APES 9.3-9.4: Ocean Warming and Acidification

Thermocline

A transition zone in which ocean temperature changes substantially with depth. It also separates regions with different nutrient movement, biological productivity, and water properties.

Ocean heat content (OHC)

The thermal energy stored by the ocean, usually reported as a change or anomaly relative to a baseline. It is an important indicator of global warming because the ocean absorbs most of the excess energy in Earth's climate system.

Specific heat capacity

The amount of heat required to raise the temperature of one unit of mass of a substance by one degree. The relationship for heating a sample is $q = mc\Delta T$, where $q$ is heat, $m$ is mass, $c$ is specific heat capacity, and $\Delta T$ is temperature change.

Why can the oceans absorb large amounts of excess energy without an equally large temperature increase?

Water has a high specific heat capacity, so a large amount of energy can be transferred to the ocean with a relatively small change in temperature. The oceans also cover about 70% of Earth's surface.

How can the equation $q = mc\Delta T$ be used to compare ocean warming with warming of land?

For the same absorbed heat and mass, a substance with a larger $c$ has a smaller $\Delta T$. Because water has a high specific heat capacity, the ocean warms more slowly than land.

What is the difference between ocean heat content and ocean heat uptake?

Ocean heat content is the energy stored in the ocean, while ocean heat uptake refers to the transfer of additional energy into the ocean. Positive changes in OHC indicate net ocean heat uptake.

Why is ocean heat content generally reported as an anomaly or change rather than as an absolute value?

A change relative to a baseline makes it easier to compare warming over time and among ocean regions. Positive anomalies indicate that the ocean contains more thermal energy than during the reference period.

What does a persistent positive imbalance in Earth's energy budget imply for the ocean?

More energy is entering the climate system than leaving it, so the ocean gains net heat over time. The added energy can be stored near the surface or transported into deeper water.

How do greenhouse gases cause ocean warming?

Increased greenhouse gases reduce outgoing infrared radiation from Earth's atmosphere, producing a positive energy imbalance. The ocean absorbs most of this added energy because of its large heat capacity and extensive volume.

What fraction of Earth's excess energy from global warming has been stored in the ocean since the 1970s?

More than 90% of the excess energy has been taken up by the ocean. This results primarily from human-caused greenhouse gas emissions reducing the rate at which infrared energy escapes to space.

How is total ocean heat content calculated conceptually?

Temperature, density, and heat capacity are combined throughout the three-dimensional volume of the ocean. Mathematically, this is a volume integral of the energy density over the measured region.

Areal density of ocean heat between two depths

The heat stored per unit horizontal area between depths $h_1$ and $h_2$ can be represented as $H = c_p\int_{h_1}^{h_2}\rho(z)\Theta(z)\,dz$, where $c_p$ is seawater heat capacity, $\rho$ is density, and $\Theta$ is conservative temperature. Its SI units are $\mathrm{J\,m^{-2}}$.

Why does calculating ocean heat content require measurements of density as well as temperature?

Heat stored depends on the amount of water in a given volume and its temperature. Density helps determine the mass of seawater, while heat capacity and temperature change determine the energy associated with that mass.

Conservative temperature in ocean heat calculations

Conservative temperature is a temperature variable referenced to a standard pressure, allowing seawater heat exchanges to be compared more consistently. Earlier calculations often used potential temperature.

What are the approximate temperature-based layers of the ocean?

The upper mixed layer extends roughly from 0–200 m, the thermocline from about 200–1500 m, and the deep ocean below approximately 1500 m. These boundaries vary with latitude and season.

Upper mixed layer

The near-surface ocean layer whose temperature is relatively uniform because waves, wind, and turbulence mix the water. It receives energy directly from sunlight and exchanges heat with the atmosphere.

Why is the thermocline generally deeper in tropical or summer conditions and shallower near the poles?

Strong solar heating and surface mixing can warm and mix a thicker upper layer, placing the thermocline deeper. In polar regions, weak sunlight and cold surface water reduce the depth of the warm upper layer.

Argo profiling floats

Argo is a global network of robotic floats that repeatedly measure ocean temperature, salinity, and pressure. During a typical cycle, a float profiles from about 2000 m to the surface, transmits its data by satellite, and repeats the process.

Why are satellite observations used together with in situ ocean measurements?

Profiling floats provide detailed measurements at particular locations and depths, whereas satellites provide broad spatial coverage or integrated properties. Combining them improves estimates of ocean heat content and sea-level change.

Why can natural climate variability make long-term ocean warming trends difficult to detect?

Phenomena such as El Niño–Southern Oscillation and volcanic eruptions can temporarily redistribute or reduce the measured heat in parts of the ocean. Limited historical spatial coverage also increases uncertainty in older data.

What long-term pattern has been observed in global ocean heat content?

Global ocean heat content has shown a sustained upward trend since at least the 1970s, with warming in the upper 700 m and very likely warming from 700–2000 m. Deep-ocean warming below 2000 m is also likely, especially in the Southern Ocean.

Approximately how much of the added ocean heat had reached below 700 m by 2020?

About one-third of the added energy had propagated below 700 m, demonstrating that warming is not confined to the surface.

Why is the Southern Ocean especially important for global ocean heat uptake?

A disproportionately large fraction of excess heat is stored in the Southern Ocean, including at depth. Its circulation transports heat into the ocean interior and influences sea ice, Antarctic ice shelves, and global overturning circulation.

How does ocean heat move below the surface?

Heat is transported downward through thermal conduction, sinking or downwelling of water, and ocean circulation. Winds and currents redistribute heat horizontally and vertically.

How can La Niña conditions change the vertical distribution of ocean heat?

Changes in wind circulation and ocean currents during La Niña can transport substantially more heat from the upper ocean into deeper layers. Thus, surface warming may temporarily slow even while heat continues accumulating in the ocean.

Why can ocean warming be considered relatively irreversible on human time scales?

The ocean stores enormous thermal energy and releases it slowly because of its high heat capacity and slow circulation. Even if atmospheric forcing decreases, returning the ocean to its previous thermal state would take a very long time.

What is the relationship between evaporation and ocean heat content?

Evaporation transfers thermal energy from the ocean to the atmosphere as water changes from liquid to gas. This energy transfer helps power the water cycle and can later influence storms and precipitation.

Why does land surface temperature generally rise faster than ocean surface temperature?

Land has lower effective thermal inertia and generally less capacity to store and redistribute heat than the ocean. Water also removes heat through mixing and evaporation, moderating surface temperature changes.

Thermal expansion

The increase in volume that occurs when a substance is heated. For a liquid such as seawater, warming increases the average molecular motion and spacing, raising sea level even without adding water.

How does ocean warming contribute to sea-level rise?

Warming seawater expands, a process called thermal expansion. This effect accounted for roughly 30–40% of global sea-level rise from 1900 to 2020; melting land ice contributes additional rise.

Why does melting sea ice have a different direct effect on sea level than melting land ice?

Floating sea ice already displaces seawater, so its melting has little direct effect on sea level. Melting land ice adds water to the ocean and therefore raises sea level.

How does ocean warming amplify polar warming through albedo changes?

Warming melts reflective sea ice and glaciers, exposing darker ocean or land. The lower albedo causes more solar radiation to be absorbed, creating a positive feedback that enhances warming.

What ecological changes are associated with warming oceans?

Ocean warming can cause coral bleaching, marine heat waves, and migration of marine species. It can also disrupt coastal ecosystems and communities that depend on ecosystem services.

What is a marine heat wave?

A marine heat wave is a prolonged period during which ocean temperatures in a region are unusually high. Such events can stress or kill marine organisms and alter species distributions.

How can high sea-surface temperatures influence extreme weather?

Warm surface water supplies energy and moisture to the atmosphere through evaporation. This can intensify tropical cyclones, atmospheric rivers, and heat waves that extend over land.

Henry's law and warming ocean water

At a given pressure, the amount of gas dissolved in a liquid generally increases with the gas's partial pressure above the liquid. Because gas solubility usually decreases as temperature rises, warmer surface water is less able to retain dissolved oxygen and absorb atmospheric gases.

Why can ocean warming reduce dissolved oxygen concentrations?

Gases are generally less soluble in warmer liquids, so warming decreases the equilibrium solubility of oxygen in surface seawater. Changes in circulation and biological processes can further contribute to ocean deoxygenation.

How can warming affect the ocean's ability to absorb carbon dioxide even if atmospheric $CO_2$ continues to increase?

Increasing atmospheric $CO_2$ raises the driving force for absorption, but warming lowers the solubility of gases in seawater. These competing effects mean warmer surface water is intrinsically less capable of dissolving gases.

How does absorption of atmospheric carbon dioxide cause ocean acidification?

Dissolved $CO_2$ reacts with water to form carbonic acid: $CO_2(aq) + H_2O(l) \rightleftharpoons H_2CO_3(aq)$. Carbonic acid dissociates to produce hydrogen ions, $H_3O^+$, lowering ocean pH.

Ocean acidification

The decrease in ocean pH caused primarily by absorption of atmospheric carbon dioxide. Increasing $[H_3O^+]$ shifts carbonate equilibria toward bicarbonate and reduces the availability of carbonate ions needed by many organisms to form calcium carbonate.

Why does ocean acidification particularly affect organisms that form calcium carbonate shells or skeletons?

Higher $[H_3O^+]$ reduces the concentration of carbonate ions, $CO_3^{2-}$, and can promote dissolution of calcium carbonate. As a result, calcification and shell or skeleton growth may become more difficult.

Why does a decrease of one pH unit represent a major change in ocean acidity?

The pH scale is logarithmic: a decrease of one pH unit represents a tenfold increase in hydrogen-ion concentration. Therefore, even a seemingly small numerical decline in ocean pH indicates a substantial increase in acidity.

How does ocean acidification alter the carbonate system?

Additional dissolved $CO_2$ increases carbonic acid and hydrogen-ion concentrations. Hydrogen ions react with carbonate ions to form bicarbonate, reducing the carbonate available for calcium carbonate formation.

What is the effect of ocean acidification on coral reefs?

Ocean acidification can reduce coral calcification and make it more difficult for reefs to build and maintain calcium carbonate structures. Combined with warming, which causes coral bleaching, acidification can increase stress on coral reef ecosystems.

Which marine organisms are especially vulnerable to ocean acidification?

Calcifying organisms such as corals, oysters, clams, mussels, sea urchins, and some plankton are especially vulnerable because they need carbonate ions to build shells or skeletons. Impacts can spread to predators and other species through marine food webs.

How can ocean acidification affect marine food webs and human societies?

Reduced survival or growth of calcifying organisms can alter prey availability and disrupt food webs. Declines in shellfish and reef ecosystems can also harm fisheries, aquaculture, tourism, and coastal communities.

What is the most direct way to reduce ocean acidification?

Reducing human emissions of carbon dioxide is the most direct long-term solution because less atmospheric $CO_2$ will be absorbed by seawater. Limiting warming also helps preserve the capacity of marine ecosystems to cope with acidification.