Carbon Cycle Changes

The global carbon cycle has been profoundly altered by human activity since the Industrial Revolution (c. 1750). This lesson examines the key human-induced changes to carbon stores and flows, including fossil fuel combustion, deforestation, agriculture, and land-use change. It also considers natural drivers of carbon cycle change and evaluates the evidence for anthropogenic climate change. Understanding these changes — and their cascading effects on the water cycle and broader Earth system — is a central requirement of the AQA A-Level specification.

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The Pre-Industrial Carbon Cycle

Before the Industrial Revolution, the carbon cycle was in approximate dynamic equilibrium: natural sources of CO₂ (respiration, decomposition, volcanic outgassing, ocean degassing) were broadly balanced by natural sinks (photosynthesis, ocean absorption, weathering, sediment burial).

Key indicators of pre-industrial equilibrium:

• Atmospheric CO₂ fluctuated between approximately 180 ppm (glacial periods) and 280 ppm (interglacial periods) over the past 800,000 years (Luthi et al., 2008, from Antarctic ice cores at EPICA Dome C).
• These fluctuations were driven by Milankovitch orbital cycles and associated feedback mechanisms (ice-albedo, ocean circulation, vegetation changes).
• The rate of change was slow — typically <0.01 ppm per year during natural transitions.

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