7.2: Ocean Acidification and Short Term Carbon Cycle

Ocean Acidification

Before we delve into the impacts of greenhouse gases, let’s stay in the ocean for just a few more moments. The excess carbon absorbed by the ocean does not come without consequences. Carbon is a very reactive element, and one of its favorite molecules to bond with is Water (H₂O), so when Carbon is absorbed into the ocean, it reacts with water. Since most of the excess carbon pumped into the atmosphere by human activities is in the form of Carbon Dioxide, most of the excess carbon absorbed by the ocean is also Carbon Dioxide. As a result, Carbon Dioxide and Water react to form a new compound, one that is familiar to anyone who has enjoyed a carbonated beverage before: Carbonic Acid. While this isn’t a Chemistry class, we will share a brief chemical formula to explain how the reaction occurs.

CO₂ + H₂O = H₂CO₃ = +H + HCO₃

The left term is Carbon Dioxide added to Water; the middle term is Carbonic Acid, which then dissociates into the right term, bicarbonate and free hydrogen ions, which are the nasty little culprits in acidification. Figure \(\PageIndex{1}\) outlines the ocean acidification process a bit more:

Figure \(\PageIndex{1}\): Effects of Ocean Acidification on Calcification. Note that in a decreased pH environment, calcium shells become deformed. (CC-BY-SA 4.0; Elizajans via Wikimedia Commons).

If you remember your high school chemistry class, the pH scale measures how acidic or alkaline a substance is. The scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic. Pure water is neutral, with a pH of 7.0. When chemicals are mixed with water, the mixture can become either acidic or alkaline to some extent. Vinegar and lemon juice are acidic substances, while laundry detergents and ammonia are alkaline. Figure \(\PageIndex{2}\) shows the change in pH of the world ocean since pre-industrial times (1770s).

Figure \(\PageIndex{2}\): Change in average Ocean pH from the 1770s to the 1990s. (CC-BY-SA 3.0; Plumbago via Wikimedia Commons). Alternative Description of Image.

While Figure \(\PageIndex{2}\) shows the global pH distribution of our oceans, Figure \(\PageIndex{3}\) is a time series of global average pH from 1988 to 2021, based on data from the Aloha Station in Hawaii, and plotted by Our World In Data.

Figure \(\PageIndex{3}\): Time series of Mean seawater pH from 1988 to 2024. Data collected by the University of Hawaii and displayed by OurWorldInData.org (CC-BY 4.0; Ocean Acidification via Our World in Data). Alternative Description of Image.

8. From Figure \(\PageIndex{3}\), while there is a seasonal cycle (hence the jaggedness and “up and down” nature of the plot), the overall trend in global average pH (which can be found in the "Annual Average” line) is:
   1. Upward
   2. Downward
   3. Neither
9. This indicates that the ocean is becoming (note: higher values indicate a more basic substance while lower values indicate a more acidic substance):
   1. More Basic
   2. More Neutral
   3. More Acidic
10. However, Figure \(\PageIndex{2}\) makes it clear that not every location in the ocean is experiencing the same decline in pH. Rather, the __________ seem to be seeing the biggest change while the _____________ is seeing the least amount of change. Note: The Southern Ocean refers to the Southern Atlantic, Pacific, and Indian Oceans.
    1. Equator; North Atlantic and Southern Ocean
    2. North Atlantic; Southern Ocean
    3. Equator; Southern Atlantic Only
    4. North Atlantic and Southern Ocean; Equator

While these numbers and figures may seem relatively insignificant to us as humans, the truth is that many marine species (such as coral) are very sensitive to changes in the pH of the ocean, so even a slight change in either direction can significantly stress an ecosystem or, in the case of more significant changes, cause an ecosystem to collapse.

Short-Term Carbon Cycle

In Figure \(\PageIndex{3}\), you may notice a “jagged” shape to the “Monthly Average” line. This is due to seasonality; many natural systems exhibit seasonal variation, with levels of a particular variable higher in one season and lower in another. While there are many causes of the ocean’s pH seasonality, one of the main causes would be Carbon Dioxide levels, which experience a similar but inverse (opposite) seasonality. This seasonality is part of the Short-Term Carbon Cycle: carbon exchanges that occur over relatively short time spans, i.e., seconds, minutes, hours, days, weeks, or months. Meanwhile, long-term chemical and geological processes often take millions of years. Figure \(\PageIndex{4}\) is CO₂ data taken from the Mauna Loa Observatory from the two-year period ending on May 27th, 2026.

Figure \(\PageIndex{4}\): Carbon Dioxide Concentrations measured at the Mauna Loa Observatory for the two-year period ending on May 27th, 2026. (CC BY 4.0; Ralph Keeling via Scripps Institute of Oceanography). Alternative Description of Image.

11. While there may be some year-to-year variation, Carbon Dioxide concentrations tend to peak around May and reach their lowest annual levels in approximately:
    1. February/March
    2. September/October
    3. December/January
12. Based on the information above, Carbon Dioxide levels tend to increase during ____________and decrease during __________.
    1. Fall/Winter; Summer
    2. Summer; Winter
    3. Spring; Fall/Winter

There is some lag between when photosynthetic activity wakes up/shuts down and when levels change, but in general, this is what happens:

• During warmer/brighter months, there is more sunlight, so vegetation can perform photosynthesis. As a result, there is more photosynthesis than respiration happening, and Carbon Dioxide Concentrations go down.
• During colder/darker months, there isn’t enough sunlight for photosynthesis, so vegetation can’t perform photosynthesis. As a result, vegetation often hibernates during those months, consuming sugars produced during the prior growing season and releasing Carbon Dioxide back into the atmosphere (via respiration). This causes Carbon Dioxide concentrations to rise.

13. Despite the seasonality shown here, comparing CO₂ concentrations from May 2021 (the far left of the graph) to May 2023, you can still see that Carbon Dioxide concentrations are:
    1. Higher in 2023 than in 2021
    2. Lower in 2023 than in 2021
    3. Exactly the same in 2023 as in 2021
14. So even though there is a seasonality to Carbon Dioxide, the overall trend in Carbon Dioxide is:
    1. Upward
    2. Downward
    3. Neither

To figure out what's causing this, we need to investigate the Long-Term Carbon Cycle, which we will do in the next section.