Climate change is driven primarily by rising concentrations of greenhouse gases — especially carbon dioxide from burning fossil fuels. When hydrocarbons burn, carbon stored for millions of years is released as CO₂, which absorbs outgoing infrared radiation and warms the planet. Understanding the chemistry explains both the cause and potential solutions.
What is the greenhouse effect and why does it matter?
The greenhouse effect is a natural process by which certain gases in the atmosphere absorb infrared radiation (heat) re-emitted from Earth's surface and radiate it back downward, warming the planet. Without any greenhouse effect, Earth's average surface temperature would be approximately −18 °C — far too cold for life as we know it.
How it works:
- Short-wavelength radiation (visible light and UV) from the Sun passes through the atmosphere and warms Earth's surface.
- Earth's surface re-emits energy as longer-wavelength infrared radiation (heat).
- Greenhouse gases absorb this infrared radiation and re-emit it in all directions — including back towards Earth's surface.
- The result is a warming of the lower atmosphere.
The problem is not the greenhouse effect itself but its enhancement: rising concentrations of greenhouse gases caused by human activities are trapping more heat than before, raising global average temperatures.
Which gases are responsible for climate change?
| Greenhouse gas | Main sources | Global warming potential (relative to CO₂) |
|---|---|---|
| Carbon dioxide (CO₂) | Burning fossil fuels, deforestation, cement production | 1 (reference) |
| Methane (CH₄) | Livestock digestion, rice paddies, landfill, natural gas leaks | ~28 over 100 years |
| Nitrous oxide (N₂O) | Fertiliser use, agriculture, combustion | ~265 over 100 years |
| Water vapour (H₂O) | Evaporation — increases with warming (a feedback effect) | Variable |
| HFCs (hydrofluorocarbons) | Refrigerants, aerosols | Thousands |
Carbon dioxide is the principal driver of current climate change because it is released in enormous quantities by burning fossil fuels and remains in the atmosphere for hundreds to thousands of years.
What is the chemistry of burning fossil fuels?
Fossil fuels (coal, oil, natural gas) are primarily hydrocarbons — compounds containing only carbon and hydrogen. When burned in a plentiful supply of oxygen, they undergo complete combustion:
General equation: Hydrocarbon + oxygen → carbon dioxide + water
Example — methane (natural gas):
CH₄ + 2O₂ → CO₂ + 2H₂O
Example — octane (a component of petrol):
2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O
Every mole of carbon in a fuel produces one mole of CO₂ upon combustion. Global combustion of fossil fuels currently releases approximately 37 billion tonnes (37 Gt) of CO₂ per year into the atmosphere.
What is the evidence that human activity is causing climate change?
GCSE students need to understand that the link between human carbon emissions and rising temperatures is supported by multiple independent lines of evidence:
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Rising atmospheric CO₂: direct measurements at Mauna Loa Observatory since 1958 show CO₂ rising from 315 ppm to over 420 ppm — a 33% increase in 65 years. Ice core records extend this back 800,000 years, showing that current CO₂ levels are unprecedented.
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Rising average temperatures: global average surface temperature has risen by approximately 1.1 °C since pre-industrial times. The ten warmest years on record (to 2024) all occurred after 1998.
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Isotopic fingerprint: the carbon in CO₂ from fossil fuels has a different ratio of carbon isotopes (less ¹⁴C and ¹³C) than naturally occurring CO₂. Atmospheric measurements show this isotopic signature shifting towards the fossil fuel pattern.
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Physical models: climate models that include only natural factors (volcanic eruptions, solar variation) cannot reproduce the observed warming. Models that include human emissions match observations closely.
What are the consequences of increasing greenhouse gas concentrations?
| Effect | Mechanism |
|---|---|
| Rising sea levels | Thermal expansion of oceans + melting of ice sheets and glaciers |
| More frequent extreme weather | Greater energy in the climate system amplifies storms, droughts, floods |
| Loss of biodiversity | Habitat shifts faster than species can adapt or migrate |
| Ocean acidification | CO₂ dissolves in seawater: CO₂ + H₂O → H₂CO₃ (carbonic acid); pH falls |
| Disruption to agriculture | Changed rainfall patterns, longer droughts, reduced crop yields |
Ocean acidification is a direct chemical consequence: as atmospheric CO₂ rises, more dissolves in the oceans. This lowers pH, which damages calcium carbonate shells of corals, molluscs, and many plankton species — threatening marine food webs.
What strategies can reduce carbon emissions?
Strategies fall into two broad categories: reducing emissions and removing existing CO₂.
Reducing emissions:
- Transition from fossil fuels to renewable energy (wind, solar, tidal, hydroelectric)
- Improving energy efficiency in buildings, transport, and industry
- Developing electric vehicles and hydrogen fuel cell vehicles
- Reducing methane emissions from agriculture and landfill
Removing CO₂ (carbon capture):
- Carbon capture and storage (CCS): CO₂ from power stations is captured before release, compressed, and stored underground in depleted oil fields or saline aquifers
- Reforestation and afforestation: growing trees absorbs CO₂ from the atmosphere via photosynthesis
- Direct air capture: industrial machines draw in air and chemically absorb CO₂
Each strategy has limitations — CCS is expensive and energy-intensive; reforestation takes decades; renewable energy requires land and infrastructure investment. Most scientists agree that multiple strategies used in combination are required.
Frequently asked questions
How does CO₂ warm the atmosphere if it is a colourless, odourless gas?
CO₂ absorbs infrared radiation (heat) because its molecular bonds vibrate at frequencies that match infrared wavelengths. When a CO₂ molecule absorbs an infrared photon, it re-emits the energy in all directions — including back towards Earth's surface. More CO₂ in the atmosphere means more infrared is absorbed and re-emitted downward, increasing the surface temperature. Nitrogen and oxygen (which make up 99% of the atmosphere) cannot absorb infrared because their molecular bonds do not vibrate in response to these wavelengths.
What is the difference between the greenhouse effect and global warming?
The greenhouse effect is the natural process by which greenhouse gases warm Earth's surface to a habitable temperature. Without it, Earth would be approximately −18 °C. Global warming refers to the enhanced greenhouse effect caused by human emissions increasing greenhouse gas concentrations, raising average temperatures above their natural level. Global warming is thus the human-caused intensification of a naturally beneficial process.
Is water vapour a greenhouse gas?
Yes — water vapour is the most abundant greenhouse gas in the atmosphere and responsible for about half of the natural greenhouse effect. However, water vapour concentration in the atmosphere is controlled by temperature (warmer air holds more moisture), not directly by human activity. As temperatures rise due to CO₂ emissions, more water evaporates, increasing water vapour concentration and amplifying the warming — a positive feedback loop. This is why scientists focus on CO₂ (and methane) as the primary drivers that humans can control.
What is ocean acidification and why does it matter?
When CO₂ dissolves in seawater, it reacts with water to form carbonic acid (H₂CO₃), which partially dissociates to release hydrogen ions, lowering the pH. Ocean pH has already fallen from 8.2 to 8.1 since industrialisation — a 26% increase in hydrogen ion concentration. Many marine organisms (corals, oysters, sea urchins, some plankton) build shells or skeletons from calcium carbonate (CaCO₃). At lower pH, calcium carbonate dissolves more readily, making it harder for these organisms to form and maintain their shells, threatening both individual species and entire marine food chains.
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