Carbon sources, sinks and stores
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Convex Seascape SurveyThe global carbon cycle involves complex interactions between Earth's major carbon reservoirs. Understanding how carbon moves between these reservoirs is crucial for comprehending climate change and environmental processes.
Carbon exists in various forms across five major planetary reservoirs. The ocean is by far the largest carbon store, holding carbon primarily as dissolved inorganic carbon. The soil (on land) and sediment (in the ocean) store include carbon in organic matter and minerals. Vegetation stores carbon in living plant biomass. Fossil fuels represent ancient organic matter transformed over millions of years. The atmosphere holds carbon primarily as carbon dioxide (CO₂) and methane (CH₄). Understanding these stores and how they interconnect is essential for teaching the carbon cycle.
Carbon stores then and now
The infographic below provides a powerful visual representation of how Earth's carbon stores have changed since pre-industrial times. The size of each store and the magnitude of change are shown to scale, helping students grasp both the relative size of different carbon reservoirs and how human activities have altered them.
These shifts in carbon stores tell a clear story of human influence on the carbon cycle. The most dramatic changes are interconnected: the substantial decrease in fossil fuel carbon (-445 GtC (Gigatonnes of carbon)) corresponds closely with increases in atmospheric carbon (+279 GtC) and ocean carbon (+173 GtC). This reflects how carbon from burned fossil fuels moves primarily into the atmosphere, with the ocean absorbing about a third of these emissions.
The slight decrease in vegetation carbon (-25 GtC) adds to this atmospheric burden, mainly through deforestation and land-use change. While soil and sediment stores remain stable overall, their role in the carbon cycle is dynamic, with ongoing exchanges that could shift under climate change. These changes are significant because the increased atmospheric carbon drives global warming, while elevated ocean carbon leads to acidification, affecting marine ecosystems. Understanding these interconnected changes helps explain both the causes and consequences of climate change.
How carbon moves
Carbon constantly moves between these different stores through various natural and human-driven processes. These movements are known as fluxes and are carefully quantified by scientists and reported in IPCC (International Panel on Climate Change) assessments. The interactive diagram below shows some of the most important current annual fluxes. The largest fluxes are human-caused: burning fossil fuels releases about 9.4 GtC to the atmosphere each year, while deforestation and other land-use changes add another 1.6 GtC. However, natural processes help offset these emissions - both the ocean (through physical and chemical processes) and land-based vegetation (through photosynthesis) act as vital carbon sinks, together removing more than half of human emissions from the atmosphere each year.
Key Definitions
Carbon Store A reservoir that holds carbon for extended periods. Examples include the ocean, soil, and vegetation. The size of a store can increase or decrease over time depending on incoming and outgoing fluxes.
Carbon Source Any process or reservoir that releases more carbon than it absorbs, resulting in a net addition of carbon to the atmosphere. Current major sources include fossil fuel combustion and land-use changes.
Carbon Sink Any process or reservoir that absorbs more carbon than it releases, resulting in a net removal of carbon from the atmosphere. The ocean and terrestrial vegetation currently act as carbon sinks, partially offsetting human emissions.