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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Fossil Fuel Combustion

The Scale of Emissions

The burning of coal, oil, and natural gas is the dominant cause of the increase in atmospheric CO₂. Fossil fuel combustion releases approximately 9.5 GtC/year (as of 2022) — roughly 35 billion tonnes of CO₂/year (Friedlingstein et al., 2023).

Decade	Average Fossil Fuel Emissions (GtC/year)
1960s	3.0
1970s	4.7
1980s	5.4
1990s	6.4
2000s	7.8
2010s	9.4
2020–22	9.5

Emissions by Sector

Sector	% of Global CO₂ Emissions
Energy (electricity & heat)	31%
Transport	16%
Manufacturing & construction	12%
Industrial processes	6%
Agriculture, forestry, land use	22%
Buildings (heating, cooking)	6%
Other	7%

Source: IPCC AR6, 2022

Emissions by Country (2022)

Country	Annual CO₂ Emissions (GtCO₂)	% of Global Total	Per Capita (tCO₂)
China	11.5	31%	8.0
USA	5.1	14%	15.3
India	2.9	8%	2.0
EU-27	2.8	8%	6.2
Russia	1.8	5%	12.5
Japan	1.1	3%	8.5
UK	0.33	0.9%	4.9

Exam Tip: When discussing emissions by country, always consider per capita emissions as well as total emissions. China has the highest total emissions, but its per capita figure is roughly half that of the USA. This distinction is crucial for evaluation of climate justice and international policy debates.

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Deforestation

Scale and Rate

• An estimated 420 million hectares of forest (an area larger than the EU) was lost between 1990 and 2020 (FAO, 2020).
• Net forest loss has slowed in recent decades — from 7.8 million ha/year in the 1990s to 4.7 million ha/year in 2010–2020 — but gross tropical deforestation remains high.
• The Amazon rainforest lost approximately 17% of its forest cover in the 50 years from 1970 to 2020 (INPE data).

Mechanisms of Carbon Release

Deforestation releases carbon through multiple pathways:

1. Biomass burning — trees are often cleared by fire (slash-and-burn), releasing stored carbon immediately as CO₂ and particulate carbon (black carbon).
2. Decomposition of residual organic matter — roots, stumps, and leaf litter left behind decompose over years, releasing CO₂.
3. Soil disturbance — deforestation exposes soil organic carbon to oxidation, releasing CO₂. Forest soils in the tropics can lose 20–50% of their carbon within a decade of clearance (Don et al., 2011).
4. Loss of future carbon sink — the removed forest can no longer sequester carbon through photosynthesis.

Case Study: Amazon Rainforest Deforestation

The Amazon contains approximately 150–200 GtC in its biomass — roughly 10% of all carbon in terrestrial vegetation.

Drivers of deforestation:

• Cattle ranching — responsible for ~80% of Amazon deforestation in Brazil (Nepstad et al., 2014).
• Soy cultivation — Brazil is the world's largest soy exporter; expansion drives forest conversion.
• Logging — both legal and illegal timber extraction.
• Road construction — the Trans-Amazonian Highway (BR-230), built from 1972, opened vast areas to settlement and clearance.
• Mining — gold mining and iron ore extraction (e.g., Carajas Mine).