When working with any problem, it is important to understand the full context. Therefore, with atmospheric carbon it is important to understand Earth’s carbon cycles and carbon stores, not just point emissions from a particular narrow industry or fuel. In common discourse it is easy to loose sight of the scales involved and mis-judge.

There are actually two separate planet-wide carbon cycles and carbon stores:

1. Deep Carbon (slow geophysical phenomenon)
2. Surface Carbon (faster biological phenomenon)

Due to frequent use of words carbon and climate change, people are often surprised to learn that atmospheric carbon represents only 0.00000002 of earth’s total. Even when only considering the surface layer, it is less than 2% of that total. (Whilst at the same time being 860 Gigatons of pure carbon, which is a lot.)

As these quantities are on planetary scale, they are difficult conceptualize. Here magnitudes are more important than detailed values. The sections below summarize main aspects of Earth’s carbon stores and carbon cycles:

1. Deep Carbon stores. Whilst uncertainties remain, it is clear that the great majority of Earth’s carbon is deep in its core, mantle and lithospheric crust – and only a very small portion remains on surface interacting with atmosphere.
2. Deep Carbon cycle. Most of the deep carbon is inert over the short-term – locked beneath the surface. However over the long-term, emissions through mantle degassing (volcanic and oceanic mid-ridge emissions) and in reverse absorption via rock weathering, create the loop that defines and balances atmospheric carbon levels – over million year time scales.
3. Earth’s VERY long-term climate. Indeed these deep carbon effects and feedback loops have been the main stabilizing factor on climate over the very long – million year – time scales. This is evident in the fossil, ocean sediment, ice core, and stalagmite studies that paleoclimatology has used in unravelling earth’s atmospheric and temperature histories.
4. Surface carbon stores. Unlike the deep carbon, surface carbon in hydrosphere and terrestrial biomes interact continuously with atmosphere. Here the magnitudes of carbon stocks are more balanced, particularly when we limit ourselves only to active carbon. However, even in the context of surface carbon, carbon in air is much smaller than that in oceanic or terrestrial stocks. Furthermore, as the stocks are magnitudes larger than their annual flows, they are critical whilst often overlooked in the climate change discussions.
5. Surface carbon cycle. From the perspective of climate change the dynamic moves among surface carbon stores is the key element. In recent thousand-year histories these have mostly been in balance, and began to change only when anthropogenic effects become visible.

In the deep carbon cycle, mantle and lithospheric carbon continually escapes through degassing (volcanoes and mid-ocean ridges) and returns back to mantle via the process of rock weathering, carbonate formation, accumulation and subduction of oceanic plates. It plays the key role in regulating Earth’s climate over very long – million year – timescales. In the words, earth’s climate histories are determined by the balance between CO₂ emitted by volcanic degassing, CO₂ removed by weathering of rocks, and stabilizing feedback loops embedded in the deep carbon cycle.

Deep Carbon Emissions. Atmospheric release from the mantle is straight forward: it is CO₂ released in volcanoes, mid-ocean ridges, and oceanic hotspots. Most visible are the occasional volcanic eruptions, but a more steady seeps comes from at tectonic plates divergence boundaries: as oceanic plates drift apart, mid-ocean ridges form allowing magma to rise and solidify as new oceanic crust thereby releasing deep carbon.

Deep Carbon Capture. Atmospheric carbon’s capture back into mantle is a bit more complicated. This carbon capture starts with rain, where atmospheric CO₂ combines with water to form weak carbonic acid. Upon contact the acid in turn dissolves rock in a process known as chemical weathering, releasing magnesium, potassium, sodium and, in particular calcium ions.¹ Rivers carry these calcium ions into the ocean where they react with dissolved bicarbonate.  The product of that reaction, solid calcium carbonate (CaCO₃), slowly sinks onto the ocean floor, where it over time becomes limestone.

Eventually as a result of continental shift, the oceanic plates collide with continental ones,. The denser oceanic plates are forced beneath the continental plates in process called subduction, and the limestone embedded carbon moves back into the lithosphere and upper mantle.

These subducted carbonates can be further transformed under mantle’s high pressure and temperature via metamorphism and mineralization. Some carbon may even be  transported into the lower mantle via (upwellings from Earth’s core-mantle boundary).  Together this creates a VERY long term (hundred million to billion year) storage of carbon within Earth’s interior, whose total size dwarfs the surface carbon.

From the perspective of atmospheric carbon, deep carbon cycle has two main characteristics: highly stabilising feedback mechanism, but very slow speed.

Stabilizing feedback mechanism. The keys aspect of deep carbon cycle is that weathering acts as a natural feedback mechanism that regulates atmospheric CO₂levels.

Higher CO₂concentrations increase the rate of weathering, which in turn draws down CO₂, stabilizing the climate.

Higher CO₂ concentrations, warmer and wetter climates enhance weathering rates, leading to increased CO₂sequestration. Similarly lower CO₂, with colder and drier climates reduced the capturing process.

Thereby for example  increased tectonic activity and CO₂release initially lead to higher atmospheric carbon, but the newly exposed surfaces along with more active weathering process slowly turn the atmospheric carbon levels back to equilibrium. The deep carbon cycle prevents long-term runaway type climate change, and has kept Earth’s climate habitable for 4 billion years. (Isbell et al. 2012, Kump 2016, Park et al. 2020, Mitchell et al. 2021).

Slow speed. Whilst deep carbon cycle stabilises the climate, it only over very long, million year timeframes. It has been estimated that carbon takes between 100-200 million years to move through the slow carbon cycle. Indeed, the current estimated magnitude of chemical weathering on Earth is approximately 0.3 Gt of removed carbon per year (Bufe  2024). Similarly, total mantle degassing form volcanoes and volcanic regions is estimated at 0.28 to 0.36 Gt of Carbon per year. (Black and Gibson 2019). Thus the current system is roughly in balance, and in magnitude about one thirtieth of anthropogenic emissions.

Therefore, whilst deep carbon cycle is assuring from the perspective that it will correct any run-away climate change over the VERY long term, it cannot offset the current rapid increase in CO2 in any biological scale.

1. Carbonate Weathering: Involves the dissolution of carbonate rocks (e.g., limestone CaCO₃) to form bicarbonate ions: CaCO₃+CO₂+H₂O→Ca²⁺+2HCO₃
   Silicate Weathering: Involves the breakdown of silicate minerals (e.g., calcium silicate CaSiO₃) to form clay minerals and bicarbonate ions: CO₂ + H₂O + CaSiO₃ -> CaCO₃ + SiO₂ + H₂O
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WRITEUP IN PROGRESS.

Earth’s carbon can be viewed from an even more fundamental perspective: All life’s structures are based on atomic carbon, and the carbon cycle of photosynthesis & cell perspiration creates all of its energy.

Carbon’s unique atomic characteristics underlie the highly versatile building blocks that allow complex life-forms:

1. Tetravalency: Carbon has four valence electrons, allowing it to form up to four covalent bonds with other atoms. This makes it highly versatile in forming complex and stable molecules.
2. Catenation and bonding with other essential elements: Carbon bonds easily with itself (catenation) forming long chains and rings. This ability to form stable chains and cyclic structures is fundamental to the complexity of organic molecules, from simple hydrocarbons to complex proteins and DNA. Carbon also bonds easily with other essential elements such as hydrogen, oxygen, nitrogen, sulfur, and phosphorus. These combinations in turn create key biological molecules like carbohydrates, proteins, lipids, and nucleic acids.
3. Versatility: Indeed, with this ease of bonding carbon forms truly a vast array of compounds, more than any other element. This diversity is crucial and organic molecules can greatly vary in size, shape, and function.
4. Allotropic Forms: Carbon also exists in different allotropic forms, such as graphite, diamond, and fullerenes, each with unique properties. This versatility extends to organic chemistry, where slight changes in molecular structure can lead to vastly different properties and functions.
5. Stability and Reactivity Balance: Carbon compounds are generally stable under a wide range of temperatures and conditions, making them suitable for the diverse environments on Earth. However, at the same time they can undergo wide range of chemical reactions. This balance is essential for metabolic processes, allowing living organisms to build and break down molecules efficiently.
6. Abundance: Carbon is also plentiful on earth and indeed the fourth most abundant element in the solar photosphere (and indeed in known universe, originally created in the centre of aging stars).

In addition to the static building blocks of life, it is the very basic carbon cycle that creates the base energy used by every living organism.

1. Photosynthesis: Plants, algae, and certain bacteria use photosynthesis to convert carbon dioxide, water, and light energy into glucose and oxygen.

• 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

2. Respiration: Animals and other organisms release CO₂ back into the atmosphere through respiration where the opposite occurs. In living cells ATP (C₁₀H₁₆N₅O₁₃P₃) is a high-energy molecule that with water can be broken down into ADP (C₁₀H₁₅N₅O₁₀P₂) and inorganic phosphate (HPO₄^2-) to release energy. Cellular respiration regenerates ATP from ADP, producing carbon dioxide and water as by-products.

• C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP

This most basic carbon cycled maintains the ecosystem balance of carbon and thus basic stability of planet’s life.