Carbon Cycle

There are two parts to the cycle:

The Fast Carbon Cycle:

This cycle moves carbon through living things and the atmosphere over a relatively short period of time, like a few years or decades.

Plants and phytoplankton take carbon dioxide out of the air to create energy they can use in a process called photosynthesis.

The carbon is passed to animals when they eat plants. Some of it is released in solid waste and some of it through respiration, which is when an animal breathes in oxygen and releases carbon dioxide.

Eventually, plants and animals die. They are broken down by microbes, releasing the carbon — and other nutrients — mostly back into the air in the form of carbon dioxide. Here, the carbon is ready to be absorbed again by plants and phytoplankton.

Plants can also burn in wildfires, which similarly release the carbon back into the air in the form of carbon dioxide.

The Slow Carbon Cycle:

This cycle moves carbon over tens to hundreds of millions of years between rocks, soil, oceans and the atmosphere. Rainwater, in combination with CO2 in the air, slowly breaks down rocks over time. This weathering process releases ions that eventually run off into the ocean and form the shells of organisms like corals. When these organisms die, the shells sink to the ocean floor. Over millions of years, these shells become rock, locking carbon deep below the surface.

Another way that carbon ends up underground is when plant and animal material are trapped in layers of mud. Over millions of years, these layers build one on top of the other. The earth’s heat and pressure eventually form the layers into rock. In certain conditions, if the plant and animal material do not decompose, this heat and pressure can turn them into fossil fuels.

Some of this carbon that is stored deep underground is released back into the atmosphere naturally by volcanoes over long periods of time.

Human impact on the carbon cycle

Fossil fuel extraction takes carbon that was previously locked away deep underground and releases it into the atmosphere. This accelerates the release of underground carbon, or causes the release of carbon that might not have ever been released into the atmosphere through natural means. This cuts the slow cycle short and throws nature into imbalance by transferring the carbon from the slow carbon cycle to the fast cycle, where living things are unable to process it quickly enough to prevent accumulation in the atmosphere. 

Activities like deforestation also alter the carbon cycle by removing a large amount of plants that have a high ability to absorb carbon dioxide from the air. 

Why the carbon cycle matters for climate change 

The carbon cycle moderates the concentration of carbon dioxide in the air, which, up until recently, had never been higher than 300 parts per million (ppm) within the last million years. Recently, the increasing release of stored carbon into the atmosphere from human activities has overwhelmed the natural carbon cycle, resulting in a CO2 concentration of over 430 ppm. High concentrations of CO2 and other greenhouse gases trap heat in the atmosphere that would otherwise escape into space. These higher concentrations have caused Earth’s average temperature to rise about 0.06 Celsius per decade since 1850.