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Welcome to GCSE Edexcel Science revision.

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Unit S C 21: Earth and atmospheric science.

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The early atmosphere developed substantially from gases released by volcanic activity.

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The GCSE model describes much carbon dioxide, water vapour, little or no free oxygen and smaller amounts of other gases.

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Its exact composition and changes over billions of years are uncertain.

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There are no direct measurements of the earliest atmosphere; evidence comes from rocks, fossils, volcanic gases and comparisons with other planets.

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Venus and Mars have C O 2-rich atmospheres that provide a useful comparison, but they do not prove Earth followed an identical history.

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Saturn is not the intended rocky-planet comparison.

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As Earth cooled, water vapour condensed and rain contributed to forming oceans.

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Carbon dioxide dissolved in the oceans, reducing atmospheric C O 2.

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Carbon was later stored in carbonate sediments, shells and rocks and in organic material that could form fossil fuels.

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These processes helped change the atmospheric carbon balance.

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A simplified process sequence, not a precisely dated or complete atmospheric history.

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Rock chemistry can indicate oxidation conditions and changes in free oxygen.

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A rock containing chemically bound oxygen is not evidence that there was already much O 2 gas; early rocks were not universally oxygen-free.

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Photosynthetic microorganisms, including cyanobacteria, used C O 2 and released oxygen.

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Photosynthesis and oxygen reactions with rocks and oceans contributed to a long, complex rise in atmospheric oxygen.

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A major rise in free oxygen occurred around 2.4 billion years ago, with further changes later.

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Do not picture today's 21 percent oxygen appearing instantly at that time.

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For GCSE revision, dry air is approximately 78 percent nitrogen, 21 percent oxygen and 1 percent other gases.

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The “dry” qualification excludes variable water vapour.

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Nitrogen 78 percent, oxygen 21 percent, other gases about 1 percent; water vapour varies.

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Argon contributes most of the remaining roughly 1 percent; carbon dioxide is around 0.04 percent and its measured concentration changes over time and location.

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Other gases occur in smaller amounts.

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Water-vapour content varies with conditions and location, so percentages for moist air differ from a simplified dry-air composition.

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Oxygen supports combustion and is tested using a glowing splint: it relights in a suitable oxygen sample.

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A lighted-splint pop instead tests hydrogen.

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Evidence from an air-oxygen practical can use removal of oxygen by a suitable reaction and a measured gas-volume fall.

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Control temperature and pressure and let equipment cool before comparing volumes.

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If 100 cubic centimetres air falls to 79 cubic centimetres after oxygen removal at the same conditions,

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oxygen represented about 21 cubic centimetres,

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or 21 percent,

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of the original volume.

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Other gases remain.

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Earth absorbs some incoming solar radiation and emits infrared radiation as its surface warms.

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Some outgoing infrared escapes directly to space.

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Greenhouse gases,

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including C O 2,

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methane and water vapour,

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absorb certain infrared wavelengths and subsequently emit radiation in different directions,

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including back towards the surface.

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This natural greenhouse effect keeps Earth warmer than it would be without these gases.

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It is essential to the climate, not inherently a problem to eliminate completely.

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Increasing greenhouse-gas concentrations enhances the effect, changing Earth's energy balance and driving warming until energy flows adjust.

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The mechanism is absorption and re-emission, not mirror-like reflection of all heat.

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Schematic energy paths: absorption and re-emission, not reflection by a solid lid.

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Methane has a stronger warming effect per unit mass than C O 2 over commonly used comparison periods, but the comparison depends on timescale and atmospheric lifetime.

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Amounts and persistence also matter.

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Do not confuse greenhouse warming with ozone-layer depletion or acid rain.

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They involve different chemistry, even where some activities contribute to more than one problem.

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Burning fossil fuels adds C O 2 from long-stored carbon to the atmosphere.

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Deforestation can release stored carbon and reduce photosynthetic uptake.

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Farm animals such as cattle produce methane while digesting food.

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Methane also escapes from fossil-fuel equipment and forms when organic material decomposes with little oxygen, for example in waterlogged soil or landfill.

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Climate is the pattern of conditions over long periods; weather is short-term conditions.

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Global warming means a long-term rise in global average temperature, not every place warming every day.

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Modern atmospheric measurements, surface temperature records, satellite observations, ocean measurements and ice cores provide different evidence about climate change.

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Ice cores contain trapped bubbles of ancient air that record past greenhouse gases.

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Other features of the ice provide evidence about past temperature; the gases are not simply described as dissolved in the ice.

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Schematic core: different evidence is used for past gas levels and temperature.

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Rising greenhouse gases and warming are correlated, but correlation alone is not sufficient proof of causation.

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Physical radiation measurements, mechanisms and multiple lines of evidence support the role of human emissions.

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Climate data have uncertainties, for example from instrument calibration, where measurements were taken and gaps in old records.

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Evidence used to estimate past conditions (proxy records) also has dating and interpretation uncertainties.

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Global averages use many measurements, not one local thermometer.

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Natural influences and variability also affect climate, but they do not explain away the evidence for human-driven recent warming.

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Distinguish uncertainty in exact projections from uncertainty about the basic greenhouse mechanism.

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Warming oceans expand, and melting land-based glaciers and ice sheets add water, contributing to sea-level rise.

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Melting floating ice is not the same direct addition of sea water.

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Floating-ice melt is not equivalent to melting a land ice sheet.

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Sea-level rise can increase coastal flooding and erosion.

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Changes in rainfall and heat can affect water supply, crop production, habitats and species distributions.

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Warming changes the likelihood or intensity of some extremes, including heatwaves and heavy rainfall.

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Do not claim every tornado, storm or single event has the same response or is solely caused by climate change.

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Ocean uptake of C O 2 increases acidity, lowering P H and changing carbonate chemistry.

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This can make it harder for some organisms to build and maintain calcium-carbonate shells or skeletons.

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Ocean acidification means becoming more acidic relative to before, not necessarily having P H below 7.

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It is a direct C O 2 chemistry effect as well as related to the broader climate problem.

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Heat stress can make corals lose the algae that live with them in a mutually beneficial relationship (symbiosis).

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This exposes their pale skeletons: coral bleaching.

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Bleaching and ocean acidification are different processes.

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Ocean warming and acidification can both affect reefs, by different mechanisms.

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Bleached coral can sometimes recover if stress ends, but prolonged or repeated stress can lead to death.

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Changes in reefs can affect many dependent organisms.

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Mitigation tackles the causes of climate change by reducing greenhouse-gas emissions or removing these gases.

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Examples include energy efficiency, low-carbon energy and transport, protecting forests and reducing methane releases.

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Livestock and food-system changes can lower some methane emissions, but evaluate practicality, scale and other environmental effects rather than proposing one universal solution.

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Carbon capture and storage aims to prevent some C O 2 reaching the atmosphere by capturing and storing it securely.

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Costs, energy needs, capacity and long-term leakage risks matter.

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Adaptation reduces the harm from climate change.

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Examples include flood defences, better drainage, buildings suited to hotter weather and changes to farming.

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Some vulnerable communities may need to move, which has social and economic costs.

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A response can reduce risks without eliminating the need to tackle causes.

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Reflecting more sunlight is a proposed solar-radiation-management approach with substantial uncertainties and risks.

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It does not remove C O 2 or directly solve ocean acidification and is not a simple substitute for emissions reduction.

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Evaluate a response using scale, cost, effectiveness, risks, timescale and environmental implications.

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Tradeoffs can include land use, habitats, materials and who receives benefits or bears costs.

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A clear explanation links activity to increased greenhouse gas, leading to stronger infrared absorption and re-emission, leading to warming and possible climate impacts.

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Include methane and other gases where relevant, not only carbon dioxide.

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That completes Earth and atmospheric science.

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Revisit the notes and test yourself on the revision website.