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The carbon cycle

Review your understanding of the carbon cycle with this free article aligned to NGSS and AP standards.

Key terms

CarbonAn essential element that forms the base of all organic matter, including the bodies of living organisms, and is a key component of fossil fuels
Carbon dioxide (COA2)A compound in the atmosphere that is taken in by photosynthetic organisms to make organic molecules and is later released back into the atmosphere through processes such as respiration and decomposition
PhotosynthesisThe process by which energy from sunlight is used to transform carbon dioxide from the atmosphere into the organic matter that fuels food webs
Cellular respirationThe process by which living organisms break down organic compounds to produce usable energy, releasing carbon dioxide into the atmosphere as a by-product
Carbon cycleThe complex flow of carbon between inorganic and organic sources within the environment

The carbon cycle and carbon reservoirs

The carbon cycle describes the continuous flow of carbon between organic and inorganic carbon reservoirs, or areas of Earth where large amounts of carbon are stored. Most of Earth’s carbon is found in inorganic reservoirs such as rocks, water, and sediments. Only a small portion is stored in organic reservoirs, such as in the bodies of living organisms.
The carbon cycle is represented in the following diagram:
A diagram shows processes within the carbon cycle connected by arrows indicating the flow of carbon within and between the atmosphere, land, and ocean. Processes that cycle carbon between the air and the surface include the burning of fossil fuels and wood, volcanic eruptions, terrestrial and marine photosynthesis, and air-sea gas exchange. Carbon cycles on land via weathering, runoff, sedimentation, and soil accumulation. Underground, fossil fuel formation and decomposition cycle carbon.
To help us understand the carbon cycle, we can think of it as two interconnected subcycles: a biological carbon cycle and a geological carbon cycle.

Biological carbon cycle

The biological carbon cycle occurs through biological processes and describes the relatively quick flow of carbon between the atmosphere and Earth’s ecosystems.
Carbon is introduced into food webs by photosynthetic organisms, which convert gaseous COA2 from the atmosphere into biomass. As organisms carry out cellular respiration, they break and re-form molecular bonds, producing usable energy and cycling COA2 back into the atmosphere. The decomposition of dead organisms and other nonliving organic matter also returns COA2 to the atmosphere.
Organic matter that is not immediately broken down accumulates and is buried. Given enough time and pressure, this matter turns into fossil fuel deposits and rock, which brings us to the geological carbon cycle.

Geological carbon cycle

The geological carbon cycle occurs by geologic processes and describes the much slower flow of carbon between Earth’s nonliving carbon reservoirs.
During the geological carbon cycle, carbon moves from rocks on land into the oceans via weathering and rainwater runoff. Carbon also enters oceans from the atmosphere as carbon dioxide dissolves into the water, forming carbonic acid and bicarbonate ions.
Bicarbonate is part of the shells of many marine creatures, and these shells and other sunken organic materials ultimately form the sediment and rocks lining the ocean floor. Carbon may remain stored deep in sediments and rocks for millions of years until an event such as a volcanic eruption returns it to the surface.

Human impacts on the carbon cycle

The amount of carbon in the atmosphere has been increasing rapidly over the past century, and this increase is largely the result of human activity. For example, the burning of fossil fuels releases a significant amount of carbon into the atmosphere, and deforestation removes photosynthetic organisms that would normally help trap excess COA2.
Although photosynthetic organisms and oceans absorb some COA2, they simply can’t keep up. As a result, carbon is entering the atmosphere faster than it can cycle back into reservoirs for long-term storage. This excess atmospheric COA2 contributes to global warming and, ultimately, climate change.

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