6.3 Human activities and perturbations to the carbon cycle

Human activities are drastically altering Earth's carbon cycle. Fossil fuel burning, deforestation, and land-use changes release massive amounts of CO2 into the atmosphere, disrupting natural carbon sinks and storage.

These perturbations have far-reaching impacts. Rising CO2 levels intensify global warming, acidify oceans, and trigger ecosystem shifts. While carbon capture technologies offer hope, the long-term consequences for climate and ecosystems remain severe.

Human Activities and Perturbations to the Carbon Cycle

Sources of carbon dioxide emissions

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• Fossil fuel combustion releases CO2 into atmosphere when coal, oil, and natural gas are burned for energy production (power plants), transportation (cars, trucks, planes), and industrial processes (manufacturing, cement production)
• Land-use changes such as deforestation and agricultural practices (livestock farming, rice cultivation) alter carbon uptake and storage in ecosystems
• Waste management activities including landfills and wastewater treatment emit CO2 and methane (CH4) during decomposition of organic matter

Deforestation effects on carbon cycle

• Deforestation reduces carbon uptake by plants through photosynthesis removes a major carbon sink from the environment
• Releases stored carbon from biomass into the atmosphere when trees are cut down and burned or left to decompose
• Decreases soil carbon storage due to erosion and degradation exposes soil organic matter to oxidation and decomposition
• Land-use change from natural ecosystems to agricultural land (cropland, pastures) reduces carbon sequestration capacity and increases soil carbon loss through tillage and erosion
• Urbanization and infrastructure development reduces vegetation cover and carbon uptake while increasing carbon emissions from construction and energy use in buildings and transportation

Potential of carbon capture technologies

• Carbon capture and storage (CCS) involves:

1. Capturing CO2 from point sources (power plants, industrial facilities) through pre-combustion capture, post-combustion capture, or oxy-fuel combustion
2. Transporting captured CO2 to storage sites via pipelines or ships
3. Storing captured CO2 in geological formations such as depleted oil and gas reservoirs, deep saline aquifers, or unmineable coal seams

• Challenges and limitations of CCS include high costs and energy requirements for capture and compression, limited storage capacity and long-term stability of storage sites, and potential leakage and environmental risks (groundwater contamination, induced seismicity)

Long-term impacts on global carbon cycle

• Increased atmospheric CO2 concentration enhances the greenhouse effect leading to global warming and climate change, as well as ocean acidification due to absorption of CO2 by seawater
• Altered plant growth and ecosystem dynamics result in changes in species composition and biodiversity, shifts in growing seasons and phenology (timing of leaf emergence, flowering, fruiting)
• Feedbacks and tipping points amplify carbon cycle perturbations:
  • Permafrost thawing releases stored methane (CH4) a potent greenhouse gas
  • Reduced ocean carbon uptake due to warming and stratification limits a major carbon sink
  • Increased wildfire frequency and intensity releases more CO2 and reduces vegetation cover
• Long-term consequences include sea-level rise due to thermal expansion and ice sheet melting, impacts on agriculture (crop yields, water availability), water resources (drought, flooding), human health (heat stress, disease spread), and ecosystem degradation and loss of ecosystem services (biodiversity loss, reduced water and air quality)