Altered Greenhouse Gas Concentrations Over Time

Greenhouse gas (GHG) concentrations in the atmosphere are not fixed — they vary on timescales ranging from seasonal cycles to millions of years. Understanding both natural and human-driven variations is essential for interpreting climate change evidence.

Seasonal Variation (Months)

CO\(_2\) concentrations fluctuate annually — most visible in the Northern Hemisphere:

• Spring/summer: Increased photosynthesis by growing vegetation removes CO\(_2\) from the atmosphere → concentrations fall
• Autumn/winter: Leaf fall, decomposition and reduced photosynthesis release CO\(_2\) → concentrations rise
• Annual oscillation: ~6–8 ppm in the Northern Hemisphere (less pronounced in Southern Hemisphere due to more ocean)

This seasonal ‘breathing’ of the biosphere is clearly visible in the Keeling Curve (Mauna Loa Observatory data), which shows a sawtooth pattern superimposed on the long-term rising trend.

Interannual Variation (Years)

GHG concentrations vary year to year due to:
- El Niño events: Drought reduces forest carbon uptake; fire frequency increases, releasing CO\(_2\)
- Volcanic eruptions: Volcanic SO\(_2\) cools climate temporarily, enhancing vegetation growth → slight CO\(_2\) decrease; large eruptions directly emit CO\(_2\)
- Ocean circulation anomalies: Affect ocean CO\(_2\) exchange with atmosphere

Centennial Variation (100–1,000 Years)

• Medieval Warm Period (~950–1250 CE): Slightly warmer conditions driven by solar variability and low volcanic activity; modest increase in natural GHGs
• Little Ice Age (~1300–1850 CE): Cooler conditions linked to solar minimum, increased volcanic activity; slight decrease in natural GHGs
• Industrial Revolution (~1850–present): Sharp, unprecedented rise in CO\(_2\), CH\(_4\) and N\(_2\)O from human activities

Millennial Variation (Thousands of Years)

Ice cores from Antarctica (e.g. Vostok, EPICA Dome C) provide continuous records of atmospheric GHG concentrations and temperature over 800,000 years.

Human Activities Causing GHG Changes

1. Combustion of Fossil Fuels

\$\(\text{C (fossil fuel)} + O_2 \rightarrow CO_2 + \text{energy}\)\$
- Largest source of CO\(_2\) emissions globally
- Coal combustion releases most CO\(_2\) per unit energy; gas combustion releases least
- Also emits methane from coal seam gas extraction and fugitive emissions

2. Cement Production

\$\(CaCO_3 \rightarrow CaO + CO_2\)\$
- Heating limestone (CaCO\(_3\)) to produce cement clinker releases CO\(_2\)
- Accounts for ~8% of global CO\(_2\) emissions — third largest industrial source

3. Agriculture

• Livestock (enteric fermentation): Methanogenic microbes in ruminant digestive systems produce CH\(_4\)
• Rice paddies: Flooded paddies are anaerobic — produce CH\(_4\) from decomposing organic matter
• Synthetic fertilisers: Nitrogen applied to soils converted by bacteria to N\(_2\)O (powerful GHG, GWP\(_{100}\) ≈ 273)
• Manure management: Both CH\(_4\) and N\(_2\)O emissions

4. Land Use Changes

• Deforestation: Removes carbon storage; burning vegetation releases CO\(_2\); decomposition of remaining organic matter continues to emit CO\(_2\)
• Wetland drainage: Draining peatlands exposes stored organic matter to aerobic decomposition → CO\(_2\) release
• Urbanisation: Replaces vegetation with impervious surfaces, eliminating carbon sequestration

The Role of Each Human Activity

VCAA FOCUS: Students need to explain GHG changes across ALL specified time scales. VCAA may ask why CO\(_2\) concentrations fluctuate seasonally (photosynthesis/decomposition) even while trending upward overall. Always specify the mechanism driving change at each timescale.