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The Earth is warm enough for life because of a natural process called the greenhouse effect — without it, the planet would be a frozen ball of ice. But human activity is now adding extra greenhouse gases to the atmosphere, strengthening this effect and driving climate change. This is one of the biggest global challenges of our time. This lesson, part of Topic C6 of OCR Gateway Science A, explains which gases are greenhouse gases, how the greenhouse effect works, how human activities increase greenhouse-gas levels, the evidence for and uncertainties about climate change, and what a carbon footprint is and how it can be reduced.
By the end of this lesson you should be able to name the main greenhouse gases, explain the greenhouse effect, describe how human activities increase greenhouse-gas levels, discuss the evidence and uncertainties around climate change, and explain what a carbon footprint is and how it can be reduced.
The main greenhouse gases in the atmosphere are:
These gases are only a small part of the atmosphere, but they have a large effect on the Earth's temperature because of the way they interact with radiation.
Exam Tip: Learn the three greenhouse gases: carbon dioxide, methane and water vapour. A frequent slip is to name oxygen or nitrogen — these are not greenhouse gases.
The greenhouse effect is the natural process that keeps the Earth warm. It works like this:
The crucial detail is that greenhouse gases let the Sun's short-wavelength radiation in, but absorb the Earth's outgoing long-wavelength infrared radiation — so they trap heat that would otherwise escape. The natural greenhouse effect is essential; the problem is that human activity is enhancing it.
Exam Tip: The key idea is direction and wavelength: greenhouse gases absorb the outgoing infrared radiation (long wavelength) from the Earth and re-radiate it back. They do not "block the Sun's heat coming in" — that is a classic misconception.
Human activities have raised the levels of greenhouse gases in the atmosphere:
| Activity | Greenhouse gas | Why |
|---|---|---|
| Burning fossil fuels | Carbon dioxide | Combustion of coal, oil and gas releases CO2 |
| Deforestation | Carbon dioxide | Fewer trees means less photosynthesis, so less CO2 is removed |
| Agriculture / livestock (e.g. cattle, rice paddies) | Methane | Animals and waterlogged fields release methane |
| Landfill sites | Methane | Decomposing waste releases methane |
Burning fossil fuels and cutting down forests are the main reasons the level of carbon dioxide has risen; farming and landfill are major sources of extra methane.
Exam Tip: Match the gas to the activity: fossil fuels and deforestation → carbon dioxide; livestock, rice fields and landfill → methane. Deforestation matters because it reduces photosynthesis that would otherwise absorb CO2.
Based on peer-reviewed scientific evidence, the mainstream scientific consensus is that the increase in greenhouse gases caused by human activity is leading to global warming — a rise in the Earth's average temperature — and to climate change. Likely consequences include:
Because the climate is a very complex system, there are genuine uncertainties — for example in the precise size and timing of future changes, which depend on many interacting factors and on future emissions. However, these uncertainties are about the details; they do not overturn the well-evidenced conclusion that human activity is warming the planet. It is important to base conclusions on peer-reviewed evidence rather than on speculation, opinion or biased reporting, and to avoid giving "false balance" to claims that are not supported by the evidence.
Exam Tip: Present the peer-reviewed consensus clearly: extra greenhouse gases from human activity cause global warming and climate change. Note genuine uncertainties in the detail (size/timing) without pretending the basic conclusion is in doubt — that is the balanced, scientific position.
The carbon footprint of something is the total amount of greenhouse gases (expressed as carbon dioxide equivalent) released over the whole life of a product, service or event. It includes the greenhouse gases from making, using and disposing of a product, not just from running it.
Ways to reduce a carbon footprint include:
These actions are difficult to put into practice for several reasons: they can be expensive; the technology is not always ready or proven (for example, large-scale carbon capture); they require lifestyle changes that people are reluctant to make; and they need international political agreement and cooperation, which is hard to achieve.
Exam Tip: Define a carbon footprint as the total greenhouse gases (as CO₂ equivalent) over the whole life of a product/service/event — not just one stage. For "why is reducing it hard?", quote cost, technology, lifestyle and politics.
A point that students often miss is that the greenhouse effect is not itself a bad thing. The natural greenhouse effect has operated throughout Earth's history and is essential for life: without any greenhouse gases the Earth's average temperature would be roughly 30 °C colder, far below freezing, and liquid water and life as we know it could not exist. The greenhouse effect is what makes the planet habitable.
The problem is the enhanced greenhouse effect. By burning fossil fuels and clearing forests, humans are adding extra greenhouse gases to the atmosphere, which strengthens the natural effect and traps more heat than before. It is this extra warming — the enhancement — that drives global warming and climate change, not the existence of the greenhouse effect itself. When you write about this, be careful to say that human activity enhances (increases) the greenhouse effect; saying "the greenhouse effect is bad" without that qualification is a misconception.
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