Carbon sinks explained, with examples

October 20, 2022
Written by
Pierre-Louis Lemaire
Carbon sinks explained, with examples
Table of contents

What is a carbon sink

A carbon sink can be natural or artificial. It is an element that actively draws CO2 from the atmosphere and stores it indefinitely. Carbon sinks play a key role in limiting global warming by absorbing a significant portion of our carbon emissions year after year. In fact, since 1959, approximately 350 billion tonnes of carbon have been emitted by humans to the atmosphere, of which about 55 percent has been stored in various carbon sinks. Globally, the two most important carbon sinks are vegetation and the ocean.

Recent studies have shown that carbon sinks absorb more carbon dioxide from the atmosphere as its concentration goes up. However, others tend to demonstrate that this doesn’t come without side effects, and as sinks store more carbon, more pressure is applied to them. Therefore, scientists do not believe that this “keeping pace” trend will continue indefinitely, as we continue to emit enormous amounts of carbon dioxide into the atmosphere.

Global carbon accumulation from 1960 to 2010 - NOAA

As this graph shows, carbon sinks have been crucial to limiting global warming, despite our emissions reaching sky levels. However, many natural carbon sinks are at risk because of human activities (e.g., deforestation, industrial agriculture, and ocean acidification). We will see that we have no choice but to take care of nature, and more generally, the living, to reach net zero.

The carbon cycle explained

Carbon is the chore of life. We often only discuss carbon only through the CO2 emission spectrum. However, this chemical compound is at the base of the living.

Like any other animals on earth, we represent a stock of carbon. Indeed, 18% of our human bodies are composed of it. The whole human population stocks about 0.06 Gtons of carbon in its billion of bodies. The entire animal population itself stocks about 2Gtons. But, we only make up a small share compared to the global living.

Carbon stock per living species (categories) - The biomass distribution on Earth


All these living species do not store carbon from the atmosphere indefinitely. On the contrary, carbon flows in and out of our bodies. It comes from the consumption of other animals or plants that have stored it, to be exhaled back into the atmosphere. Plants, on the other hand, use photosynthesis to absorb CO2 from the atmosphere and then release it through cellular respiration.

Vegetation is, therefore, the basis of the inland carbon cycle, and photosynthesis is, in fact the largest biological carbon absorption phenomenon. The following NASA video shows that carbon dioxide concentration in the atmosphere changes as vegetation cycles.

Seasonal Vegetation and its effect on Earth - NASA

In the spring and early summer, carbon dioxide accumulates in the atmosphere due to the release of carbon from the cellular respiration of plants, which allows them to grow. As we enter the second phase of the year (the degrowth phase), vegetation absorbs more carbon dioxide.

Additionally, oceans constantly capture enormous amounts of carbon from the atmosphere. Like for any other gas, when reaching the surface of a water body, a share of it gets dissolved into the water. As the gas pressure in the atmosphere increases, the ocean will absorb more of that gas to keep the pressure balanced. Meaning, that when emissions start to reduce, we may face a decrease in carbon absorption from the ocean. Nevertheless, it would still be 100% beneficial.

These interactions constitute what scientists call the "fast carbon cycle." As we explained in a recent LinkedIn post, the carbon dioxide we emit today does not lock in future temperature increases in the climate pipeline. Meaning, we can start dramatically reducing emissions now, quickly reach net zero, and stop global warming almost immediately.

Fast carbon cycle - Wikipedia

Different types of carbon sink

Forest

Everyone knows that trees absorb carbon dioxide from the atmosphere. Massive tree-planting projects are commonly touted as an easy solution to climate change. In the United States in 2004, forests sequestered about 10% of the annual carbon dioxide emissions from fossil fuel combustion.

However, while forests represent an important carbon sink and an opportunity to help us fight the climate crisis, they are also under increasing pressure. With massive deforestation continuing, scientists fear that forests will become a carbon source in the future (emitting more carbon dioxide than they capture).

In response to deforestation and corporate demand for "carbon offsets," many industrial tree-planting projects have sprung up, promising to reforest the Earth and absorb millions of tons of carbon from the atmosphere. To plant trees on an industrial scale, project coordinators use monocultures and populations of non-native species to maximize forest cover as quickly as possible, ignoring forests’ basic principles of carbon sequestration.

By relying on fast-growing non-native trees in monoculture systems, these "forests" lack the healthy vegetation layer necessary to develop a rich and resilient ecosystem. Ground flora, intermediate stages, dead wood, and rich fauna are needed to make a forest, one that massively captures carbon, does not burn after 3 years, and resists biological diseases.

Shares of carbon stored in forest environments - Wikipedia

Even worst, some studies have shown that mismanaged tree-planting projects resulted in additional carbon emissions over the years.

Nevertheless, tree planting is vital for our society to reach a net zero emissions level. Firstly, to compensate for the losses due to deforestation, which must be stopped. Secondly, because we need to absorb more carbon dioxide than we produce to reverse climate change and return to normal as quickly as possible. This is why tree planting must be done in a way that enhances all parts of a forest, from the trees to the small vegetation, to the litter, to the soil, and allows for the development of rich life within it to help store carbon and protect the forest.

Ocean

About 70% of the Earth is covered by water, which absorbs huge amounts of carbon from the atmosphere. This is "blue carbon", i.e. carbon captured by ocean and coastal ecosystems. The oceans currently store 38 000 Gtons of carbon, while the atmosphere contains only 750 Gtons. In addition, the oceans have captured 38% of all the carbon we have emitted over the past 200 years, and continue to absorb 7 Gtons of carbon dioxide per annum.

Like any other sink, the oceans soak up carbon from the atmosphere and also release a certain amount, inhaling, exhaling. Theoretically, oceans must keep an equilibrium between the pressure of carbon in the atmosphere and within Earth water bodies. Hence, as we continue to increase the atmosphere’s stock of carbon, the oceans should soak up more carbon dioxide. When absorbed, carbon dioxide will either be dissolved in water forming other molecules, or be used by photosynthesis for plant-like organisms.

However, recent studies are showing that it may not be as simple as that. In fact, scientist Sarmiento says “I think it’s possible that the Southern Ocean sink is slowing down,”. Oceans ecosystems are under pressure. Ocean-forests are also suffering deforestation, and therefore absorb less carbon. For instance, seagrass contains 10% of the oceans carbon stock, but facing an annual habitat loss rate of 1.5%, seagrass is disappearing faster than rainforests. A study also showed that mangroves store enormous amounts of carbon, and by being wiped out of the Earth, are releasing tremendous amounts of carbon in the atmosphere. Between 2000 and 2015, up to 122 million tons of this carbon was released due to mangrove forest loss – roughly equivalent to the annual emissions of Brazil.

Disruptive studies are also showing that oceans may rely on various biological and climatic factors for carbon sequestration.

Soil

Soil is mainly composed of organic matter. Organic matter from vegetation above the ground that soak up carbon from the atmosphere through photosynthesis, died  and slowly decomposed into the soil. Hence, soils around the world contains astononishing amounts of carbon. In fact, soil stores globally 2 500 Gtons of carbon.

However, changes in land-use over the past hundred years (mainly from farming practices) has degraded soils around the world, resulting in the release of 110 billion metric tons of carbon, equivalent to 80 years’ of present-day U.S. emissions. Around half of the whole carbon stores in the soil lies in its first meter, making it extremely vulnerable to human activities.

Industrial farming practices such as intensive tilling, abusive pesticides and fertilizers usage, are releasing carbon stored in the soil and demeaning its capturing capacities.

Intuitively, this means that changing our agricultural practices could result in sequestrating a lot of additional carbon dioxide. Scientists have estimated that soils could capture, if treated correctly, 1 Gtons of additional carbon dioixide every single year. To be put in perspective, this represents over a quarter of the annual carbon emissions, without sequestration from carbon sinks. Regenerative farming will be necessary for our society to reach net zero. More than only capturing carbon, it would also make our food system more resilient to climate change and droughts, less dependent on fossil fuels and pesticides, and bring enormous health benefits.