Atmospheric Carbon and the Greenhouse Effect

This lesson examines atmospheric carbon, the greenhouse effect, and the evidence for rising CO₂ concentrations, addressing the Edexcel A-Level Geography (9GE0) specification, Topic 6 Enquiry Question: "How does the carbon cycle operate to maintain planetary health?"

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Greenhouse Gases

The Earth's atmosphere contains several gases that absorb and re-emit longwave (infrared) radiation — radiation emitted by the Earth's surface after absorbing incoming solar (shortwave) radiation. These are called greenhouse gases (GHGs).

Greenhouse Gas	Chemical Formula	Pre-industrial Concentration	Current Concentration (2024)	Global Warming Potential (100-yr, relative to CO₂)	Main Sources
Carbon dioxide	CO₂	~280 ppm	~424 ppm	1	Fossil fuel combustion, deforestation, cement production
Methane	CH₄	~700 ppb	~1,920 ppb	28	Wetlands, agriculture (rice paddies, livestock), fossil fuel extraction, landfill
Nitrous oxide	N₂O	~270 ppb	~336 ppb	265	Agriculture (fertilisers), industrial processes, biomass burning
Water vapour	H₂O	Variable	Variable (~0.4%)	N/A (feedback gas)	Evaporation; concentration depends on temperature
Ozone (tropospheric)	O₃	Variable	Variable	~62	Photochemical reactions involving pollutants
Halocarbons (CFCs, HFCs)	Various	0 (synthetic)	Variable	1,000–23,000	Industrial refrigerants, aerosol propellants (now banned/restricted)

Exam Tip: Water vapour is the most abundant greenhouse gas and responsible for the largest share of the natural greenhouse effect (~60%), but it is not directly increased by human activity. It acts as a positive feedback: warming → more evaporation → more water vapour → more greenhouse warming. Edexcel expects you to distinguish between direct GHG emissions (CO₂, CH₄, N₂O) and water vapour as a feedback amplifier.

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The Natural Greenhouse Effect

Without the natural greenhouse effect, Earth's average surface temperature would be approximately −18°C instead of the actual average of +15°C — a difference of 33°C. The greenhouse effect makes life on Earth possible.

How it works:

1. Incoming solar radiation — The Sun emits shortwave radiation (mainly visible light and ultraviolet). Approximately 340 W/m² of solar energy reaches the top of the atmosphere on average.
2. Reflection — About 30% of incoming solar radiation is reflected back to space by clouds, aerosols and the Earth's surface (especially ice and snow). This is the albedo effect. The remaining ~70% (~240 W/m²) is absorbed by the Earth's surface and atmosphere.
3. Re-emission — The Earth's surface warms and emits longwave (infrared) radiation upward. If all this radiation escaped to space, the surface would cool to −18°C.
4. Absorption by GHGs — Greenhouse gas molecules in the atmosphere absorb longwave radiation and re-emit it in all directions — including back towards the Earth's surface.
5. Surface warming — The re-emitted radiation warms the surface further, raising the average temperature to +15°C.

flowchart TD
    A["☀️ Incoming Solar Radiation<br>340 W/m²"] --> B["~30% reflected<br>(albedo)"]
    A --> C["~70% absorbed by<br>surface & atmosphere"]
    C --> D["Earth's surface<br>warms and emits<br>longwave IR radiation"]
    D --> E["GHGs absorb<br>longwave radiation"]
    E --> F["Re-emit in<br>all directions"]
    F --> G["↓ Back to surface<br>(+33°C warming)"]
    F --> H["↑ Some escapes<br>to space"]

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The Enhanced Greenhouse Effect

Human activities have increased the atmospheric concentrations of greenhouse gases beyond their natural levels, strengthening the greenhouse effect. This is the enhanced greenhouse effect — the additional warming caused by anthropogenic GHG emissions.

Gas	Pre-industrial	Current	Increase	% Contribution to Enhanced GHE
CO₂	280 ppm	424 ppm	+51%	~64%
CH₄	700 ppb	1,920 ppb	+174%	~19%
N₂O	270 ppb	336 ppb	+24%	~6%
Halocarbons	0	Various	New	~11%

CO₂ is the dominant driver of the enhanced greenhouse effect because:

• It is emitted in the largest quantities (~40 GtCO₂/year, or ~11 GtC/year)
• It has a very long atmospheric lifetime (centuries to millennia for the perturbation)
• It absorbs infrared radiation in specific wavelength bands that are not fully saturated

Radiative Forcing

Radiative forcing measures the change in energy balance at the top of the atmosphere caused by a factor such as GHG increase or aerosol change. It is measured in watts per square metre (W/m²).

Factor	Radiative Forcing (W/m²) since 1750	Effect
CO₂	+2.16	Warming
CH₄	+0.54	Warming
N₂O	+0.21	Warming
Halocarbons	+0.41	Warming
Tropospheric ozone	+0.47	Warming
Aerosols (direct + indirect)	−1.3 (approximate)	Cooling (partially offsets GHG warming)
Solar irradiance changes	+0.05	Warming (very small)
Net anthropogenic	~+2.7	Warming

The net positive radiative forcing of ~2.7 W/m² means the Earth is currently absorbing more energy than it emits — the energy imbalance that drives ongoing warming.

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The Keeling Curve

The Keeling Curve is the most famous dataset in climate science. It records continuous measurements of atmospheric CO₂ concentration at the Mauna Loa Observatory in Hawaii, begun by Charles David Keeling in 1958.

Key features of the Keeling Curve: