10.1 Carbon cycle and its role in climate regulation

The carbon cycle plays a crucial role in regulating Earth's climate. It involves the exchange of carbon between the atmosphere, biosphere, hydrosphere, and geosphere through processes like photosynthesis, respiration, and weathering.

Human activities, especially fossil fuel combustion, have disrupted the natural carbon cycle. This has led to increased atmospheric CO2, causing global warming and ocean acidification. Understanding the carbon cycle is key to addressing climate change.

Carbon Cycle Components

Carbon Storage and Movement

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• Carbon cycle consists of the biogeochemical processes by which carbon is exchanged between the biosphere, geosphere, hydrosphere, and atmosphere
• Carbon sinks are reservoirs that absorb and store more carbon than they release (oceans, soils, and forests)
  • Oceans store dissolved carbon and calcium carbonate in shells of marine organisms
  • Soils store carbon in organic matter from decomposed plant and animal remains
  • Forests store carbon in biomass through photosynthesis and accumulation in wood and leaves
• Carbon sources are processes or activities that release more carbon into the atmosphere than they absorb (combustion of fossil fuels, respiration, and deforestation)
  • Combustion of fossil fuels (coal, oil, natural gas) releases stored carbon back into the atmosphere
  • Respiration by living organisms breaks down organic compounds and releases CO2
  • Deforestation reduces carbon storage in biomass and releases CO2 through burning or decomposition

Biological Processes in the Carbon Cycle

• Photosynthesis is the process by which plants and other autotrophs convert CO2 and water into glucose and oxygen using energy from sunlight
  • Photosynthesis removes CO2 from the atmosphere and incorporates it into biomass
  • Photosynthetic organisms (plants, algae, cyanobacteria) are primary producers in ecosystems
• Respiration is the process by which organisms break down organic compounds to release energy, producing CO2 and water as byproducts
  • Cellular respiration occurs in all living organisms and releases stored carbon back into the atmosphere
  • Decomposition of dead organic matter by microorganisms also releases CO2 through respiration

Human Impacts on Carbon Cycle

Fossil Fuel Combustion and Atmospheric CO2

• Fossil fuels are formed from the remains of ancient organisms and store large amounts of carbon removed from the atmosphere millions of years ago
• Combustion of fossil fuels releases CO2 into the atmosphere, increasing atmospheric CO2 concentrations
  • Atmospheric CO2 levels have increased from ~280 ppm before the Industrial Revolution to over 400 ppm today
  • Increased atmospheric CO2 enhances the greenhouse effect, contributing to global warming
• Ocean acidification occurs when excess atmospheric CO2 dissolves in seawater, forming carbonic acid and lowering ocean pH
  • Lower pH can impair shell formation in marine organisms and disrupt ocean food webs
  • Coral reefs are particularly vulnerable to ocean acidification and warming temperatures

Greenhouse Effect and Climate Change

• Greenhouse effect is the process by which atmospheric gases (CO2, water vapor, methane) absorb and re-emit infrared radiation, warming Earth's surface
  • Greenhouse gases allow shortwave radiation from the sun to pass through but trap longwave radiation emitted by Earth
  • Enhanced greenhouse effect due to human activities is the main driver of current climate change
• Increased atmospheric CO2 and other greenhouse gases from human activities amplify the greenhouse effect, leading to global warming and climate change
  • Rising temperatures cause melting of glaciers and ice sheets, sea level rise, and changes in precipitation patterns
  • Climate change impacts ecosystems, agriculture, and human societies through more frequent extreme weather events, shifts in species ranges, and altered growing seasons

Carbon Cycle Regulation

Natural Carbon Sequestration

• Carbon sequestration is the process of capturing and storing atmospheric CO2 in long-term reservoirs (oceans, soils, vegetation)
  • Photosynthesis by plants and algae removes CO2 from the atmosphere and stores it in biomass
  • Burial of organic matter in sediments can store carbon for millions of years
  • Weathering of silicate rocks removes CO2 from the atmosphere and stores it as carbonate rocks
• Carbonate-silicate cycle is a long-term (millions of years) carbon cycle regulation mechanism involving weathering of silicate rocks and formation of carbonate rocks
  • Weathering of silicate rocks by carbonic acid removes CO2 from the atmosphere and releases calcium and magnesium ions
  • Calcium and magnesium ions combine with bicarbonate in the ocean to form carbonate rocks (limestone), storing carbon for long periods
  • Volcanic activity and metamorphism release CO2 back into the atmosphere, completing the cycle

Anthropogenic Carbon Sequestration

• Reforestation and afforestation (planting trees in previously non-forested areas) can increase carbon storage in biomass and soils
  • Forests act as carbon sinks, removing CO2 from the atmosphere through photosynthesis
  • Sustainable forest management practices can enhance carbon sequestration while providing other ecosystem services
• Carbon capture and storage (CCS) technologies aim to capture CO2 from industrial sources and store it in geological formations or use it in industrial processes
  • CCS can reduce CO2 emissions from power plants and industrial facilities
  • Captured CO2 can be injected into depleted oil and gas reservoirs or saline aquifers for long-term storage
• Soil carbon sequestration can be enhanced through agricultural practices that increase soil organic matter (no-till farming, cover cropping, and use of biochar)
  • Soil organic matter improves soil structure, fertility, and water-holding capacity while storing carbon
  • Regenerative agriculture practices can help mitigate climate change while improving soil health and productivity