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Title Image Credit: Andy Schmid/@andy.schmid



Sea Level Rise is Only Part of the Story



Spilhaus projection of the world's oceansFigure 1  Spilhaus map projection of the World Ocean    Image Credit: Hannah Blondin, Hopkins Marine Station


June is NOAA Ocean Month, which reminded CAC staff of the numerous posts in which we noted the moderating effect of the ocean on anthropogenic (human-caused) climate change—storing much of the heat trapped by greenhouse gases (GHGs) and absorbing millions of tons of carbon dioxide (CO2). This article takes a deep dive (couldn’t resist) into the long term impact of these processes on the ocean itself.

While we routinely talk about the North Atlantic, Pacific Ocean, Caribbean Sea, etc. as if they were independent entities, Figure 1, above, reminds us that there is really just one global ocean. Water is moving continuously throughout the global ocean, powered by winds and the currents of the ThermoHaline Circulation. Although we often take it for granted, the global ocean provides essential services to the global biosphere, including food, energy and climate regulation.

The global ocean is a major component of the climate system. The oceans help regulate climate by taking up heat and redistributing it across the globe, by providing moisture to the atmosphere, and by absorbing CO2 from the atmosphere. The oceans are by far the largest active carbon reservoir on the planet, storing about 38,000 billion tonnes of carbon. By comparison, this is 16 times more than the quantity of carbon stored by the land biosphere and about 60 times the amount of carbon in the preindustrial atmosphere.

In previous posts, we investigated the impact of warming oceans on sea level rise, Antarctic glaciers, marine heatwaves and coral bleaching. Here are a few more reasons why the global ocean needs to be a key part of our efforts to manage climate change:

    • the ocean covers more than 70% of the earth’s surface, roughly 140 million square miles (362 square kilometers)
    • it represents 95% of the earth’s biosphere
    • it is the biggest source of wild or domestic protein in the world. Ocean fisheries bring in over 100 million tons of fish annually.
    • the ocean absorbed 91% of the excess heat energy trapped by anthropogenic GHGs over the last 50 years
    • the ocean absorbs nearly 30% of anthropogenic CO2
    • heat stored in the deep ocean will be released eventually, melting ice, evaporating water, or directly reheating the atmosphere.
    • without the ocean, Earth would be nearly 65°F/36°C hotter

The ocean’s ongoing interaction with our GHG-boosted climate is causing:

  1. extreme heating,
  2. loss of oxygen as the ocean warms, and
  3. increasing acidification from dissolved CO2

Ocean heating drives both declining oxygen and acidification, so let’s start there.

Ocean Heating

Solar radiation warms the surface waters of the ocean, augmented by the excess heat energy trapped by anthropogenic GHGs. The ocean is constantly in motion, redistributing the warming surface water and mixing it with cooler, deeper water.

The ocean is able to moderate (but not prevent) climate warming driven by the excess heat energy trapped by anthropogenic greenhouse gases for three reasons:

  • the ocean covers 70% of the earth’s surface, allowing it to capture much of the energy reaching the earth,
  • the large volume of the ocean means it has the capacity to absorb a great deal of energy without reaching its limit, and
  • water has a uniquely high heat capacity (it takes a lot of energy to increase its temperature).

However, the inevitable consequence of absorbing all that energy is ocean heating. Since interaction with the incoming energy begins at the surface of the ocean, sea surface temperatures (SSTs) around the world are rising quickly in response. Figure 2 is a plot of daily global average SSTs since 1979 (between 60N and 60S, i.e., the ice-free ocean), showing just how much and how rapidly temperatures have increased in recent years. (Note that the graph covers  12 month periods from June through May.)


sea surface temperature anomaly 2023-2024

Figure 2  The most recent 12 months, June 2023 through May 2024, with global SST at record levels.


May 2024 highlights:

  • May 2024 was the fourteenth month in a row that the global average SST has been the warmest in the ECMWF and NOAA data records for the respective month of the year. 
  • in the Northern Hemisphere,  the ocean temperature was again record-high by a wide margin (0.25°C/0.45°F warmer than the previous record set in 2020)
  • In the Southern Hemisphere, the May SST average was the highest on record.

Meanwhile, global average temperatures (land and ocean) for the June 2023-May 2024 period were the highest on record, an astounding 1.63°C (2.93°F) above the 1850-1900 preindustrial average. Preliminary analysis suggests that there is a 60% chance that 2024 will be the hottest year on record.

We are rapidly overtaking the Paris Agreement goal of a 1.5°C long term average, with no sign of slowing down.

Figure 3 shows the stunning breadth of warming across the globe in the last 12 months. Looking at the average temperatures for the 2023-2024 period relative to the 1991-2020 average (the most recent 30 year meteorological average) shows massive swaths of record high temperatures on land and sea, and very few locations cooler than the reference period.

map showing record temperatures from June 2023 through May 2024

Figure 3: Deep red indicates areas that experienced the hottest June 2023 through May 2024 (36% of Earth’s surface).  Credit: Brian Brettschneider/@climatologist49


The heat energy crossing the atmosphere/ocean boundary gradually diffuses through the ocean, aided by currents and overturning processes. Just as the ocean is warming slower than the atmosphere, so is the deep ocean warming slower than the upper ocean, as shown in Figure 4, below. The quantity of thermal energy accumulated in the ocean is referred to as the Ocean Heat Content, or OHC. The heat content is expressed in joules, an international standard unit of energy. (For comparison, a 60 watt light bulb uses 60 joules per second.)

Global ocean heat content graph 1950-2023

Figure 4: Annual global ocean heat content accumulated since 1950 (in zettajoules – billion trillion joules, or 10²¹ joules) for the 0-700 meter and 700-2,000 meter layers. Data from Cheng et al. (2024). Image Credit:  Carbon Brief.


In 2023 OHC in the upper 2000 meters increased by about 15 zettajoules. To put that into perspective, our total global energy  consumption totals about 0.6 zettajoules annually. Since 1950, OHC has increased by about 475 zettajoules.

Figure 4 also shows an accelerated rate of warming in the upper 2000 meters starting in the early 1990’s, in sync with the observed acceleration of GHG emissions.

The trend in the heat content of the upper 2000 m of ocean from 1993 to 2023 (Figure 5) shows a worldwide increase in OHC. The highest rates of warming are seen in the Gulf of Mexico and the Caribbean, and in the warm, deep and fast-flowing “boundary currents” that form on the west side of wind-driven gyres in the Atlantic (the Gulf Stream) and the Pacific (the Kuroshio Current). High rates of heating are also seen in the Arctic Ocean and along the Antarctic circumpolar current. The North Atlantic Ocean is one of only a few areas showing decreases in heat storage, thanks in part to the persistent “cold blob” just south of Greenland.

ECMW-global heat content trend through 2023Figure 5:  Trend in the ocean heat content in the upper 2000 m, from 1993 to 2023  Credit: ECMWF/C3S.


Closer to home for those of you in Florida, temperatures in the Caribbean are extraordinarily high this year, running around 84.7°F by late May, a weekly temperature not seen in 2023 until August 2nd. Another warning sign for the 2023 hurricane season.

Caribbean Ocean heat content as of June 24, 2024

Figure 6:  Caribbean OHC setting monthly records. June OHC is already at a level not reached until early August, 2023 (the red dashed line).  Credit: Brian McNoldy/@BMcNoldy


Loss of Oxygen

Oxygen is just as important to life in the ocean as it is on land. Humans rely on oxygen gas inhaled from the atmosphere, while fish absorb oxygen dissolved in water through their gills, directly into their bloodstream.

That dissolved oxygen enables a global commercial fishing industry, harvesting over 179 million tons of wild and farmed seafood each year. In the U.S. alone, commercial fishermen  landed 9.3 billion pounds of seafood valued at $5.5 billion in 2019. The aquaculture industry added another 110 million pounds, valued at $1.5 billion. So the potential impact of ocean heating on the oxygen content of the ocean is of great interest.

Fish are very sensitive to lower oxygen levels in warming waters. As the ocean gets warmer, their metabolism ramps up, requiring more oxygen, yet oxygen levels are dropping as the water warms. Larger fish are more affected simply because their gills are relatively small compared to their body size. On the other hand, large fish are often highly migratory, and increased warming could result in the species moving colder waters nearer the poles or deeper in the ocean. A recent study of migratory predatory fish such as tuna, marlin and swordfish predicts that those species could lose 70% of suitable habitat by the end of the century.

Oxygen in the Ocean

Ocean warming affects the ocean and its dissolved oxygen content in several ways. The ease with which oxygen is absorbed by the ocean is determined by its “solubility.” In general, the solubility of gases in liquids decreases as temperature increases. The added heat gives the gas molecules more energy to escape or resist attraction to water molecules. So the warmer the water, the lower the oxygen content.

Oxygen in the atmosphere enters the ocean at the air-sea interface, diffusing slowly along the surface or mixing in through turbulence caused by wind, waves, currents or tides. The near surface waters in the upper 100 meters of the ocean are usually saturated with oxygen from the atmosphere. More oxygen is generated in the surface mixed layer by oceanic plankton—drifting plants, algae, and bacteria capable of photosynthesis, which produce oxygen in much the same way as land-based plants. Excess oxygen is released into the atmosphere. Roughly half of the global oxygen production originates in the upper 100 meters of the ocean.

The ocean is stratified into different layers, defined by temperature. The surface mixed layer is the warmest, while the deep water is much colder. The transition layer between the surface water is called the thermocline, a zone in which the temperature transitions rapidly from the warm mixed layer temperature to the colder deep water temperature. Oxygen levels in the thermocline are typically the lowest in the ocean water column for two reasons:

  • organic matter falling from the mixed layer is consumed by aerobic bacteria, using oxygen in the process
  • water in the thermocline doesn’t usually circulate back to the mixed layer to replenish the lost oxygen.

Most of the sinking organic matter is consumed in the top 1,000 m (3,300 ft) of the ocean. Thus, the deeper ocean below 1,000 m has more oxygen because the rate of oxygen consumption is low, while it is supplied with cold, oxygen-rich deep water from polar regions.

In the absence of active circulation to bring in more oxygenated water (e.g., currents, upwelling), an “Oxygen Minimum Zone” (OMZ) can develop.

Oxygen Minimum Zones

OMZs are low-oxygen areas in the open ocean that occur naturally, between 100 and 1,500 m. They are often found between the more oxygen-rich layers, the surface mixed layer and the deep ocean. Pronounced stratification, or separation into horizontal layers, and reduced mixing in these layers allows oxygen minimum zones to persist for many years, their extent often varying with the seasons.

In coastal waters and estuaries, increased injection of nutrients and organic matter by runoff from agricultural areas and sewer discharge accelerates microbial consumption of oxygen, often leading to low-oxygen zones.

Figure 7 shows the global distribution of open ocean (blue) and coastal (red) low oxygen zones. Permanent OMZs exist in the Eastern South Pacific, and the Eastern Tropical North Pacific in the Pacific Ocean, and the Arabian Sea and the Bay of Bengal in the Indian Ocean. Seasonal OMZs occur in the West Bering Sea and the Gulf of Alaska. All told, the permanent OMZs cover 30.4 million square km (11.7 million square miles)—8% of the global ocean.

Oxygen Minimum Zones

Figure 7:  Global distribution of low O2 areas (i.e., O2 < 62 μmol kg–1) in the coastal and global ocean (from Breitburg et al, 2018). In the coastal area, more than 500 sites have been found with low O2 conditions in the past half century (red dots) while in the open ocean the extent of low O2 waters amounts to several million km3 (the blue dots refer to conditions at 300 m).

Warming of the Global Ocean

Climate warming is, of course, driving ocean warming. A warming ocean reduces the ability of near-surface waters to hold high concentrations of oxygen. Warming at the surface can also reduce circulation between ocean layers by increasing stratification, which decreases the rate at which oxygen is replenished in lower layers. The upper boundary of an OMZ then becomes closer to the surface and encroaches on organisms that require higher concentrations of oxygen to survive. In the tropical northeast Atlantic, this process is driving tuna into a narrower layer of water as oxygen declines—losing 15 percent of their habitat from 1960 to 2010.

Between 1960 and 2010, global average oxygen levels in the ocean dropped by more than 2% (as much as 40% in some tropical locations), and the volume of OMZs increased by 3-8%.  At this rate of climate warming, the tropical Pacific OMZ could expand by 6-8 million cubic kilometers (1.4-1.9 million cubic miles.) Oxygen levels are expected to drop a further 1–7% by 2100.

Figure 8 shows the distribution of the 1960-2010 2% global oxygen loss in the upper ocean (surface-1,200 m) and the lower ocean (1,200 m to the ocean floor).

Maps of oxygen depletion
Figure 8:  Dissolved Oxygen (DO) change in the ocean. Observational estimate of the 50yr (1960 to 2010) oxygen change in the upper (0-1,200 m) and deep (1,200 m – sea floor) ocean in micromole per kilogram per decade (µmol/kg/decade).  Lines indicate boundaries of oxygen minimum zones with less than 80 µmol/kg of oxygen anywhere within the water column (dash-dotted), less than 40 µmol/kg (dashed) and less than 20 µmol/kg (solid).  Image Credit: Oschlie et al (2018)


Recall that the effect of climate warming on the oceans lags well behind the rest of the planet. A recent study suggests that we have only seen a quarter of the committed ocean deoxygenation response to GHG emissions to date.

The inertia in the ocean’s response means that ocean warming to date is irreversible in this century. By 2100, the projected warming of the upper 2,000 m of the ocean is projected to be 2 to 6 times the warming observed so far.

At present we can expect continued warming and further deoxygenation in the upper ocean. More extreme heating in the mixed surface layer will lead to more stratification, further inhibiting oxygen transfer below the surface mixed layer.

On a larger scale, warming is changing ocean circulation, which will slow the large-scale mixing of oxygen-rich surface waters with deeper water. As the ocean warms, organisms will respond with accelerated metabolisms and respiration, thus increasing oxygen consumption.


Ocean acidification, heating and deoxygenation are mechanistically linked by the increase in atmospheric CO2. Emissions of GHGs, principally CO2, lead to ocean heating, which promotes deoxygenation. Meanwhile the ocean is absorbing 30% of anthropogenic CO2 emissions, rendering it more acidic.

Ocean acidification refers to the ongoing decrease in the pH of the global ocean. When CO2 is absorbed by the ocean, it forms carbonic acid and bicarbonate, which contribute to the ocean’s increased acidity. Figure 9 shows the link between atmospheric CO2, CO2 absorbed by the ocean and the acidity of ocean water
graph of CO2 in the oceanFigure 9:  Graph illustrating the correlation between rising levels of carbon dioxide (CO2) in the atmosphere (red) at the Mauna Loa observatory off Hawaii with rising CO2 levels in the ocean. As more CO2 accumulates in the ocean (green), the pH of the ocean (blue) decreases.   Image Credit: NOAA/PMEL


Over the last million years, average surface seawater pH has been relatively stable, varying from 8.3 during cold periods (e.g. during the last glacial maximum 20,000 years ago), and 8.2 during warm periods (e.g. just prior to the industrial revolution). The Anthropogenic emissions have increased atmospheric CO2 concentration by more than 40% above the preindustrial level. According to the European Environment Agency, half of that increase has occurred since the 1980s, causing global average surface sea water pH to decrease from 8.11 to below 8.05 in 2021 corresponding to a 15% increase in acidity since 1985 and a 40% increase since preindustrial levels.

Lowering the pH of seawater affects the ability of shell-building organisms to build and maintain their shells. These include crab, oysters, shrimp, clams, lobsters, sea urchins, corals and some types of plankton. Pteropods, one of the foundational elements of the marine food chain in Alaska, are expected to be particularly sensitive to ocean acidification. Almost half the seafood in the US comes from Alaskan waters. Both direct and indirect effects of ocean acidification could have serious implications, both for the species being harvested and for the food web that supports them.


Looking Ahead

We’ll use two cautionary events from the past to see what we might expect in the oceans going forward. One from 252 million years ago, and one from a decade ago.


The geological record reveals skyrocketing ocean heat, increasing acidification and falling oxygen levels at the end of the Permian period, about 252 million years ago, when Earth experienced the largest known extinction event in history—the Great Dying.

By the end of the Permian period, oceans were so hostile to life that an estimated 96% of marine species died off. Recent research shows that the “Great Dying” was the consequence of greenhouse gas-driven global warming. Before the die-off, the Permian ocean had temperatures and oxygen levels similar to the present day. Then extensive volcanic eruptions in Siberia, spanning the Triassic-Permian boundary, created a greenhouse gas planet, not dissimilar to today’s. As Permian ocean temperatures rose, and the metabolism of ocean animals accelerated, the warmer waters could not hold enough oxygen for the majority of them to survive.

We are a long way from the volcanic GHG scenario of the “Great Dying.” But to quote the authors:

Because similar environmental alterations are predicted outcomes of current climate change, we would be wise to take note.

The Blob

Much more recently than the Great Dying, the eastern Pacific Ocean was host to a massive marine heat wave that came to be known as… “The Blob.” (Not to be confused with the North Atlantic “Cold Blob“)

The Blob began in 2013 as an anomalous pool of warm water off the coast of Alaska. The pool persisted through the winter in the sub-Arctic Bering Sea and the Gulf of Alaska, and then rapidly expanded south along the Pacific coast. By the summer of 2014 the enormous heat wave, now named The Blob, encompassed the Pacific coast from Mexico to Alaska. It persisted through 2015, paired with a stationary high pressure ridge off the coast that blocked the winter storms that typically churn up the water off the coast, encouraging upwelling of the nutrient-rich waters of the deep ocean. But conditions were ideal for toxic algae blooms.

Hakai Magazine  summed it up: “[The] vast and potent heatwave wreaked havoc on marine ecosystems: thousands of seabirds died, while blooms of harmful algae poisoned marine mammals and shellfish. The suddenly warmed water also brought an influx of new animals to the northeast Pacific: ocean sunfish appeared in Alaska,…”

Different circumstances than 2024, but another cautionary example of the potential havoc that lies within a warming ocean.



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