We are pumping more carbon into our atmosphere than nature can recapture, and while emission cuts are paramount, carbon sequestration may be necessary for total decarbonization. This article will give you a brief overview of the state of natural carbon sinks, and the artificial alternatives we have discovered so far. 

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What are Carbon Sinks?

Carbon sinks are reservoirs that absorb more carbon dioxide (CO2) from the atmosphere than they release. In this process called “carbon sequestration”, natural or artificial deposits are created where carbon is stored for various periods of time depending on the medium.

Natural Sinks

Oceans are one of the main natural carbon sinks. According to the World Economic Forum, they are able to absorb around one-third of global CO2 emissions. The main mechanism of absorbing CO2 is called the “physical carbon pump”. 90% of atmospheric CO2 is transferred to the surface seawater by diffusion. The dissolved CO2 will then be transported by ocean currents to the deep layers of the ocean.

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ocean carbon storage

Source: ocean.climate.org

Seagrass plays a key role, as it accounts for 10% of the ocean’s capacity to store CO2, capturing it up to 35 times faster than forests. However, seagrass meadow area has been decreasing by 7% per year since 1990, due to coastal development and dredging, and reduced water quality.

In forests, carbon sequestration is done through photosynthesis. Under normal conditions, there is a net absorption of CO2 and a net release of oxygen in plants. According to the Global Forest Resources Assessment 2020, 99% of the forest carbon is found in living biomass and soil organic matter, with the remainder in dead wood and litter. Deforestation’s rise in recent decades has decreased the total forest carbon stock from 668 billion tonnes in 1990 to 662 billion tonnes in 2020. Fortunately, cities have increasingly incorporated green infrastructures to create natural carbon sinks into the urban landscape.

forest carbon storage

Source: Food and Agriculture Organizations of the United Nations, Global Forest Resources Assessment 2020.

Artificial Carbon Sinks 

In view of the diminishing roles of oceans and forests as natural carbon sinks, scientists now seek artificial techniques to extract CO2 from the atmosphere.

Iron fertilization for phytoplankton blooms 

Phytoplankton are microscopic algae that generally float on the surface and nourish themselves of nutrients like nitrates, phosphates, or iron. They absorb CO2 through photosynthesis and produce 50 to 85% of the oxygen in our atmosphere. Given the significance of phytoplankton, scientists proposed adding vast amounts of iron to the ocean to facilitate the numbers of phytoplankton photosynthesizing. However, iron fertilization could lead to harmful algal blooms and other unforeseen ecosystem effects. It would be unwise to tamper with our environment without a more robust understanding of the consequences. 

Carbon Capture, Utilization, and Storage (CCUS)

CCUS is an emission reduction technology that removes and recycles CO2 for utilization. CO2  is captured and permanently stored in geologic layers over 4500 feet underground. As of April 2020, 20 CCUS power generation projects are under development globally, with around US $20 billion invested in the sector. This is only 1% of what has gone into clean energy since 2004, and heavier funding, along with favorable policy will be necessary for CCUS to take off. 

Enhanced Rock Weathering

A recent paper published in Nature describes the carbon storage ability of crushed rock dust sprinkled over agricultural soils. In this state, the rock slowly dissolves and reacts with CO2 to form carbonates; this could remove 0.5 to 2 billion tonnes of CO2 from the atmosphere each year. 

In conclusion, while there are some very interesting carbon-capturing methods gaining popularity, their storage capacity falls well short of the 36 billion tonnes of CO2 emitted every year. Smart investment in scalable technologies will be key to clean decarbonization in the future, but emission mitigation remains the top priority for now. 

This article was written by Jennie Wong and Owen Mulhern. 

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