Changes in the Carbon Cycle Over Time: Human Causes

Introduction

Over the last two centuries, human activity has changed the carbon cycle faster than at any point in Earth’s recent history. The largest changes involve releasing carbon that has been stored for millions of years, reducing the size of natural carbon sinks, and altering global land cover.

Approximately 90% of human-related carbon emissions originate from the burning of fossil fuels, with the remainder primarily attributed to land-use change and agricultural practices.

1. Hydrocarbon Fuel Extraction and Burning

Extraction

Coal, oil and natural gas hold carbon that has been locked away in rock stores for millions of years. When these fuels are extracted, transported and processed, methane (a powerful greenhouse gas) can leak into the atmosphere.

Combustion

Burning fossil fuels transfers carbon extremely quickly:

• Carbon moves from a geological store into the atmosphere in the form of CO₂.
• This shift has increased atmospheric CO₂ concentrations from ~280 ppm before the Industrial Revolution to over 410 ppm today.
• Globally, coal accounts for the largest share of fossil-fuel CO₂ emissions, followed by oil and natural gas.

2. Farming Practices

Agriculture affects carbon stores through soil disturbance, livestock emissions and fertiliser use.

Soil disturbance

Ploughing exposes soil carbon to oxygen, increasing decomposition and releasing CO₂.

Livestock

Cattle and sheep produce methane during digestion. Methane traps far more heat than CO₂ over a short timescale.

Rice cultivation

Flooded paddies create anaerobic conditions, which produce methane.

Fertilisers and machinery

Manufacturing and using fertilisers, transporting crops and running farm machinery all depend on fossil fuels.

UK example:
Some UK farms are now adopting regenerative agriculture to store more carbon in soils. Increasing soil organic matter by just 0.1% across a large farm can store hundreds of tonnes of carbon, offsetting a significant share of machinery and fertiliser emissions.

3. Deforestation

When forests are cleared:

• Carbon stored in biomass is released through burning or decay.
• Removing trees reduces photosynthesis, lowering the rate of carbon uptake.
• If land is converted to pasture or cropland, soils can be disturbed and lose carbon through erosion.

In natural systems, decomposing trees release carbon slowly, and new vegetation absorbs it. Deforestation disrupts this balance, turning forests from carbon sinks into sources.

4. Urbanisation and Land-use Change

Urban surfaces such as tarmac and concrete replace soils and vegetation.
This reduces opportunities for photosynthesis and soil carbon storage.
Urban growth also increases emissions from:

• Transport
• Cement production
• Energy use in homes and industry

Cities cover a small proportion of the global land area (around 2–3%), but are responsible for the majority of CO₂ emissions due to their concentrated industry, transport, and energy demand.

Human Causes: Overall Impact

Human activity has:

• Increased atmospheric CO₂ rapidly
• Reduced size of natural sinks (e.g. forests, soils)
• Accelerated transfers from long-term geological stores to the atmosphere

These changes have pushed the carbon cycle out of long-term equilibrium and are driving current global warming.