5.2: Biological and Physical Processes Impacting CO2

Processes Influencing CO₂ Concentrations on Land

Before we can understand what drives variation in ocean CO2 concentrations, we need to first understand the processes that drive variation in atmospheric CO₂ concentrations.  The ocean and atmosphere are coupled systems that are constantly exchanging dissolved gasses that are very important to living organisms, particularly oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂). The ocean absorbs about 30% of the CO₂ that is released in the atmosphere. As levels of atmospheric CO₂ increase from human activity such as burning fossil fuels (e.g., car emissions) and changing land use (e.g., deforestation), the amount of  CO₂ absorbed by the ocean also increases.

 

Exercise \(\PageIndex{1}\)

6. If you were to create a Keeling curve for the ocean, how do you think it would compare to the curve shown for atmospheric CO₂?

• The CO₂ concentration in the ocean would not change at all over time
• The CO₂ concentration in the ocean would change faster than the atmospheric CO₂ concentration
• The CO₂ concentration in the ocean would change slower than the atmospheric CO₂ concentration
• The CO₂ concentration in the ocean would change at a similar rate to the atmospheric CO₂ concentration

Answer

The CO₂ concentration in the ocean would change at a similar rate to the atmospheric CO₂ concentration

 

Biological Impacts on CO₂ in the Atmosphere

CO₂ is essential for photosynthesis—a biological process through which plants and other photosynthetic organisms use sunlight as energy to combine carbon dioxide and water to produce sugars (like glucose) and oxygen – substances that all animals need to survive! Animals (and plants, too!) then break down this sugar through cellular respiration, a process that releases energy for biological functions and returns CO₂ to the atmosphere.

Now let’s get a bit more comfortable with the basic photosynthesis equation. Given what we’ve discussed so far, do your best to place the components of this equation in the correct spot.

Plants and other photosynthetic organisms (algae) consume CO₂ in the surface ocean, where sunlight is readily available, which mitigates increased CO₂ concentrations in the atmosphere. This process also produces oxygen, helping to keep the planet hospitable for life. The growth of plants and algae varies throughout the year due to seasonal changes in sunlight and nutrients.

 

Human Population’s Influence on CO₂ Concentrations in the Atmosphere

While CO₂ is a naturally occurring and biologically important gas, increased burning of fossil fuels (such as coal and oil) due to industrialization since the last century has contributed to an atmospheric level of CO₂ that Earth has not experienced since roughly 3 million years ago (Lindsey 2025). Deforestation, the cutting down of trees to make way for development or for lumber, also contributes to rising CO₂ levels as we are removing plants which are a natural “sink” of CO₂. Every year, human activities are adding more CO₂ to the atmosphere than is being removed, and so the total amount of CO₂ in the atmosphere goes up. These influences that human beings have on climate or other aspects of nature are known as anthropogenic effects.

 

The Physical Processes that Link Atmosphere and Ocean 

As mentioned above, the ocean and the atmosphere are intimately linked and are constantly exchanging gases like O₂ and CO₂. Now we are going to dive a bit deeper into these interactions and discuss how we can tell when the ocean is a sink versus a source of CO₂. The ocean is said to be a sink for CO₂ when the net movement of CO₂ is from the atmosphere to the ocean (when the ocean serves as a net absorber of atmospheric CO₂). The ocean is a source of CO₂ when the net movement of CO₂ is from the ocean to the atmosphere (when the ocean is releasing CO₂ to the atmosphere).

The figure above helps to summarize these interactions within the ocean carbon uptake system. We will learn about the role of biological processes soon. For now, let’s focus on gas exchange between the atmosphere and ocean. Before we look at some data, check your understanding of sinks and sources below.

 

Examining Fluxes at Two Oceanographic Observatories Initiative Sites

The top panel shows CO₂ concentrations in the atmosphere (in gold) and CO₂ concentrations in seawater (in blue). The principle of diffusion states that substances move from areas of higher concentration to areas of lower concentration. If CO₂ concentrations in seawater are higher than CO₂ concentrations in the atmosphere, there will be a net movement of CO₂ out of the ocean and into the atmosphere; under those conditions the ocean is a source of CO₂.

The bottom panel shows how much CO₂ is being transferred from the ocean to the atmosphere – also called the flux of CO₂ from the ocean to the atmosphere. Positive flux values indicate that CO₂ is moving from the ocean to the atmosphere and that the ocean is a source of CO₂. Negative flux values indicate the opposite: that CO₂ is moving from the atmosphere into the ocean and that the ocean is a sink for CO₂. Negative flux occurs when CO₂ concentrations in seawater are lower than CO₂ concentrations in the atmosphere. During the time period shown, atmospheric CO₂ was approximately 405 ppm. First review your understanding of flux and the movement of CO₂, then analyze the graphs below and answer the questions that follow.

 

Exercise \(\PageIndex{2}\)

What are the units for CO₂ concentration in seawater?

Answer

ppm

Exercise \(\PageIndex{3}\)

If you averaged across the year, do you think that the ocean at these two locations would be a net source or sink for CO2.

Answer

The ocean is a net sink for CO2 (more negative values in the bottom panel than position values)

 

Solubility, Saturation, and Ocean Gasses

The amount of each gas that can dissolve in the ocean depends on the solubility and saturation of the gas in water. Solubility refers to the amount of a dissolved gas that the water can hold under a particular set of conditions, which are usually defined as 0^o C and 1 atmosphere of pressure. The solubility of a gas increases with increasing pressure, decreased temperature, and decreased salinity. Saturation refers to the amount of gas currently dissolved in the water, relative to the maximum possible content. If the water is undersaturated, more gas can dissolve. If the water is saturated or supersaturated, gas may be released. Most atmospheric gases are saturated in the ocean, but O₂ and CO₂ are not saturated because they are rapidly used by living organisms.