What you'll learn
Chemistry of the atmosphere: greenhouse gases and climate change traces the Earth's atmosphere from its violent origins to the air we breathe, and then examines how human activity is changing it again. It is unusual within AQA GCSE Chemistry in that much of the evidence is indirect — there are no measurements of the atmosphere from billions of years ago — so the topic doubles as a lesson in how scientists reason from incomplete evidence. It also contains more extended-writing questions than most, and rewards structured answers. By the end of this guide you should be able to state the composition of dry air, describe how the atmosphere evolved and why oxygen rose and carbon dioxide fell, explain the greenhouse effect using the precise mechanism, evaluate the evidence for human-caused climate change, define and assess carbon footprint, and name each combustion pollutant with its effect.
Key terms and definitions
Atmosphere — the layer of gases surrounding the Earth, roughly four-fifths nitrogen and one-fifth oxygen for the last 200 million years
Greenhouse gas — a gas that absorbs outgoing long wavelength radiation and re-radiates some of it back towards the Earth
Greenhouse effect — the natural process keeping the Earth warm enough to support life
Global climate change — changes in the Earth's climate caused by increased levels of greenhouse gases
Carbon footprint — the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event
Carbon offsetting — compensating for emissions by activities that remove carbon dioxide, such as tree planting
Particulate — a small solid particle of carbon or unburnt hydrocarbon released by incomplete combustion
Global dimming — a reduction in sunlight reaching the Earth's surface, caused partly by particulates
Acid rain — rain made acidic by dissolved sulfur dioxide and oxides of nitrogen
Peer review — the checking of scientific work by other scientists before publication
Core concepts
The atmosphere today
For the last 200 million years the proportions of gases in the atmosphere have been much as they are now. Dry air is approximately four-fifths nitrogen and one-fifth oxygen, or about 80 per cent and 20 per cent. Small proportions of other gases are present, including carbon dioxide, water vapour and the noble gases.
The early atmosphere
Theories about the early atmosphere have changed over time and continue to change, because the evidence is limited. The Earth is about 4.6 billion years old, and very little direct evidence survives from its first billion years.
The current best theory is that intense volcanic activity during that first billion years released the gases that formed the early atmosphere, along with water vapour that condensed as the planet cooled to form the oceans.
The early atmosphere is thought to have consisted mainly of carbon dioxide with little or no oxygen, resembling the atmospheres of Mars and Venus today. Volcanoes also produced nitrogen, which built up gradually, together with small proportions of methane and ammonia.
Because the evidence is incomplete, these ideas should be presented as the current best theory rather than as established fact, and questions frequently reward that caution explicitly.
How oxygen increased
Algae first produced oxygen by photosynthesis about 2.7 billion years ago, and the proportion of oxygen in the atmosphere began to rise from that point.
Algae came first, and over the following billion years plants evolved. As the quantity of photosynthesising life increased, so did the oxygen level, eventually reaching a proportion sufficient for animal life to evolve.
The photosynthesis reaction is the same one met in biology: carbon dioxide and water, using light, producing glucose and oxygen.
How carbon dioxide decreased
Several processes contributed, and a full-mark answer normally requires more than one.
Photosynthesis removed carbon dioxide from the atmosphere and locked the carbon into the bodies of algae and plants.
Carbon dioxide dissolved in the oceans as they formed. Some of the dissolved gas produced carbonate precipitates, which settled as sediments on the sea bed and became sedimentary rock.
Carbon became locked away in fossil fuels and sedimentary rocks. Coal is a sedimentary rock formed from thick plant deposits buried and compressed. Crude oil and natural gas formed from the remains of plankton deposited in mud on the sea floor. Limestone formed largely from the shells and skeletons of marine organisms.
In every case, carbon that was once atmospheric carbon dioxide ended up in the ground, which is precisely why burning fossil fuels returns it.
The greenhouse effect
The main greenhouse gases are carbon dioxide, methane and water vapour, and they maintain temperatures on Earth high enough to support life.
The mechanism must be stated precisely, because vague versions score poorly. Short wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it. The Earth then emits radiation of a longer wavelength. Greenhouse gases absorb this outgoing long wavelength radiation and re-radiate some of it back towards the surface, so less energy escapes to space and the Earth remains warmer than it otherwise would be.
The greenhouse effect itself is natural and essential; without it the Earth would be far too cold for life. The concern is that human activity has increased the concentration of greenhouse gases, enhancing the effect.
Human activities and climate change
Carbon dioxide levels rise through the burning of fossil fuels and through deforestation, which both releases carbon and removes trees that would otherwise absorb it. Methane levels rise through livestock farming, particularly cattle, and through decomposition in landfill sites.
Based on peer-reviewed evidence, many scientists believe that human activities will cause the Earth's atmospheric temperature to increase, resulting in global climate change.
It is difficult to model such complex systems, which leads to simplified models, speculation and opinions presented in the media that may be biased or incomplete. A strong answer acknowledges this genuine uncertainty while being clear that the peer-reviewed scientific consensus supports human-caused warming.
The potential effects include rising sea levels as ice melts and seawater expands, causing flooding and coastal erosion; more frequent and more severe storms; changes in the amount, timing and distribution of rainfall; changes to the distribution of species; and changes to crop yields and food production patterns.
Carbon footprint
The carbon footprint of a product, service or event is the total amount of carbon dioxide and other greenhouse gases emitted over its whole life cycle, from obtaining raw materials through manufacture and use to disposal.
It can be reduced by using less fossil fuel and more renewable energy, by increasing efficiency so less energy is needed overall, by capturing and storing carbon dioxide, and through carbon taxes, licences and offsetting schemes such as tree planting.
Reduction is difficult in practice, and the reasons are examinable in their own right: the necessary changes may be expensive, there may be disagreement over the scientific evidence, individuals may be unwilling to change their lifestyles, alternative technology may not yet exist, and governments may be reluctant to act in ways that damage their economies or that require international agreement to be effective.
Atmospheric pollutants from combustion
Burning fuels releases pollutants as well as energy, and each has a specific origin and effect.
Carbon dioxide comes from complete combustion and contributes to global climate change.
Carbon monoxide comes from incomplete combustion in a limited oxygen supply. It is toxic, and because it is colourless and odourless it cannot be detected by the senses.
Particulates, solid particles of carbon and unburnt hydrocarbons, also come from incomplete combustion. They cause global dimming and damage human health by harming the lungs.
Sulfur dioxide comes from sulfur impurities present in the fuel, which are oxidised when the fuel burns.
Oxides of nitrogen do not come from the fuel at all. They form when the high temperatures inside an engine cause nitrogen and oxygen from the air to react together.
Both sulfur dioxide and oxides of nitrogen dissolve in atmospheric water to produce acid rain, which damages buildings and statues, harms plants and trees, and acidifies lakes and rivers. Both also cause respiratory problems in humans.
Worked examples
Example 1: Explaining the greenhouse effect (4 marks)
Explain how greenhouse gases keep the Earth's surface warm.
Short wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, which warms and then emits radiation of a longer wavelength.
Greenhouse gases such as carbon dioxide, methane and water vapour absorb this outgoing long wavelength radiation. They re-radiate some of it back towards the Earth's surface, so less energy is lost to space. As a result the temperature at the surface is higher than it would be without these gases.
Example 2: Accounting for falling carbon dioxide (4 marks)
Describe two processes by which the proportion of carbon dioxide in the atmosphere decreased over billions of years.
Algae and later plants carried out photosynthesis, absorbing carbon dioxide from the atmosphere and converting the carbon into glucose and then into their own tissues, so it was no longer in the air.
Carbon dioxide also dissolved in the oceans that formed as water vapour condensed. Some of the dissolved carbon dioxide formed carbonate precipitates, which settled as sediments and eventually became sedimentary rock such as limestone. In both cases carbon was locked away rather than remaining atmospheric.
Example 3: Evaluating a media claim (3 marks)
A television programme claims that scientists disagree about whether human activity causes climate change. Discuss how this claim should be assessed.
Climate systems are very complex and difficult to model, so simplified models are used and genuine uncertainty exists about the detail and scale of future predictions. Media reports may present speculation or opinion rather than peer-reviewed evidence, and may be biased or incomplete, sometimes giving disproportionate coverage to minority views.
However, the peer-reviewed evidence considered by many scientists supports the conclusion that human activities are causing the Earth's temperature to rise. Disagreement about details should not be presented as disagreement about the central conclusion.
Common mistakes and how to avoid them
The most frequent error is describing greenhouse gases as trapping heat or forming a blanket. The specification requires absorption of long wavelength radiation and re-radiation towards the Earth, and metaphors do not earn the marks.
Students often confuse the greenhouse effect with the hole in the ozone layer. They are separate phenomena, and mentioning ozone signals a misunderstanding.
Another common slip is stating definitively what the early atmosphere contained. The evidence is limited, so answers should refer to what the current best theory suggests.
In pollutant questions, many candidates name a pollutant without giving its effect. Marks are usually split between the two, so always pair them.
Finally, the origins of sulfur dioxide and oxides of nitrogen are routinely merged. Sulfur dioxide comes from impurities in the fuel; oxides of nitrogen come from air reacting at high temperature.
Exam technique for "Chemistry of the atmosphere: greenhouse gases and climate change"
Use the phrase long wavelength radiation rather than heat when describing the greenhouse effect. It is the wording the mark scheme uses.
For six-mark questions on climate change, plan three points with a consequence for each rather than listing everything you know. Structure earns more than volume in this topic.
Where a question asks for evaluation, give both sides and then a judgement. Questions on reducing carbon footprint expect the political and economic obstacles as well as the technical methods.
Learn the pollutants as cause-and-effect pairs, and rehearse the two different origins of sulfur dioxide and oxides of nitrogen until they are automatic.
Use tentative language about the early atmosphere and definite language about the present composition. The distinction is itself credited.
Quick revision summary
Dry air today is about four-fifths nitrogen and one-fifth oxygen with small amounts of carbon dioxide, water vapour and noble gases. The early atmosphere came from intense volcanic activity, rich in carbon dioxide with little oxygen, while condensing water vapour formed the oceans. Oxygen rose once algae began photosynthesising about 2.7 billion years ago, followed by plants. Carbon dioxide fell through photosynthesis, dissolving in the oceans, carbonate precipitation, and burial as fossil fuels and sedimentary rocks. Greenhouse gases — carbon dioxide, methane and water vapour — absorb outgoing long wavelength radiation and re-radiate it back, keeping the Earth warm; human activity has increased them through burning fossil fuels, deforestation, livestock and landfill. Peer-reviewed evidence links this to climate change, with effects on sea level, storms, rainfall, species distribution and food production, though complex models create genuine uncertainty and media coverage may be biased. Carbon footprint covers the whole life cycle and is hard to cut for reasons of cost, technology, lifestyle and international agreement. Combustion pollutants are carbon dioxide, carbon monoxide, particulates, sulfur dioxide from fuel impurities, and oxides of nitrogen from air at high temperature.