6.1 Global carbon cycle and carbon reservoirs

Carbon is the foundation of life and climate on Earth. It moves through various reservoirs, including the atmosphere, oceans, and terrestrial biosphere. Understanding these reservoirs and the processes that transfer carbon between them is crucial for grasping climate change.

The carbon cycle regulates Earth's climate through greenhouse effects and feedback loops. Carbon sinks and sources determine atmospheric CO2 levels, with human activities currently tipping the balance towards warming. Timescales of carbon cycling vary widely, from years in the atmosphere to millions of years in rocks.

Carbon Reservoirs and the Global Carbon Cycle

Carbon reservoirs on Earth

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• Atmosphere contains approximately 800 gigatons of carbon (GtC) represents the smallest reservoir but plays a crucial role in the carbon cycle by facilitating the exchange of carbon between other reservoirs (oceans, terrestrial biosphere)
• Oceans largest active reservoir containing about 38,000 GtC divided into the surface ocean (approximately 1,000 GtC) and deep ocean (approximately 37,000 GtC) with the surface ocean rapidly exchanging carbon with the atmosphere while the deep ocean stores carbon for longer periods
• Terrestrial biosphere contains around 2,000 GtC includes living organisms (plants, animals, microbes) and dead organic matter in soils (leaf litter, roots, humus) with plants playing a key role in removing atmospheric CO2 through photosynthesis
• Fossil fuels contain about 10,000 GtC includes coal, oil, and natural gas reserves formed from the burial and transformation of ancient organic matter over millions of years with human extraction and combustion releasing this stored carbon back into the atmosphere
• Sedimentary rocks largest reservoir containing approximately 100,000,000 GtC includes carbonate rocks (limestone, dolomite) formed by the precipitation of calcium carbonate from seawater and organic carbon (kerogen) in sediments derived from the burial of dead organisms

Carbon transfer processes

• Photosynthesis converts atmospheric CO2 into organic compounds (glucose, cellulose) by plants and phytoplankton using sunlight energy transfers carbon from the atmosphere to the terrestrial biosphere and surface ocean
• Respiration releases CO2 back into the atmosphere through cellular respiration by organisms (plants, animals, microbes) as they break down organic compounds for energy transfers carbon from the terrestrial biosphere and surface ocean to the atmosphere
• Decomposition breaks down dead organic matter (leaf litter, animal carcasses) by microorganisms (bacteria, fungi) releasing CO2 back into the atmosphere transfers carbon from the terrestrial biosphere to the atmosphere and soil
• Ocean-atmosphere gas exchange involves CO2 dissolving in the surface ocean and being released back into the atmosphere with the net transfer depending on the relative partial pressures of CO2 in the atmosphere and ocean (higher atmospheric CO2 leads to net ocean uptake)
• Ocean circulation transfers carbon between the surface and deep ocean through thermohaline circulation (deep water formation in polar regions, upwelling of nutrient-rich waters) affects the storage and distribution of carbon in the oceans with the deep ocean acting as a long-term carbon sink
• Weathering and sedimentation involve the chemical weathering of rocks (silicates, carbonates) consuming atmospheric CO2 to form carbonate rocks (limestone) and the burial of organic carbon in sediments transfers carbon from the atmosphere to sedimentary rocks over long timescales (millions of years)
• Volcanic emissions release CO2 from the Earth's interior (mantle, crust) into the atmosphere through volcanic eruptions and geothermal activity transfers carbon from the Earth's mantle to the atmosphere offsetting some of the carbon removed by weathering and sedimentation

Carbon cycle and climate regulation

• Greenhouse effect atmospheric CO2 acts as a greenhouse gas by absorbing and re-emitting infrared radiation trapping heat and warming the Earth's surface changes in atmospheric CO2 levels can affect the Earth's energy balance and climate with higher CO2 levels leading to increased warming
• Climate feedback loops involve processes that amplify (positive feedbacks) or counteract (negative feedbacks) climate change examples of positive feedbacks include the release of CO2 from thawing permafrost and increased water vapor in a warmer atmosphere while negative feedbacks include increased CO2 uptake by plants due to higher atmospheric CO2 levels (CO2 fertilization effect)
• Carbon sinks and sources refer to processes that remove CO2 from the atmosphere (sinks) or add CO2 to the atmosphere (sources) examples of carbon sinks include photosynthesis, ocean absorption, and weathering of silicate rocks while sources include respiration, fossil fuel combustion, and volcanic emissions the balance between sinks and sources determines the net effect on atmospheric CO2 levels and climate with human activities (deforestation, fossil fuel use) currently tipping the balance towards increased atmospheric CO2 and warming

Timescales of carbon cycling

• Atmosphere relatively short residence time around 5 years with rapid exchange of CO2 with the terrestrial biosphere (photosynthesis, respiration) and surface ocean (gas exchange)
• Terrestrial biosphere short to medium residence times ranging from years to decades depends on the lifespan of organisms (annuals vs. perennials) and the decomposition rate of dead organic matter (litter, soil carbon)
  • 1. Fast cycling pool: Leaves, fine roots, and litter with turnover times of months to a few years
  • 1. Slow cycling pool: Woody biomass and soil organic matter with turnover times of decades to centuries
• Surface ocean medium residence time around 20-30 years with rapid exchange of CO2 with the atmosphere (gas exchange) and slower mixing with the deep ocean (ocean circulation)
• Deep ocean long residence time hundreds to thousands of years due to slow circulation and mixing with the surface ocean (thermohaline circulation) acts as a long-term carbon sink
• Sedimentary rocks very long residence times millions to hundreds of millions of years with slow processes of weathering (chemical dissolution), sedimentation (burial), and rock formation (diagenesis, lithification)
• Fossil fuels long residence times millions of years formed from the burial and transformation of ancient organic matter (dead plants, algae) under high pressure and temperature release of carbon through human extraction and combustion (industrial revolution, modern energy use) occurs on much shorter timescales (decades to centuries)