The Greenhouse Effect Explained: KS3 Science Guide

The greenhouse effect is the process by which certain gases in Earth's atmosphere trap heat from the Sun, keeping the planet warm enough to support life. Without it, Earth's average temperature would be around −18°C rather than the current +15°C. Understanding the greenhouse effect — and how human activity intensifies it — is core KS3 science, typically taught in Year 9.

How does the greenhouse effect work?

Energy from the Sun arrives at Earth mainly as visible light and ultraviolet radiation, which passes easily through the atmosphere. The Earth's surface absorbs this radiation and warms up. The warm surface then re-emits energy as infrared radiation (heat) directed back towards space.

This is where greenhouse gases play their role. Molecules of greenhouse gases absorb outgoing infrared radiation and re-emit it in all directions — including back down towards Earth's surface. This traps heat in the lower atmosphere, raising surface temperatures. The process works in a similar way to how a glass greenhouse traps warmth, which is where the name comes from.

Step-by-step: the greenhouse effect

Solar radiation arrives. Visible light and UV radiation from the Sun pass through the atmosphere and reach Earth's surface.

Earth absorbs radiation. The land and oceans absorb the incoming solar energy and warm up.

Earth re-emits infrared radiation. The warm surface radiates energy back upwards as infrared (heat) radiation.

Greenhouse gases absorb infrared. Greenhouse gas molecules in the atmosphere absorb the outgoing infrared radiation rather than allowing it to escape to space.

Heat is re-emitted downward. The greenhouse gas molecules re-emit infrared radiation in all directions. Some escapes to space; much is directed back to the surface, warming it further.

This natural process is essential for life. The problem arises when human activities increase the concentration of greenhouse gases, amplifying the effect beyond what Earth's systems can easily balance.

What are the key greenhouse gases?

Several gases contribute to the greenhouse effect. They differ in how much infrared radiation they absorb and for how long they persist in the atmosphere.

Greenhouse gas | Main human source | Relative warming potential (vs CO2 over 100 years)
Carbon dioxide (CO2) | Burning fossil fuels, deforestation, cement production | 1 (reference)
Methane (CH4) | Livestock digestion, landfill, natural gas leaks, rice paddies | ~28
Nitrous oxide (N2O) | Agricultural fertilisers, combustion processes | ~265
Water vapour (H2O) | Natural evaporation (amplifies warming set by other gases) | Varies; acts mainly as a feedback
Hydrofluorocarbons (HFCs) | Refrigerants, aerosols | Hundreds to thousands

Carbon dioxide is the most significant driver of human-caused warming because it is produced in such large quantities, even though methane and nitrous oxide are more potent molecule for molecule.

What is the enhanced greenhouse effect?

The natural greenhouse effect has kept Earth's climate relatively stable for millions of years. The enhanced (anthropogenic) greenhouse effect refers to the additional warming caused by human activities that increase greenhouse gas concentrations above natural levels.

Three main human activities are responsible:

Burning fossil fuels — Coal, oil, and natural gas are formed from ancient organic matter rich in carbon. When burned for electricity, transport, and heating, they release CO2 that was locked away underground for millions of years, adding it to the active carbon cycle.

Deforestation — Trees absorb CO2 through photosynthesis and store carbon in their biomass. When forests are cleared or burned, this stored carbon is released as CO2. Fewer trees also means less capacity to absorb future CO2 emissions.

Agriculture — Livestock (particularly cattle and sheep) produce methane as a by-product of digestion. Nitrogen-based fertilisers release nitrous oxide from soils. As global food demand grows, the greenhouse gas contribution from agriculture increases.

According to the Met Office, the global average surface temperature has risen approximately 1.1°C above pre-industrial levels as a result of these emissions — a change happening faster than any natural climate shift in the past 10,000 years. The Met Office's causes of climate change pages explain how these emissions are driving significant changes to weather patterns, sea levels, and ecosystems worldwide.

What are the consequences of enhanced warming?

Even a relatively small average temperature rise has large consequences for Earth's systems:

Rising sea levels — Warming causes seawater to expand (thermal expansion) and accelerates the melting of glaciers and ice sheets, raising global sea levels and threatening low-lying coastal areas.

More extreme weather — Higher temperatures increase the energy in the atmosphere, making heatwaves, heavy rainfall events, and droughts more frequent and more intense.

Ecosystem disruption — Species that evolved in stable climates face habitat loss as temperature and rainfall patterns shift. Coral reefs, for example, are severely stressed by water temperatures only 1–2°C above their usual range.

Ocean acidification — CO2 dissolves in seawater to form carbonic acid, lowering ocean pH and threatening marine organisms with calcium carbonate shells and skeletons.

What is the UK doing about climate change?

The UK was the first major economy to set a legally binding net-zero target: net-zero greenhouse gas emissions by 2050, enshrined in the Climate Change Act (amended 2019). The UK was also a signatory to the Paris Agreement, the 2015 international treaty that committed countries to limiting global warming to well below 2°C above pre-industrial levels, aiming for 1.5°C.

In practice, this means phasing out coal power generation (completed in 2024), expanding offshore wind capacity, electrifying transport, and improving energy efficiency in buildings. These policy choices are directly linked to the science of the greenhouse effect studied at KS3.

Frequently asked questions

Is the greenhouse effect natural?

Yes — the greenhouse effect is a natural and essential feature of Earth's atmosphere. Without it, Earth would be far too cold to support liquid water or life as we know it. What is not natural is the rate at which human activities since the Industrial Revolution (around 1750) have increased greenhouse gas concentrations, intensifying the effect and causing the rapid warming now observed. Scientists refer to this human-caused intensification as the enhanced greenhouse effect to distinguish it from the natural baseline process.

What are the main greenhouse gases?

The main greenhouse gases in Earth's atmosphere are water vapour, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Water vapour is the most abundant, but its concentration is controlled mainly by temperature (it acts as a feedback rather than a driver). Carbon dioxide is the most important driver of current human-caused warming because it is produced in such large volumes by burning fossil fuels and is long-lived in the atmosphere (persisting for centuries to millennia). Methane is more potent molecule for molecule but shorter-lived. Nitrous oxide from agriculture is also a significant contributor.

How does the greenhouse effect cause climate change?

The greenhouse effect causes climate change when the concentration of greenhouse gases in the atmosphere increases. More greenhouse gas molecules means more infrared radiation is absorbed and re-emitted back to Earth's surface, raising average surface temperatures. This warming then drives a cascade of changes: altered rainfall patterns, melting ice, rising sea levels, and shifts in when seasons begin and end. These changes collectively constitute climate change — a long-term shift in global and regional weather patterns. The Met Office has documented how observed changes in temperature, precipitation, and extreme weather events are consistent with the predictions made by climate science.

Why does burning fossil fuels increase CO2 levels?

Fossil fuels — coal, oil, and natural gas — were formed over millions of years from the compressed remains of ancient plants and marine organisms. The carbon in those organisms was originally absorbed from the atmosphere by photosynthesis and then locked underground when they died and were buried. When we burn fossil fuels, we reverse that process almost instantaneously on a geological timescale: carbon that took millions of years to accumulate underground is released as CO2 in decades. The atmosphere's natural carbon sinks (forests and oceans) cannot absorb this carbon fast enough, so atmospheric CO2 concentration rises — currently above 420 parts per million, compared to around 280 ppm before industrialisation.

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