Human activity is changing the carbon cycle mainly by moving large amounts of carbon from long-term underground storage into the atmosphere much faster than natural processes can remove it. Burning coal, oil, and natural gas releases carbon dioxide that was stored underground for millions of years. Deforestation and other land-use changes also release carbon stored in vegetation and soils while reducing the ability of ecosystems to absorb carbon from the atmosphere.
The result is a major shift in the balance among the atmosphere, land, oceans, living organisms, and long-term carbon stores.
What the carbon cycle does naturally
Carbon is constantly moving through Earth’s atmosphere, oceans, soils, rocks, plants, animals, and microorganisms. This movement is called the carbon cycle.
Plants, algae, and some microorganisms remove carbon dioxide from the atmosphere through photosynthesis. They use sunlight to convert carbon dioxide and water into organic compounds, incorporating carbon into their tissues. When plants are eaten, that carbon moves into animals and other organisms.
Carbon returns to the atmosphere through respiration, the process by which organisms release carbon dioxide as they obtain energy from organic molecules. When organisms die, decomposers break down their remains, returning some of the carbon to the atmosphere and storing some in soils.
The oceans are another enormous part of the cycle. Carbon dioxide continually moves between the atmosphere and surface seawater. Some dissolved carbon is transported into deeper water, while marine organisms incorporate carbon into their bodies and shells. Over long periods, some carbon becomes buried in sediments.
A much slower part of the cycle operates through rocks and geological processes. Carbon can be stored in carbonate rocks and sediments for millions of years. Under particular geological conditions, some ancient organic matter was transformed into fossil fuels such as coal, oil, and natural gas.
Natural carbon cycling is not static. Carbon has always moved among these reservoirs, and the amount of carbon in each one can change. What makes modern human activity significant is the speed and scale at which people are shifting carbon from long-term reservoirs into the atmosphere.
Burning fossil fuels adds ancient carbon to the atmosphere
The largest human influence on the carbon cycle comes from burning fossil fuels.
Coal, oil, and natural gas contain carbon that was removed from the atmosphere by ancient organisms and eventually buried and transformed over geological time. Under natural conditions, much of this carbon would remain underground for extremely long periods.
When people extract and burn these fuels, that stored carbon is converted primarily into carbon dioxide and released into the atmosphere.
This changes the carbon cycle in a fundamental way. Instead of carbon remaining in a relatively isolated geological reservoir, a portion is rapidly transferred into the atmosphere and then redistributed among the oceans, vegetation, soils, and other reservoirs.
Some of the additional carbon is absorbed by the land and oceans. These natural sinks slow the accumulation of carbon dioxide in the atmosphere, but they do not immediately return the carbon to the geological reservoirs from which it came.
That difference in timescale matters. Carbon removed from the atmosphere by a forest or absorbed by the ocean may remain there for years, decades, centuries, or longer, depending on the process. Carbon released from fossil fuels can therefore accumulate in the faster parts of the carbon cycle while the slow geological cycle cannot keep pace.
Deforestation changes both sides of the carbon balance
Cutting and clearing forests affects the carbon cycle in two related ways: it can release stored carbon and reduce future carbon uptake.
Trees and other plants contain carbon accumulated through photosynthesis. Forest soils and dead organic matter can also hold substantial amounts of carbon. When forests are cleared, some of this carbon eventually returns to the atmosphere through burning, decomposition, or disturbance of the soil.
At the same time, a forest that has been removed generally absorbs less carbon dioxide than the vegetation that replaced it, particularly when the replacement land contains relatively little vegetation.
The effect depends on what happens after clearing. A forest converted to cropland, pasture, or another land use does not necessarily lose all of its stored carbon, and vegetation can regrow in some places. But widespread land-use change has altered the distribution and storage of carbon across terrestrial ecosystems.
Human activities also affect grasslands, wetlands, peatlands, and other ecosystems that store carbon belowground. Disturbing these systems can expose previously protected organic matter to decomposition and change how much carbon ecosystems retain.
Agriculture changes carbon movement through soils and ecosystems
Agriculture has reshaped large areas of land and therefore changed the movement of carbon between plants, soils, and the atmosphere.
Plants continually move carbon into soils through roots, fallen leaves, and other organic material. Soil organisms then consume and decompose this material, releasing some carbon dioxide and incorporating some carbon into soil organic matter.
Cultivation can disturb this balance. Tilling and other practices can accelerate the decomposition of organic material and alter how carbon is stored in soils. Removing crop biomass also changes how much organic carbon returns to the soil.
Agricultural systems can therefore either release carbon or retain more of it depending on vegetation, soil conditions, management practices, and how land is used. The carbon cycle is affected not simply by farming itself but by the particular ways agricultural landscapes are managed.
Urbanization changes local carbon storage and emissions
Cities transform natural ecosystems into built environments containing roads, buildings, parking areas, and other infrastructure. This changes the local carbon cycle by replacing vegetation and soils with surfaces that store carbon differently and by concentrating energy use and transportation.
Urban areas also generate substantial carbon dioxide emissions because buildings, vehicles, industry, and electricity production depend on energy sources that can involve fossil fuels.
Urbanization therefore affects the carbon cycle both through direct emissions and through changes in the land that previously stored and exchanged carbon with the atmosphere.
The oceans absorb much of the additional carbon
The ocean is one of Earth’s major carbon reservoirs, and it absorbs a significant portion of the carbon dioxide added to the atmosphere by human activities.
When atmospheric carbon dioxide dissolves in seawater, it participates in a series of chemical reactions involving dissolved carbon dioxide, carbonic acid, bicarbonate, and carbonate ions. These reactions allow the ocean to take up additional carbon from the atmosphere.
Ocean circulation can transport dissolved carbon from surface waters into deeper parts of the ocean, where it may remain for much longer periods.
But absorbing additional carbon dioxide also changes seawater chemistry. The increased concentration of dissolved carbon dioxide lowers ocean pH, a process known as ocean acidification. It also changes the availability of carbonate ions, which many marine organisms use to build shells and skeletons.
Thus, the ocean’s role as a carbon sink helps slow the increase of atmospheric carbon dioxide while simultaneously altering the chemistry of marine environments.
Plants and soils are responding to higher carbon dioxide
Higher atmospheric carbon dioxide can affect the terrestrial carbon cycle because carbon dioxide is a raw material for photosynthesis.
Under suitable conditions, increased carbon dioxide can stimulate photosynthesis and plant growth. This effect is sometimes called carbon dioxide fertilization. However, the response varies among plant species and ecosystems and depends on factors such as water, nutrients, temperature, and growing conditions.
Plant growth alone does not determine whether an ecosystem becomes a long-term carbon sink. Carbon taken up by plants can later return to the atmosphere through respiration, decomposition, fire, harvesting, or other disturbances.
Climate change itself can also alter these processes. Changes in temperature, rainfall, drought, wildfire, and other environmental conditions can affect how much carbon ecosystems absorb and how much they release.
Human activity is altering the balance between carbon sources and sinks
A carbon source adds more carbon to the atmosphere than it removes over a given period. A carbon sink absorbs more carbon than it releases.
Fossil-fuel combustion is a major human-caused carbon source. Land-use changes can also act as sources when they release more stored carbon than the affected ecosystems absorb.
Forests, soils, and oceans can function as carbon sinks by taking up some of the additional carbon dioxide. But their ability to absorb carbon is not unlimited, and their behavior can change as environmental conditions change.
This means the modern carbon cycle is not simply a matter of humans adding carbon to the atmosphere while nature removes it. Instead, human emissions have altered the flows among several reservoirs, while natural systems redistribute and absorb part of the added carbon.
Why the speed of change matters
Carbon has always moved through Earth’s systems, so the important question is not whether humans have introduced carbon into the atmosphere at all. The key issue is how rapidly carbon is being transferred between reservoirs.
The geological carbon cycle normally moves carbon on timescales ranging from thousands to millions of years. Human societies can transfer comparable forms of stored carbon much more rapidly through mining, extraction, combustion, and land-use change.
Natural sinks then redistribute some of that carbon through the atmosphere, oceans, vegetation, and soils. But these processes operate on different timescales and cannot simply reverse the original geological storage on human timescales.
The modern carbon cycle is therefore characterized by a large, rapid transfer of carbon from long-term geological storage into the active carbon cycle.
Human changes to the carbon cycle are connected to climate change
Carbon dioxide is a greenhouse gas, meaning it absorbs and re-emits infrared radiation and helps regulate the amount of heat retained in Earth’s climate system.
Increasing the concentration of carbon dioxide in the atmosphere strengthens the greenhouse effect, contributing to global warming. Other human activities also add greenhouse gases, but carbon dioxide from fossil fuels and land-use change is central to the long-term human alteration of the carbon cycle.
Warming then feeds back into the carbon cycle. For example, changes in temperature, precipitation, vegetation, wildfire, and ocean conditions can influence how quickly carbon moves between ecosystems, the atmosphere, and the ocean.
The carbon cycle and climate system are therefore tightly linked: changing the movement of carbon changes atmospheric greenhouse-gas concentrations, while a changing climate can in turn modify the movement and storage of carbon.
The carbon cycle has not stopped—it has been shifted
Human activity has not created a new carbon cycle. It has changed the existing one.
Fossil-fuel extraction and combustion move carbon from geological storage into the atmosphere. Deforestation and land-use change alter carbon stored in vegetation and soils. Agriculture and urbanization modify exchanges between land and atmosphere. Meanwhile, oceans and terrestrial ecosystems absorb and redistribute part of the additional carbon.
The central change is a disruption of the long-standing balance among carbon reservoirs and the rates at which carbon moves between them. Understanding that shift is essential for understanding why atmospheric carbon dioxide has risen and why human changes to the carbon cycle have become a major driver of modern climate change.
