The neglected carbon “bank” on the Earth — Ocean

Carbon plays a critical role in Earth’s life and climate system. Carbon not only plays a central role in Earth’s life but also is an important greenhouse gas that regulates the temperature of the atmosphere.

However, the amount of carbon in the atmosphere is surprisingly small. The largest carbon reservoir is solid Earth, which holds nearly 100,000 times as much carbon as the atmosphere, and the second-largest carbon reservoir is the often-ignored ocean, with roughly 60 times as much carbon as the atmosphere. The exchange of carbon between these major reservoirs and the atmosphere and biosphere constitutes the global carbon cycle. Understanding and quantifying this cycle are among the primary aims of modern Earth system science.

The ocean pulls carbon from the atmosphere through two pump-like mechanisms: solubility pump and biological pump.

First, the solubility pump is caused by the deep ocean’s cold temperatures and increased solubility. The ocean’s deep and bottom waters typically range in temperature from 2-4°C. These waters are almost twice as soluble as typical surface waters, which are about 20°C in temperature. This causes the amount of dissolved carbon to rise with depth.

Second, the biological pump is caused by some species, like numerous microscopic marine algae that can be found in seawater depths where sunlight can reach, where they collect carbon dioxide through photosynthesis (a process using sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar). When fish and other marine life live at greater depths where there is no sunshine, they swim to the upper layers of the water at night to feed, and they release carbon through feces that sink to the bottom of the sea, which also can carry carbon into the deeper water.

The third and smallest factor that changes dissolved carbon is anthropogenic carbon (the carbon produced directly by human activities). Although the impact of anthropogenic factors on carbon uptake in the ocean is small compared to natural causes, the ocean is extremely important for the uptake of anthropogenic CO2.

Since the start of the industrial revolution, human activities like industrial production and logging have emitted significant amounts of CO2, but about 60% of the CO2 has been absorbed by the oceans and terrestrial biosphere.

But there is a limit to how much carbon the ocean can absorb and absorbing large amounts of carbon can cause ocean water to acidify, leading to the death of corals and other marine life. The average pH of surface seawater has declined by 0.1 during the industrial era, most of which has occurred in the last 50 years.

The greatest declines in pH occurred in the surface ocean, where the concentration of anthropogenic carbon is the highest. Because of the slow ventilation times of the deep ocean, anthropogenic carbon has not penetrated into the deep layers of many parts of the ocean, which limits the rate of oceanic CO2 uptake.

In the future, to improve the ocean’s ability to absorb carbon to alleviate CO2-induced warming, we also need to further understand the ocean carbon cycle and enhance the rate at which carbon is transferred from the surface ocean or atmosphere to the deep ocean.

Citation:

DeVries, Tim. “The Ocean Carbon Cycle.” Annual Review of Environment and Resources 47 (2022): 317-341.

Auther:

Chengran Di