Carbon sink

A carbon sink is anything, natural or otherwise, that accumulates and stores some carbon-containing chemical compound for an indefinite period and thereby removes carbon dioxide (CO₂) from the atmosphere. These sinks form an important part of the natural carbon cycle. An overarching term is carbon pool, which is all the places where carbon can be (the atmosphere, oceans, soil, plants, and so forth). A carbon sink is a type of carbon pool that has the capability to take up more carbon from the atmosphere than it releases.

Globally, the two most important carbon sinks are vegetation and the ocean. Soil is an important carbon storage medium. Much of the organic carbon retained in the soil of agricultural areas has been depleted due to intensive farming. "Blue carbon" designates carbon that is fixed via the ocean ecosystems. Coastal blue carbon includes mangroves, salt marshes and seagrasses which make up a majority of ocean plant life and store large quantities of carbon. Deep blue carbon is located in the high seas beyond national jurisdictions and includes carbon contained in "continental shelf waters, deep-sea waters and the sea floor beneath them. As a main carbon sink, the ocean removes excess greenhouse gas emissions such as heat and energy."

Many efforts are being made to enhance natural carbon sinks, mainly soils and forests, to mitigate climate change. These efforts counter historical trends caused by practices like deforestation and industrial agriculture which depleted natural carbon sinks; land use, land-use change, and forestry historically have been important human contributions to climate change. In addition to enhancing natural processes, investments in artificial sequestration initiatives are underway to store carbon in building materials or deep underground.

Definition

In the context of climate change and in particular mitigation, a sink is defined as "Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere".

In the case of non-CO₂ greenhouse gases, sinks need not store the gas. Instead they can break it down into substances that have a reduced effect on global warming. For example, nitrous oxide can be reduced to harmless N₂.

Related terms are "carbon pool, reservoir, sequestration, source and uptake". The same publication defines carbon pool as "a reservoir in the Earth system where elements, such as carbon [...], reside in various chemical forms for a period of time."

Both carbon pools and carbon sinks are important concepts in understanding the carbon cycle, but they refer to slightly different things. A carbon pool can be thought of as the overarching term, and carbon sink is then a particular type of carbon pool: A carbon pool is all the places where carbon can be (for example the atmosphere, oceans, soil, plants, and fossil fuels). A carbon sink, on the other hand, is a type of carbon pool that has the capability to take up more carbon from the atmosphere than it releases.

Types

The amount of carbon dioxide varies naturally in a dynamic equilibrium with photosynthesis of land plants. The natural carbon sinks are:

Soil is a carbon store and active carbon sink.
Photosynthesis by terrestrial plants with grass and trees allows them to serve as carbon sinks during growing seasons.
Absorption of carbon dioxide by the oceans via solubility and biological pumps.

Artificial carbon sinks are those that store carbon in building materials or deep underground (geologic carbon sequestration). No major artificial systems remove carbon from the atmosphere on a large scale yet.

Public awareness of the significance of CO₂ sinks has grown since passage of the 1997 Kyoto Protocol, which promotes their use as a form of carbon offset.

Natural carbon sinks

Soils

Soils represent a short to long-term carbon storage medium, and contain more carbon than all terrestrial vegetation and the atmosphere combined. Plant litter and other biomass including charcoal accumulates as organic matter in soils, and is degraded by chemical weathering and biological degradation. More recalcitrant organic carbon polymers such as cellulose, hemi-cellulose, lignin, aliphatic compounds, waxes and terpenoids are collectively retained as humus.

Organic matter tends to accumulate in litter and soils of colder regions such as the boreal forests of North America and the Taiga of Russia. Leaf litter and humus are rapidly oxidized and poorly retained in sub-tropical and tropical climate conditions due to high temperatures and extensive leaching by rainfall. Areas where shifting cultivation or slash and burn agriculture are practiced are generally only fertile for two to three years before they are abandoned. These tropical jungles are similar to coral reefs in that they are highly efficient at conserving and circulating necessary nutrients, which explains their lushness in a nutrient desert.

Grasslands contribute to soil organic matter, stored mainly in their extensive fibrous root mats. Due in part to the climatic conditions of these regions (e.g. cooler temperatures and semi-arid to arid conditions), these soils can accumulate significant quantities of organic matter. This can vary based on rainfall, the length of the winter season, and the frequency of naturally occurring lightning-induced grass-fires. While these fires release carbon dioxide, they improve the quality of the grasslands overall, in turn increasing the amount of carbon retained in the humic material. They also deposit carbon directly to the soil in the form of biochar that does not significantly degrade back to carbon dioxide.

Organic matter in peat bogs undergoes slow anaerobic decomposition below the surface. This process is slow enough that in many cases the bog grows rapidly and fixes more carbon from the atmosphere than is released. Over time, the peat grows deeper. Peat bogs hold approximately one-quarter of the carbon stored in land plants and soils.

Enhancing soil carbon sinks

Much organic carbon retained in many agricultural areas worldwide has been severely depleted due to intensive farming practices. Since the 1850s, a large proportion of the world's grasslands have been tilled and converted to croplands, allowing the rapid oxidation of large quantities of soil organic carbon. Methods that significantly enhance carbon sequestration in soil include no-till farming, residue mulching, cover cropping, and crop rotation, all of which are more widely used in organic farming than in conventional farming.

Forests

Forests are generally carbon dioxide sinks when they are high in diversity, density or area. However, they can also be carbon sources if diversity, density or area decreases due to deforestation, selective logging, climate change, wildfires or diseases. One study in 2020 found that 32 tracked Brazilian non-Amazon seasonal tropical forests declined from a carbon sink to a carbon source in 2013 and concludes that "policies are needed to mitigate the emission of greenhouse gases and to restore and protect tropical seasonal forests". In 2019 forests took up a third less carbon than they did in the 1990s, due to higher temperatures, droughts and deforestation. The typical tropical forest may become a carbon source by the 2060s.

FAO reported that: "The total carbon stock in forests decreased from 668 gigatonnes in 1990 to 662 gigatonnes in 2020". However, another study finds that the leaf area index has increased globally since 1981, which was responsible for 12.4% of the accumulated terrestrial carbon sink from 1981 to 2016. The CO₂ fertilization effect, on the other hand, was responsible for 47% of the sink, while climate change reduced the sink by 28.6%. In Canada's boreal forests as much as 80% of the total carbon is stored in the soils as dead organic matter.

Carbon offset programs are planting millions of fast-growing trees per year to reforest tropical lands, for as little as $0.10 per tree; over their typical 40-year lifetime, one million of these trees will fix 1 a million tons of carbon dioxide.

Life expectancy of forests varies throughout the world, influenced by tree species, site conditions and natural disturbance patterns. In some forests, carbon may be stored for centuries, while in other forests, carbon is released with frequent stand replacing fires. Forests that are harvested prior to stand replacing events allow for the retention of carbon in manufactured forest products such as lumber. However, only a portion of the carbon removed from logged forests ends up as durable goods and buildings. The remainder ends up as sawmill by-products such as pulp, paper and pallets, which often end with incineration (resulting in carbon release into the atmosphere) at the end of their lifecycle. For instance, of the 1,692 megatonnes of carbon harvested from forests in Oregon and Washington from 1900 to 1992, only 23% is in long-term storage in forest products.

Changes in albedo effect

Forests generally have a low albedo because the majority of the ultraviolet and visible spectrum is absorbed through photosynthesis. For this reason, the greater heat absorption by trees could offset some of the carbon benefits of afforestation (or offset the negative climate impacts of deforestation). In other words: The climate change mitigation effect of carbon sequestration by forests is partially counterbalanced in that reforestation can decrease the reflection of sunlight (albedo).

In the case of evergreen forests with seasonal snow cover albedo reduction may be great enough for deforestation to cause a net cooling effect. Trees also impact climate in extremely complicated ways through evapotranspiration. The water vapor causes cooling on the land surface, causes heating where it condenses, acts a strong greenhouse gas, and can increase albedo when it condenses into clouds. Scientists generally treat evapotranspiration as a net cooling impact, and the net climate impact of albedo and evapotranspiration changes from deforestation depends greatly on local climate.

Mid-to-high-latitude forests have a much lower albedo during snow seasons than flat ground, thus contributing to warming. Modeling that compares the effects of albedo differences between forests and grasslands suggests that expanding the land area of forests in temperate zones offers only a temporary mitigation benefit.

Deep ocean, tidal marshes, mangroves and seagrasses

Blue carbon is a term used in the climate change mitigation context that refers to "biologically driven carbon fluxes and storage in marine systems that are amenable to management." Most commonly, it refers to the role that tidal marshes, mangroves and seagrasses can play in carbon sequestration. Such ecosystems can contribute to climate change mitigation and also to ecosystem-based adaptation. When blue carbon ecosystems are degraded or lost they release carbon back to the atmosphere.

Blue carbon management methods can be grouped into ocean-based biological carbon dioxide removal (CDR) methods. They are a type of biologic carbon sequestration.

There is increasing interest in developing blue carbon potential. Research is ongoing. In some cases it has been found that these types of ecosystems remove far more carbon per area than terrestrial forests. However, the long-term effectiveness of blue carbon as a carbon dioxide removal solution remains contested. The term deep blue carbon is also in use and includes efforts to store carbon in the deep ocean waters.

Enhancing natural carbon sinks

Purpose in the context of climate change

An important mitigation measure is one the IPCC Sixth Assessment Report calls "preserving and enhancing carbon sinks". This refers to the management of Earth's natural carbon sinks in a way that preserves or increases their capability to remove CO₂ from the atmosphere and to store it durably. We call this carbon sequestration. In the context of climate change mitigation, the IPCC defines a sink as "Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere". Globally, the two most important carbon sinks are vegetation and the ocean.

To enhance the ability of ecosystems to sequester carbon, changes are necessary in agriculture and forestry. Examples are preventing deforestation and restoring natural ecosystems by reforestation. Scenarios that limit global warming to 1.5 °C typically project the large-scale use of carbon dioxide removal methods over the 21st century. There are concerns about over-reliance on these technologies, and their environmental impacts. But ecosystem restoration and reduced conversion are among the mitigation tools that can yield the most emissions reductions before 2030.

Land-based mitigation options are referred to as "AFOLU mitigation options" in the 2022 IPCC report on mitigation. The abbreviation stands for "agriculture, forestry and other land use" The report described the economic mitigation potential from relevant activities around forests and ecosystems as follows: "the conservation, improved management, and restoration of forests and other ecosystems (coastal wetlands, peatlands, savannas and grasslands)". A high mitigation potential is found for reducing deforestation in tropical regions. The economic potential of these activities has been estimated to be 4.2 to 7.4 gigatonnes of carbon dioxide equivalent (GtCO₂ -eq) per year.

Carbon sequestration techniques in oceans

To enhance carbon sequestration processes in oceans the following technologies have been proposed but none have achieved large scale application so far: Seaweed farming, ocean fertilisation, artificial upwelling, basalt storage, mineralization and deep sea sediments, adding bases to neutralize acids. The idea of direct deep-sea carbon dioxide injection has been abandoned.

Artificial carbon sinks

Geologic carbon sequestration

Wooden buildings

Mjøstårnet, one of the tallest timber buildings, at its opening 2019

Broad-base adoption of mass timber and their role in substituting steel and concrete in new mid-rise construction projects over the next few decades has the potential to turn timber buildings into carbon sinks, as they store the carbon dioxide taken up from the air by trees that are harvested and used as mass timber. This could result in storing between 10 million tons of carbon per year in the lowest scenario and close to 700 million tons in the highest scenario. For this to happen, the harvested forests would need to be sustainably managed and wood from demolished timber buildings would need to be reused or preserved on land in various forms.