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The Fast and Slow Carbon Cycles

The Earth system stores carbon in several primary carbon reservoirs, namely the marine and terrestrial biospheres, the atmosphere, the ocean, sediments, and rocks. The Earth system has two distinct, but coupled, carbon-cycling dynamics: the fast carbon cycle and the slow carbon cycle, a framing that has been put forward by NASA [Riebeek, 2011] amongst others.

Fast Carbon Cycle

The fast carbon cycle consists of carbon that flows into and out of reservoirs continuously or up to a decadal timescale, including the reservoir of atmospheric carbon in the form of CO₂. The fast carbon cycle encompasses the daily and seasonal cycling of carbon in the biosphere, atmosphere, and surface ocean primarily through photosynthesis and respiration. The fast carbon cycle is dynamic and volatile, and can be best understood as the flow of carbon through living ecosystems and the atmosphere.

Slow Carbon Cycle

The slow carbon cycle consists of the movement of carbon through longer-duration pathways and reservoirs via natural processes including chemical weathering, sedimentary burial, and ocean overturning circulation. These processes move carbon from living ecosystems into geological and deep ocean reservoirs, such as sediments, hydrocarbon deposits (oil, gas, coal), and deep waters. Carbon moves through slow cycle reservoirs over centuries to geologic timescales [Prentice et al., 2021].
The fast and slow carbon cycles are loosely coupled, and the global carbon cycle operates through a variety of response and feedback mechanisms which maintain a balance between these cycles, keeping the Earth’s atmosphere and ocean in a “Goldilocks zone” (i.e., the narrow range of conditions in which ecosystems and communities can thrive). Prior to the Industrial Revolution, carbon cycling between the atmosphere, ocean, biosphere, and geologic reservoirs, in both the fast and slow carbon cycles, was generally balanced in a manner that promoted stable climate, ocean chemistry, and ecosystems over human timescales.

The Necessity of Fast-to-Slow Framing

Because of the interconnectedness of these fast and slow carbon cycles, a singular focus on atmospheric carbon in existing carbon removal accounting practices is not consistent with the best available science and presents an incomplete framing of complex Earth system dynamics.
When anthropogenic activity transfers slow carbon to the fast carbon cycle through fossil emissions as CO₂, that increase in fast carbon is distributed throughout the fast carbon cycle, including in the atmosphere, the ocean, soils, and aboveground terrestrial biomass. That gross transfer of slow carbon to the fast carbon cycle represents a carbon liability. This carbon liability cannot be discounted based on the fate of where that fast carbon ends up – i.e., whether that fast carbon ends up in the soil, the surface ocean, or is naturally transferred back to the slow carbon cycle. To fully resolve that carbon liability – and to mitigate the harm caused by that liability, whether that be atmospheric warming, ocean acidification, or other – a carbon removal activity must occur. Functionally, this carbon removal activity is the inverse of the carbon liability; until an activity has occurred that reverses (i.e., removes) the damage, a liability remains on the emitters' (and the Earth system’s) “balance sheet”, and a total system imbalance will remain. The only way to mitigate anthropogenic emissions and rebalance the global carbon cycle is to remove the totality of those carbon liabilities — the more than 2.4 trillion metric tons of anthropogenic carbon that have been released from the slow carbon cycle since the onset of the Industrial Revolution [IPCC, 2022], and which include those pooled in the atmosphere, biosphere, or upper ocean — from the fast to the slow carbon cycle.
The activity of carbon removal can be defined as the intentional movement of carbon from the fast carbon cycle to the slow carbon cycle, where the total fast carbon removed exceeds the total slow carbon emitted within a given project boundary.
A fast-to-slow framing is inherently conservative in early-year calculations of net carbon removed due to the current atmosphere-centric design of emissions accounting standards and models used to quantify emissions. This conservatism considers both fossil carbon (slow-to-fast) and fast cycle fluctuations in evaluating the net carbon impact of a project, and as such allows for the implementation of an accounting approach that promotes the rebalancing of the full carbon cycle in line with the best available science as carbon accounting standards mature.
Figure 1: The fast and slow carbon cycles.