Carbon Cycle and Sequestration

The Carbon Cycle

Overview

The carbon cycle describes the continuous exchange of carbon between Earth's atmosphere, biosphere, hydrosphere, lithosphere, and anthroposphere. Carbon exists in several forms: carbon dioxide ($\mathrm{CO_2}$), methane ($\mathrm{CH_4}$), organic carbon (in living and dead organisms), inorganic carbon (carbonate rocks, dissolved inorganic carbon in the ocean), and fossil carbon (coal, oil, natural gas).

Natural Carbon Fluxes

Flux	Magnitude (GtC/year)	Description
Gross primary productivity (photosynthesis)	Approximately 120	Plants absorb $\mathrm{CO_2}$ from the atmosphere and convert it to organic carbon
Respiration and decomposition	Approximately 120	Plants, animals, and decomposers release $\mathrm{CO_2}$ through cellular respiration and decomposition of organic matter
Ocean-atmosphere exchange	Net approximately 2 (absorption)	The ocean absorbs approximately 92 GtC/year and releases approximately 90 GtC/year; net absorption of approximately 2 GtC/year
Volcanic outgassing	Approximately 0.1	Release of $\mathrm{CO_2}$ from volcanic activity and mid-ocean ridges
Weathering	Approximately 0.3	Chemical weathering of silicate rocks absorbs $\mathrm{CO_2}$, converting it to bicarbonate ions transported to the ocean and eventually deposited as carbonate sediments

Anthropogenic Carbon Fluxes

Source	Magnitude (GtC/year, 2023)	Proportion of Total
Fossil fuel combustion and industrial processes	Approximately 10.8	Approximately 89%
Land-use change (deforestation, agriculture)	Approximately 1.3	Approximately 11%
Total anthropogenic emissions	Approximately 12.1	100%

Carbon Sinks

Carbon sinks are reservoirs that absorb more carbon than they release. The three major natural carbon sinks are:

1. The ocean. The ocean absorbs approximately 26% of anthropogenic $\mathrm{CO_2}$ emissions, through dissolution at the surface and subsequent transport to deep water via the thermohaline circulation and the biological pump (phytoplankton fix carbon through photosynthesis; when they die, organic carbon sinks to the deep ocean). However, ocean absorption causes acidification, which impairs the ability of calcifying organisms to build shells and skeletons.

2. The terrestrial biosphere (land). The terrestrial biosphere absorbs approximately 29% of anthropogenic $\mathrm{CO_2}$ emissions, through increased plant growth (the $\mathrm{CO_2}$ fertilisation effect), forest regrowth on abandoned agricultural land, and afforestation. However, this sink is vulnerable to climate change: warming increases respiration (releasing $\mathrm{CO_2}$), drought reduces plant growth, and wildfires release stored carbon.

3. Geological storage. Over geological timescales (millions of years), carbon is removed from the active carbon cycle through the formation of carbonate sediments (limestone, chalk) and fossil fuels (coal, oil, natural gas). This sink operates too slowly to be relevant to human timescales.

The remaining approximately 45% of anthropogenic $\mathrm{CO_2}$ accumulates in the atmosphere, increasing atmospheric $\mathrm{CO_2}$ concentration by approximately 2.5 ppm per year.

Carbon Sequestration

Carbon sequestration is the process of removing $\mathrm{CO_2}$ from the atmosphere and storing it in a stable form. Sequestration methods are classified as natural or technological.

Natural Sequestration

Afforestation and reforestation. Planting trees on previously non-forested land (afforestation) or replanting trees on deforested land (reforestation) removes $\mathrm{CO_2}$ from the atmosphere through photosynthesis and stores it as biomass and soil organic carbon.

• Capacity: a hectare of tropical forest can sequester approximately 5--10 tonnes of carbon per year during the growth phase. Global afforestation and reforestation could potentially sequester approximately 5--10 GtC per year, though land availability, opportunity costs (agricultural land), and climate impacts (warming may reduce forest carbon uptake) constrain the potential.
• Limitations: carbon stored in forests is not permanent -- it can be released by wildfires, pest outbreaks, drought, or future deforestation. The permanence of forest carbon sinks is therefore uncertain over long timescales.
• Case study: China's afforestation programme. China has planted approximately 80 billion trees since 1978 under the Three-North Shelter Forest Programme (the "Great Green Wall"), increasing forest cover from approximately 12% to approximately 23% of land area. However, many planted forests are monocultures of fast-growing species (poplar, pine) with lower biodiversity and carbon storage capacity than natural forests, and survival rates are low in arid areas (approximately 15% in some regions).

Soil carbon sequestration. Agricultural practices that increase soil organic carbon (SOC) include no-till farming, cover cropping, crop residue retention, organic amendments (compost, manure), agroforestry, and improved grazing management.

• Capacity: global agricultural soils have lost approximately 50--70% of their original organic carbon. Restoring SOC could sequester approximately 1.5--5 GtC per year globally.
• Limitations: SOC sequestration reaches saturation after 20--40 years; the stored carbon is vulnerable to release if management practices change (e.g., conversion of no-till land to conventional tillage); measurement and verification of SOC changes are technically challenging.

Wetland and mangrove restoration. Wetlands (peatlands, marshes, mangroves) store approximately 20--30% of global soil carbon despite covering only approximately 5--8% of land area. Peatlands alone store approximately 600 GtC, approximately twice the carbon stored in the world's forests.

• Capacity: mangroves sequester carbon at rates of 2--4 times greater than terrestrial forests on a per-area basis. Restoring degraded mangroves and peatlands could make a significant contribution to carbon sequestration.
• Limitations: degraded peatlands can become net carbon sources (drained Indonesian peatlands emit approximately 500 Mt $\mathrm{CO_2}$ per year due to oxidation and fire); restoration requires ongoing water management.

Technological Sequestration

Carbon Capture and Storage (CCS). CCS involves capturing $\mathrm{CO_2}$ from point sources (power plants, cement factories, steel mills), compressing it, transporting it (by pipeline or ship), and injecting it into deep geological formations (saline aquifers, depleted oil and gas reservoirs) for permanent storage.

• Current capacity: approximately 40 million tonnes of $\mathrm{CO_2}$ captured per year globally (2023), a tiny fraction of annual emissions.
• Cost: approximately USD 50--150 per tonne of $\mathrm{CO_2}$ for point-source capture; approximately USD 250--600 per tonne for direct air capture (DAC).
• Limitations: high energy penalty (capturing $\mathrm{CO_2}$ from a power plant reduces its net efficiency by 15--25%); geological storage carries a risk of leakage over long timescales; limited infrastructure for transport and storage; high capital cost; public acceptance concerns.

Bioenergy with Carbon Capture and Storage (BECCS). BECCS involves growing biomass (trees, energy crops), burning it for energy, capturing the $\mathrm{CO_2}$ emitted during combustion, and storing it geologically. In theory, BECCS can achieve "negative emissions" because the carbon captured during biomass growth exceeds the carbon emitted during combustion.

• Potential: many integrated assessment models (IAMs) rely heavily on BECCS to achieve net-zero emissions. The IPCC's 1.5$^\circ$C pathway assumes approximately 5--15 Gt $\mathrm{CO_2}$ per year of BECCS by 2100.
• Limitations: massive land requirements (achieving 10 Gt $\mathrm{CO_2}$/year via BECCS could require 300--1000 million hectares, equivalent to 20--70% of current global cropland), creating competition with food production; water requirements for energy crops; uncertain permanence of geological storage; currently operates at only a small fraction of the assumed scale.

Direct Air Capture (DAC). DAC extracts $\mathrm{CO_2}$ directly from ambient air using chemical solvents or solid sorbents, followed by release of concentrated $\mathrm{CO_2}$ for storage or use.

• Current facilities: the largest DAC plant (Climeworks' Mammoth plant in Iceland, operational 2024) captures approximately 36 000 tonnes of $\mathrm{CO_2}$ per year.
• Cost: approximately USD 250--600 per tonne, projected to fall to USD 100--200 per tonne by 2040 with scale and learning.
• Limitations: extremely energy-intensive (requiring heat and electricity); current capacity is negligible relative to emissions; very high cost.

REDD+ Framework

REDD+ (Reducing Emissions from Deforestation and Forest Degradation, plus the sustainable management of forests, and the conservation and enhancement of forest carbon stocks) is a UN framework that provides financial incentives for developing countries to reduce deforestation and forest degradation.

Mechanism. Under REDD+, developing countries receive payments (from developed country governments, multilateral funds, or carbon markets) for verified reductions in forest carbon emissions below a reference level (baseline). The mechanism requires countries to establish national forest monitoring systems, reference emission levels, and safeguards to protect biodiversity and indigenous rights.

Results framework. The Warsaw Framework (2013) established the rules for REDD+. As of 2023, over 50 countries have developed REDD+ strategies or action plans, and several countries (Brazil, Indonesia, Colombia) have received results-based payments for verified emission reductions. However, the scale of finance (approximately USD 5 billion committed cumulatively) is small relative to the drivers of deforestation.

Limitations:

1. Leakage: reducing deforestation in one area may displace it to another (if logging companies simply move to unprotected forests).
2. Permanence: carbon stored in forests is vulnerable to release by wildfires, drought, or political changes.
3. Measurement uncertainty: estimating forest carbon stocks and changes is technically challenging, particularly in remote tropical forests.
4. Governance: REDD+ requires strong governance, land tenure security, and anti-corruption measures that are lacking in many forested developing countries.
5. Indigenous rights: there is concern that REDD+ could restrict indigenous land use without adequate consultation or compensation.

Carbon Trading

Cap-and-Trade (Emissions Trading Systems)

Cap-and-trade systems set a total cap on greenhouse gas emissions for participating sectors and allocate or auction emission allowances (each allowance permits the emission of one tonne of $\mathrm{CO_2}$ equivalent). Allowances can be traded among participants: firms that can reduce emissions cheaply sell excess allowances; firms facing high reduction costs buy allowances. The cap is reduced over time, driving progressively deeper emission cuts.

Major emissions trading systems:

System	Region	Coverage	Price (2023)
EU Emissions Trading System (EU ETS)	European Union (plus EEA and Switzerland)	Power generation, heavy industry, aviation (approximately 40% of EU emissions)	Approximately EUR 80--100 per tonne $\mathrm{CO_2}$
California Cap-and-Trade	California, USA	Power generation, industry, transportation fuel (approximately 80% of California emissions)	Approximately USD 30--35 per tonne $\mathrm{CO_2}$
China National ETS	China	Power sector (approximately 30% of China's emissions)	Approximately CNY 60--80 per tonne $\mathrm{CO_2}$

Carbon Offset Markets

Carbon offset markets allow entities (governments, companies, individuals) to compensate for their emissions by purchasing credits generated by emission reduction or removal projects elsewhere. Offset projects include afforestation, renewable energy installation, methane capture from landfills, and cookstove distribution.

Criticism of offsets:

1. Additionality: many offset projects would have occurred anyway without the offset revenue (e.g., renewable energy projects that are already commercially viable). If the offset does not represent additional emission reductions, it does not compensate for the purchaser's emissions.
2. Permanence: forest carbon offsets are vulnerable to reversal by wildfires, drought, or future deforestation.
3. Verification challenges: estimating the actual emission reductions or removals from offset projects is technically difficult and susceptible to over-crediting.
4. Ethical concerns: offset markets allow wealthy countries and corporations to continue emitting by purchasing cheap offsets in developing countries, rather than reducing their own emissions.

Common Pitfalls: Equating Carbon Sequestration with Emission Reduction

Carbon sequestration and emission reduction are complementary but distinct strategies. Sequestration removes $\mathrm{CO_2}$ from the atmosphere; emission reduction prevents $\mathrm{CO_2}$ from entering the atmosphere. Both are needed to achieve net-zero emissions, but sequestration cannot substitute for emission reduction at the required scale. The IPCC makes clear that the primary pathway to limiting warming to 1.5$^\circ$C is rapid, deep emission reductions across all sectors, supplemented by carbon removal to address residual emissions. Over-reliance on future carbon removal technologies (particularly BECCS and DAC, which currently operate at negligible scale) risks delaying the emission reductions that are needed now. When discussing sequestration, always frame it as a complement to, not a substitute for, emission reduction.

For related topics, see ./atmospheric-systems and ./climate-adaptation-and-mitigation. The parent topic page is at ../climate-change.