Lesson 3 of 6 · ocean heat and chemistry
Distinguish ocean warming from changing chemistry
10 minute lesson · 3 checks along the way
Your goal for this lessonInterpret heat storage and acidification with correct quantities, scales and measurement limits.
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Temperature does not inventory all the energy
Earth’s ocean exchanges energy with the atmosphere and stores a large share of the extra energy accumulating in the climate system. Its temperature varies with depth, location, and time. A surface temperature image therefore cannot measure the entire ocean’s energy change by itself. A thin warm layer and a deep, moderately warmed layer can have very different heat-content changes even if the surface looks similar.
Warming generally expands seawater and contributes to sea-level rise, but the expansion coefficient depends on temperature, , and pressure. There is no universal conversion from one degree of surface warming to a fixed global sea-level change. Profiling instruments and physical models help connect temperature measurements over depth with stored energy. Mixing can redistribute that energy without requiring an equal new input from the atmosphere.
Check 1 of 3
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Work through a decision
Worked heating: specify the material
Use an invented, well-mixed parcel containing one million kilograms of seawater. Approximate its as 4,000 joules per kilogram per kelvin and supply a net two billion joules. Assume no phase change, no mass exchange, and negligible work or energy loss during this step. Dividing energy by mass times heat capacity gives a temperature increase of 0.5 kelvin, numerically the same increase in degrees Celsius.
Spread the same energy through twice as much water with the same constant specific heat capacity and the increase becomes 0.25 kelvin. The smaller temperature change does not mean less energy was supplied. Our simplified parcel differs from the actual ocean, where currents, mixing, radiation, , and changing density interact. The calculation identifies what must be known before a measured warming can be converted into a heat-content estimate.
Carbon dioxide changes chemical equilibria
Carbon dioxide entering seawater participates in reactions among dissolved carbon dioxide, bicarbonate, carbonate, and hydrogen ions. Under comparable conditions, increased carbon dioxide generally lowers and reduces available carbonate ions. Many organisms use carbonate in shells or skeletons, although biological responses depend on species and other conditions. describes the direction of chemical change; seawater can become more acidic while remaining on the alkaline side of neutral.
pH is logarithmic, not a linear percentage scale. In an idealized comparison on the same activity-based scale, a decline from pH 8.2 to 8.0 corresponds to a ratio of 10 raised to 0.2, approximately 1.58. That is about a 58 percent increase, not a 0.2 percent increase. This invented change is not a current global measurement, nor does pH alone specify the total dissolved carbon inventory.
Check 2 of 3
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Use complementary observations
Warming and acidification share links to rising atmospheric carbon dioxide, but they are different mechanisms and require different measurements. Temperature profiles constrain energy storage; chemical samples and sensors constrain carbonate chemistry. Salinity, alkalinity, biology, and mixing influence local conditions. A color photograph of the ocean cannot diagnose its pH, and a pH measurement is not a thermometer for the amount of heat stored beneath it.
To compare two surveys, match the depth range, season, location, reference period, and chemical scale. Check calibration and whether the reported quantity is directly measured or calculated from other variables. A brief coastal excursion can differ from an offshore average without invalidating either record. Connecting the physical and chemical budgets reveals how one ocean can absorb energy and carbon while changing in several measurable ways.
This July 2015 EPIC composite uses visible red, green and blue bands. It offers a global view of ocean-covered Earth, while measurements through the water column are needed to estimate ocean heat content. Visible blueness is not a temperature or pH scale.
Look closely
Could you rank the heat content of two ocean regions from their blue shades in this image? What measurements would be needed instead?
See what to notice
Visible color is affected by illumination, atmosphere and water properties. A heat-content estimate needs temperature over depth and the amount and thermal properties of water; chemical measurements are needed separately to establish pH.
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Put it together
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