2.6 The Carbon Cycle and Earth's Atmosphere

The carbon cycle is a crucial Earth system that moves carbon between the atmosphere, oceans, land, and living things. It plays a key role in regulating our planet's climate by controlling CO2 levels in the air.

Human activities like burning fossil fuels and deforestation have disrupted the natural carbon cycle. This has led to rising CO2 in the atmosphere, causing global warming and climate change. Understanding these impacts is vital for addressing environmental challenges.

The Carbon Cycle

Overview of the Carbon Cycle

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• The carbon cycle is the biogeochemical cycle that exchanges carbon between the Earth's atmosphere, oceans, land, and living organisms
• Carbon dioxide (CO2) is a greenhouse gas that traps heat in the atmosphere leading to warmer global temperatures when concentrations are higher
• Photosynthesis by plants and other autotrophs (algae) removes CO2 from the atmosphere and stores it as organic carbon in biomass while cellular respiration and decomposition release this carbon back into the atmosphere as CO2
• The oceans absorb large amounts of CO2 from the atmosphere, acting as a carbon sink, however, as ocean temperatures rise, their capacity to absorb CO2 decreases
• Volcanic eruptions release CO2 into the atmosphere from the Earth's interior while weathering of silicate rocks (limestone) removes CO2 from the atmosphere over long timescales

Regulating Earth's Climate

• The balance between various sources and sinks of carbon helps regulate Earth's climate over long timescales and disruptions to this balance can lead to changes in global temperature
• Changes in the balance between carbon sources and sinks can lead to changes in atmospheric CO2 levels, which in turn affect Earth's climate
• The atmospheric concentration of CO2 has increased from around 280 ppm (parts per million) in pre-industrial times to over 410 ppm today, primarily due to human activities
• This increase in atmospheric CO2 has led to an enhanced greenhouse effect, causing global temperatures to rise
• Positive feedback loops can amplify the effects of increased atmospheric CO2, for example, as permafrost thaws due to warming temperatures, it releases stored carbon into the atmosphere, further enhancing the greenhouse effect

Carbon Reservoirs and Fluxes

Major Carbon Reservoirs

• The atmosphere, oceans, land, and living organisms are the major reservoirs of carbon in the Earth system
• The largest reservoir of carbon is the deep ocean, which stores carbon in the form of dissolved inorganic carbon and organic matter (marine snow)
• Soil and permafrost are significant reservoirs of organic carbon on land with permafrost storing large amounts of carbon that can be released as it thaws due to warming temperatures
• Fossil fuels, such as coal, oil, and natural gas, are reservoirs of carbon that formed from the remains of ancient organisms over millions of years

Carbon Fluxes

• The major fluxes of carbon include photosynthesis, respiration, decomposition, ocean-atmosphere exchange, weathering, and volcanic emissions
• Photosynthesis by plants and other autotrophs removes CO2 from the atmosphere and stores it as organic carbon in biomass
• Cellular respiration and decomposition release carbon back into the atmosphere as CO2
• Ocean-atmosphere exchange involves the absorption and release of CO2 by the oceans
• Weathering of silicate rocks removes CO2 from the atmosphere over long timescales
• Volcanic emissions release CO2 into the atmosphere from the Earth's interior
• Anthropogenic fluxes, such as fossil fuel combustion and land use changes (deforestation), have significantly altered the natural balance of the carbon cycle

Human Impact on the Carbon Cycle

Fossil Fuel Combustion and Deforestation

• The burning of fossil fuels releases large amounts of CO2 into the atmosphere that had been stored in the Earth's crust for millions of years
• Deforestation reduces the amount of CO2 removed from the atmosphere by photosynthesis and releases the carbon stored in plant biomass back into the atmosphere
• Agriculture, particularly livestock farming and rice cultivation, releases methane (CH4), another potent greenhouse gas, into the atmosphere
• Cement production involves the calcination of limestone, which releases CO2 as a byproduct

Consequences of Altered Carbon Cycle

• These human activities have led to a significant increase in the concentration of greenhouse gases in the atmosphere, primarily CO2, since the start of the Industrial Revolution
• The increased greenhouse gas concentrations enhance the greenhouse effect, leading to global warming and climate change
• The atmospheric concentration of CO2 has increased from around 280 ppm (parts per million) in pre-industrial times to over 410 ppm today, primarily due to human activities
• This increase in atmospheric CO2 has led to an enhanced greenhouse effect, causing global temperatures to rise

Carbon Cycle and Earth's Atmosphere

Interactions between Carbon Cycle and Atmosphere

• The carbon cycle plays a crucial role in regulating the concentration of CO2 in the Earth's atmosphere
• Changes in the balance between carbon sources and sinks can lead to changes in atmospheric CO2 levels, which in turn affect Earth's climate
• The oceans act as a buffer for atmospheric CO2, absorbing excess CO2 from the atmosphere, however, as ocean temperatures rise and acidification increases, the oceans' capacity to absorb CO2 may decrease, potentially accelerating the rate of atmospheric CO2 increase

Importance of Understanding Carbon Cycle-Atmosphere Relationship

• Understanding the complex interactions between the carbon cycle and Earth's atmosphere is crucial for predicting future climate change and developing effective mitigation strategies
• Monitoring atmospheric CO2 concentrations and studying the various components of the carbon cycle can help scientists better understand the mechanisms driving climate change
• This knowledge can inform policy decisions and guide efforts to reduce greenhouse gas emissions and mitigate the impacts of climate change