The carbon cycle describes the natural movement of carbon among Earth’s major reservoirs: the atmosphere, the terrestrial biosphere, the hydrosphere, and the lithosphere. Before the industrial era, this global system existed in a state of approximate balance, where the amount of carbon dioxide (CO₂) entering the atmosphere was nearly equal to the amount being removed by natural processes like photosynthesis and ocean absorption. Human activities have profoundly disrupted this long-standing balance by accelerating the movement of carbon from the slow, geologic reservoir and the terrestrial biosphere into the atmosphere. This perturbation has fundamentally altered the fluxes, or annual exchange rates, between all major carbon pools, creating an imbalance that drives global environmental change.
Mobilizing Geologic Carbon Reserves
The largest disruption to the natural carbon cycle is the rapid mobilization of carbon that had been inertly stored in the Earth’s crust for millions of years. This reservoir is composed of fossil fuels—coal, oil, and natural gas. These resources represent carbon that was effectively locked out of the active atmospheric-oceanic system over geologic timescales.
The extraction and subsequent combustion of these fossil fuels rapidly transfers this sequestered carbon back into the atmosphere in the form of carbon dioxide. This process is occurring at a rate hundreds to thousands of times faster than nature can remove it.
The pre-industrial atmospheric concentration of CO₂ was approximately 280 parts per million (ppm). Continuous burning of fossil fuels since the mid-18th century has injected massive quantities of new carbon into the active cycle, causing atmospheric CO₂ concentrations to rise to levels exceeding 420 ppm.
This rapid transfer shifts carbon from the lithosphere’s slow cycle directly into the atmosphere’s fast cycle, overwhelming the natural balancing mechanisms.
Altering Carbon Exchange Through Land Use
Human land use practices constitute the second major way carbon is rapidly transferred into the atmosphere. This involves the disruption of the terrestrial biosphere, which includes all living biomass and the vast amount of carbon stored in soils.
Land use change, primarily through deforestation and agricultural methods, shifts large areas from being carbon sinks (absorbers) to carbon sources (releasers). The removal of forests eliminates a significant portion of the terrestrial carbon sink, as the stored carbon in the trees’ biomass is released through decay or burning.
In addition to the above-ground impact, the disturbance of soil, particularly through intensive tillage in agriculture, accelerates the decomposition of soil organic matter. Tilling breaks apart soil aggregates, which physically protect stored carbon from microbes. This physical disruption exposes previously protected organic matter to oxygen, allowing aerobic bacteria to rapidly decompose it and release the stored carbon as CO₂. Land use changes contribute to atmospheric loading both by removing the biological capacity to absorb carbon and by oxidizing carbon already stored in the soil.
The Consequence of Atmospheric Loading
The combined effect of mobilizing geologic carbon and disrupting terrestrial reservoirs is the rapid increase in the atmospheric concentration of carbon dioxide. This atmospheric loading is the direct cause of the enhanced greenhouse effect.
Earth’s surface absorbs incoming solar energy and re-radiates it back toward space as infrared radiation. Certain gases in the atmosphere, including CO₂, methane, and water vapor, are capable of absorbing specific wavelengths of this outgoing infrared energy. This process traps heat within the lower atmosphere, acting like an insulating blanket.
However, the continuous, human-caused increase in CO₂ concentration amplifies this effect, leading to an enhanced retention of heat. This long-term increase in atmospheric CO₂ is clearly demonstrated by the Keeling Curve, a continuous record of measurements that shows a steady, accelerating rise from pre-industrial levels. The concentration of atmospheric CO₂ has risen by more than 40% since the start of the Industrial Revolution, reaching levels not seen in millions of years.
Ocean Acidification and Sink Saturation
The planet attempts to re-establish its carbon balance by utilizing its natural carbon sinks, primarily the oceans and the remaining terrestrial biosphere, to absorb the excess atmospheric CO₂. The ocean has absorbed roughly 25 to 30 percent of the carbon dioxide released by human activities in recent decades, which has significantly slowed the rate of atmospheric warming. This absorption occurs as CO₂ dissolves into the surface water.
However, this absorption comes at a cost to the ocean’s chemistry. When CO₂ dissolves in seawater, a portion of it reacts with water molecules to form carbonic acid. This increase raises the water’s acidity, a process known as ocean acidification.
Since the start of the industrial era, the pH of surface ocean waters has fallen by approximately 0.1 units, representing a substantial 30 percent increase in acidity. Meanwhile, the capacity of the terrestrial sink—plants and soil—to absorb additional CO₂ is also showing signs of stress and potential saturation.
While higher CO₂ levels can initially stimulate plant growth, known as the CO₂ fertilization effect, this uptake is often limited by other factors, such as the availability of nutrients or water. Furthermore, rising global temperatures increase the rate of decomposition of organic matter in the soil and trigger events like permafrost thaw, which releases previously locked-away carbon. This weakening of natural sinks means a greater proportion of new human-caused emissions remains in the atmosphere, accelerating the overall planetary imbalance.