What is the Carbon Cycle?
The carbon cycle describes the continuous movement of carbon atoms between the atmosphere, oceans, terrestrial biosphere, and geosphere. Carbon exists in different forms — as CO2 in the atmosphere, dissolved in ocean water, stored in plant biomass and soils, and locked in fossil fuels and carbonate rocks. Natural fluxes move roughly 210 GtC per year between these reservoirs. Human activities have disrupted this balance by releasing approximately 11 GtC annually from fossil fuels and land use changes — carbon that was sequestered over millions of years, injected into the atmosphere in decades.
Why It Matters
Understanding the carbon cycle is fundamental to effective climate strategy. The rate at which atmospheric CO2 increases depends not just on emissions but on how efficiently natural sinks — primarily oceans and terrestrial ecosystems — absorb those emissions. Currently, natural sinks remove about 54% of annual anthropogenic CO2, with oceans absorbing roughly 26% and land ecosystems 28%. The remaining 46% accumulates in the atmosphere. If sink efficiency declines — a real possibility under warming conditions — the same emission levels would produce faster atmospheric CO2 growth.
The ocean carbon sink is both essential and threatened. The ocean has absorbed approximately 30% of all human-emitted CO2 since the industrial revolution — roughly 170 GtC. This absorption has slowed the pace of atmospheric warming but at a cost: ocean acidification. Surface ocean pH has dropped from 8.2 to 8.1 since pre-industrial times — a 26% increase in acidity. This threatens calcifying organisms (corals, shellfish, plankton) that form the foundation of marine food webs and support fisheries worth $100 billion annually.
For businesses, the carbon cycle defines both the problem and the solution space. Carbon removal strategies — from reforestation to direct air capture — work by enhancing natural or creating artificial carbon sinks. Blue carbon ecosystems (mangroves, seagrasses, salt marshes) sequester carbon 2–4 times faster per unit area than terrestrial forests. Understanding which carbon pools are stable, which are vulnerable, and which can be enhanced is essential for evaluating climate strategies and offset investments.
The vulnerability of carbon stocks creates material climate risk. Permafrost contains an estimated 1,500 GtC — nearly twice the amount currently in the atmosphere. As permafrost thaws, microbial decomposition releases this carbon as CO2 and methane. Tropical forests store roughly 250 GtC but are increasingly losing carbon through deforestation, fire, and drought stress. If these stocks become net sources rather than sinks, they would accelerate warming beyond what emission reduction alone can manage.
How It Works / Key Components
The fast carbon cycle operates on timescales of days to centuries through photosynthesis, respiration, decomposition, and ocean gas exchange. Plants absorb atmospheric CO2 through photosynthesis, incorporating carbon into biomass. Respiration, decomposition of organic matter, and fires return carbon to the atmosphere. Ocean surface waters exchange CO2 with the atmosphere based on partial pressure differentials — cold, productive waters absorb CO2 while warm, upwelling waters release it.
The slow carbon cycle operates over millions of years through geological processes. Weathering of silicate rocks consumes CO2, which is eventually transported to the ocean and incorporated into carbonate sediments. Organic matter buried in sediments becomes fossil fuels over geological time. Volcanic eruptions return carbon to the atmosphere. By extracting and burning fossil fuels, humans are short-circuiting the slow carbon cycle — releasing in decades what took millions of years to sequester.
The biological pump in the ocean transfers carbon from surface to deep waters. Phytoplankton fix CO2 through photosynthesis; when they die or are consumed, organic carbon sinks to depth. This biological pump exports roughly 10 GtC per year to deep ocean storage. Zooplankton fecal pellets, dead organisms, and dissolved organic carbon all contribute. Changes in ocean circulation, temperature, and nutrient availability affect pump efficiency — modeling suggests that warming could reduce biological pump carbon export by 10–20% by 2100.
Terrestrial carbon dynamics are increasingly volatile. Intact tropical forests absorb roughly 7.6 GtCO2 per year, but the Amazon's carbon sink has weakened significantly since the 2000s. Boreal forests, the largest terrestrial carbon store, face increased wildfire and insect outbreaks that release stored carbon. Soil organic carbon — the largest terrestrial pool at approximately 2,400 GtC — responds to temperature and moisture changes. Warming accelerates decomposition, potentially creating a positive feedback loop where soils release more carbon, amplifying warming further.
The Carbon Cycle in Practice
The Monterey Bay Aquarium Research Institute studies how the ocean carbon cycle responds to climate change, tracking how warming and acidification affect the biological pump in the California Current system. Their research has documented shifts in phytoplankton communities that affect carbon export efficiency — findings with direct implications for fisheries, marine conservation, and blue carbon strategies.
Norway's Northern Lights project — the world's first open-source CO2 transport and storage infrastructure — exemplifies industrial-scale intervention in the carbon cycle. The project will inject captured CO2 into geological formations beneath the North Sea, permanently returning carbon to the slow geological cycle. Initial capacity is 1.5 million tons annually, with plans to scale to 5 million tons.
Council Fire's Approach
The carbon cycle sits at the heart of Council Fire's ocean and climate practice. We help organizations understand how marine carbon dynamics — from blue carbon ecosystems to ocean acidification to the biological pump — create both risks and opportunities. Our work in coastal ecosystem conservation directly enhances natural carbon sinks, while our climate strategy practice helps clients develop carbon management approaches grounded in cycle science rather than simplistic offset accounting. We bring the ocean literacy that ensures carbon strategies account for the 70% of the Earth's surface that most sustainability consultancies overlook.
Frequently Asked Questions
What is blue carbon and why does it matter?
Blue carbon refers to carbon captured and stored by coastal and marine ecosystems — primarily mangroves, seagrasses, and salt marshes. These ecosystems sequester carbon at rates 2–4 times higher per hectare than terrestrial forests and store it in sediments for centuries to millennia. Despite covering less than 2% of ocean area, coastal wetlands account for roughly 50% of carbon burial in marine sediments. Their destruction releases stored carbon and eliminates ongoing sequestration capacity, making protection and restoration high-value climate interventions.
Is the ocean's ability to absorb carbon declining?
Evidence is mixed and regional. The Southern Ocean carbon sink weakened between 1990 and 2006 but partially recovered through 2020. The North Atlantic sink has shown more consistent weakening. As ocean surface waters absorb more CO2, they become more acidic and approach chemical saturation, reducing absorption efficiency. Warming also reduces CO2 solubility. Current projections suggest the ocean sink will weaken as a proportion of emissions through the century, though it will continue absorbing carbon in absolute terms under most scenarios.
How does deforestation affect the carbon cycle?
Deforestation releases the carbon stored in tree biomass (immediately through burning, gradually through decomposition) and disrupts soil carbon that can take decades to centuries to accumulate. Tropical deforestation emits roughly 4.8 GtCO2 per year — approximately 10% of global anthropogenic emissions. Beyond direct emissions, deforestation reduces regional rainfall recycling (a single Amazonian tree can transpire 1,000 liters per day), potentially triggering dieback feedback loops that release additional stored carbon.
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