

[33] Guy Claireaux and Denis Chabot, “The Significance of Ocean Deoxygenation for the Physiology of Marine Organisms,” in Ocean Deoxygenation: Everyone’s Problem, ed. D. Laffoley and J. M. Baxter (Gland, Switzerland: IUCN, 2019), 461.

[34] Lijing Cheng et al., “Record-Setting Ocean Warmth Continued in 2019,” Advances in Atmospheric Sciences, February 2020.

[35] Kate S. Zaital, “Disrupting the Deep: Ocean Warming Reaches the Abyss,” Earth, March 8, 2018.

[36] Chinese Academy of Sciences, “Record-setting Ocean Warmth Continued in 2019,” News Release, January 14, 2020.

[37] Lijing Cheng et al., “Record-Setting Ocean Warmth Continued in 2019,” Advances in Atmospheric Sciences, February 2020.

[38] Grant R. Bigg, The Oceans and Climate, 2nd ed. (Cambridge Univ. Press, 2006), x.

[39] S. E. Perkins-Kirkpatrick and S. C. Lewis, “Increasing Trends in Regional Heatwaves,” Nature Communications 11 (July 2020)

[40] A. Pearce et al., The “Marine Heat Wave” Off Western Australia During the Summer of 2010/11 (Western Australian Fisheries and Marine Research Laboratories, 2011), 1. The quote marks around “Marine Heat Wave” indicate that this was not yet the accepted term.

[41] Mark R. Payne, “Metric for Marine Heatwaves Suggests How These Events Displace Ocean Life,” Nature 584 (August 8, 2020), 43.

[42] Intergovernmental Panel on Climate Change, Special Report on the Ocean and Cryosphere in a Changing Climate(IPCC, 2019), 67, 607.

[43] Charlotte Laufkötter, Jakob Zscheischler, and Thomas L. Frölicher, “High-impact Marine Heatwaves Attributable to Human-induced Global Warming,” Science 389 (September 25, 2020), 1621.

[44] Dan A. Smale et al., “Marine Heatwaves Threaten Global Biodiversity and the Provision of Ecosystem Services,” Nature Climate Change 9, no. 4 (March 04, 2019).

[45] Eric C. J. Oliver et al., “Projected Marine Heatwaves in the 21st Century and the Potential for Ecological Impact,” Frontiers in Marine Science 6 (December 2019). doi:10.3389/fmars.2019.00734). The study used the IPCC emissions scenarios RCP4.5 and RPC8.5, and the climate modelling system CMIP5.

[46] Jan Zalasiewicz and Mark Williams, “The Anthropocene Ocean in Its Deep Time Context,” in The World Ocean in Globalisation, ed. Davor Vidas and Peter Johan Schei (Leiden: Brill, 2011), 34.

[47] “Oceans Turning From Friend to Foe, Warns Landmark UN Climate Report,” Agence France Presse, August 29, 2019.