Natural vs anthropogenic causes
Big idea: Climate has always changed naturally, but current warming is primarily caused by anthropogenic emissions of greenhouse gases.
Natural factors
- Milankovitch cycles: Changes in Earths orbit and tilt (cause ice ages over ~100,000 year cycles)
- Volcanic eruptions: Can cause short-term cooling (aerosols reflect sunlight) or warming (CO₂ release)
- Solar output variations: Small changes in solar energy (0.1% variation over 11-year cycle)
- Ocean circulation changes: El Niño/La Niña cause short-term climate shifts
Natural factors cannot explain the rapid warming since 1950. Solar output has been relatively stable, and volcanic activity hasnt increased — but CO₂ has risen dramatically.
Anthropogenic factors
- Fossil fuel combustion: Burning coal, oil, gas releases CO₂ (largest source)
- Deforestation: Removes carbon sinks; burning releases stored carbon
- Agriculture: Livestock produce CH₄; rice paddies release CH₄; fertilisers release N₂O
- Industrial processes: Cement production, refrigerants (CFCs/HFCs)
- Land use change: Urbanisation increases local temperatures; changes albedo
Exam tip: Be able to link specific human activities to specific greenhouse gases.
Greenhouse gases in detail
Big idea: Different greenhouse gases have different sources, atmospheric lifetimes, and global warming potential (GWP).
The major greenhouse gases
- Carbon dioxide (CO₂): GWP = 1 (baseline). Lasts ~100–300 years. Main sources: fossil fuels, deforestation. Responsible for ~75% of warming.
- Methane (CH₄): GWP = 28–36. Lasts ~12 years. Sources: livestock, rice, landfills, natural gas leaks.
- Nitrous oxide (N₂O): GWP = 265–298. Lasts ~121 years. Sources: fertilisers, combustion, industrial processes.
- Fluorinated gases (CFCs, HFCs): GWP = 1,000–23,000. Last centuries. Sources: refrigerants, aerosols.
CO₂ has the biggest total impact because of sheer quantity. CH₄ is more potent per molecule but breaks down faster.
Carbon sinks and sources
Carbon sources (release CO₂)
- Fossil fuel combustion
- Deforestation and burning
- Cement production
- Respiration
- Decomposition
Carbon sinks (absorb CO₂)
- Forests (photosynthesis)
- Oceans (dissolve CO₂)
- Soil (organic matter)
- Peat bogs
Exam tip: Questions often ask why deforestation is a double impact — it removes a sink AND releases stored carbon as a source.