Mr Kidman's Classroom← Back

Edexcel Separate Sciences · Chemistry · Paper 2

SC21 · Earth and atmospheric scienceTopic 8 — Fuels and Earth science

The atmosphere, climate and resources

Revise the key ideas

The early atmosphere and its evidence

  • The early atmosphere developed substantially from gases released by volcanic activity. The GCSE model describes much carbon dioxide, water vapour, little or no free oxygen and smaller amounts of other gases.
  • Its exact composition and changes over billions of years are uncertain. There are no direct measurements of the earliest atmosphere; evidence comes from rocks, fossils, volcanic gases and comparisons with other planets.
  • Venus and Mars have CO₂-rich atmospheres that provide a useful comparison, but they do not prove Earth followed an identical history. Saturn is not the intended rocky-planet comparison.
  • As Earth cooled, water vapour condensed and rain contributed to forming oceans. Carbon dioxide dissolved in the oceans, reducing atmospheric CO₂.
  • Carbon was later stored in carbonate sediments, shells and rocks and in organic material that could form fossil fuels. These processes helped change the atmospheric carbon balance.
    Atmosphere evolvesVolcanic gases: CO₂ and water vapour; little O₂ → Cooling: water condenses to oceans → CO₂ dissolves and becomes stored in carbon reservoirs → Photosynthesis uses CO₂ and releases O₂Volcanic gases: CO₂ and water vapour; little O₂Cooling: water condenses to oceansCO₂ dissolves and becomes stored in carbon reservoirsPhotosynthesis uses CO₂ and releases O₂
    A simplified process sequence, not a precisely dated or complete atmospheric history.
  • Rock chemistry can indicate oxidation conditions and changes in free oxygen. A rock containing chemically bound oxygen is not evidence that there was already much O₂ gas; early rocks were not universally oxygen-free.
  • Photosynthetic microorganisms, including cyanobacteria, used CO₂ and released oxygen. Photosynthesis and oxygen reactions with rocks/oceans contributed to a long, complex rise in atmospheric oxygen.
  • A major rise in free oxygen occurred around 2.4 billion years ago, with further changes later. Do not picture today's 21% oxygen appearing instantly at that time.

The atmosphere today and oxygen testing

  • For GCSE revision, dry air is approximately 78% nitrogen, 21% oxygen and 1% other gases. The “dry” qualification excludes variable water vapour.
    Approximate dry-air compositionOne hundred cells: first seventy-eight nitrogen, next twenty-one oxygen, final one other gases; water vapour excluded.78 nitrogen21 oxygen1 other gases100 cells represent approximate dry-air percentages
    Nitrogen 78%, oxygen 21%, other gases about 1%; water vapour varies.
  • Argon contributes most of the remaining roughly 1%; carbon dioxide is around 0.04% and its measured concentration changes over time and location. Other gases occur in smaller amounts.
  • Water-vapour content varies with conditions and location, so percentages for moist air differ from a simplified dry-air composition.
  • Oxygen supports combustion and is tested using a glowing splint: it relights in a suitable oxygen sample. A lighted-splint pop instead tests hydrogen.
  • Evidence from an air-oxygen practical can use removal of oxygen by a suitable reaction and a measured gas-volume fall. Control temperature/pressure and let equipment cool before comparing volumes.
  • If 100 cm³ air falls to 79 cm³ after oxygen removal at the same conditions, oxygen represented about 21 cm³, or 21%, of the original volume. Other gases remain.

The greenhouse effect

  • Earth absorbs some incoming solar radiation and emits infrared radiation as its surface warms. Some outgoing infrared escapes directly to space.
  • Greenhouse gases, including CO₂, methane and water vapour, absorb certain infrared wavelengths and subsequently emit radiation in different directions, including back towards the surface.
  • This natural greenhouse effect keeps Earth warmer than it would be without these gases. It is essential to the climate, not inherently a problem to eliminate completely.
  • Increasing greenhouse-gas concentrations enhances the effect, changing Earth's energy balance and driving warming until energy flows adjust. The mechanism is absorption and re-emission, not mirror-like reflection of all heat.
    Greenhouse absorption and re-emissionIncoming solar radiation warms Earth's surface. Surface infrared partly escapes and is partly absorbed by greenhouse gases, which emit radiation upward and downward.Incoming sunlightSome infrared escapesAtmosphereGreenhouse gases absorb IRWarm surface emits infrared (IR)Re-emitted IR
    Schematic energy paths: absorption and re-emission, not reflection by a solid lid.
  • Methane has a stronger warming effect per unit mass than CO₂ over commonly used comparison periods, but the comparison depends on timescale and atmospheric lifetime. Amounts and persistence also matter.
  • Do not confuse greenhouse warming with ozone-layer depletion or acid rain. They involve different chemistry, even where some activities contribute to more than one problem.

Human activities and climate evidence

  • Burning fossil fuels adds CO₂ from long-stored carbon to the atmosphere. Deforestation can release stored carbon and reduce photosynthetic uptake.
  • Farm animals such as cattle produce methane while digesting food. Methane also escapes from fossil-fuel equipment and forms when organic material decomposes with little oxygen, for example in waterlogged soil or landfill.
  • Climate is the pattern of conditions over long periods; weather is short-term conditions. Global warming means a long-term rise in global average temperature, not every place warming every day.
  • Modern atmospheric measurements, surface temperature records, satellite observations, ocean measurements and ice cores provide different evidence about climate change.
  • Ice cores contain trapped bubbles of ancient air that record past greenhouse gases. Other features of the ice provide evidence about past temperature; the gases are not simply described as dissolved in the ice.
    Air bubbles in an ice coreLayered ice contains trapped air bubbles. Their gas composition records ancient atmosphere; other properties of ice indicate past climate.Trapped airBubbles preserve gases; ice also holds climate clues
    Schematic core: different evidence is used for past gas levels and temperature.
  • Rising greenhouse gases and warming are correlated, but correlation alone is not sufficient proof of causation. Physical radiation measurements, mechanisms and multiple lines of evidence support the role of human emissions.
  • Climate data have uncertainties, for example from instrument calibration, where measurements were taken and gaps in old records. Evidence used to estimate past conditions (proxy records) also has dating and interpretation uncertainties. Global averages use many measurements, not one local thermometer.
  • Natural influences and variability also affect climate, but they do not explain away the evidence for human-driven recent warming. Distinguish uncertainty in exact projections from uncertainty about the basic greenhouse mechanism.

Potential effects on people and ecosystems

  • Warming oceans expand, and melting land-based glaciers and ice sheets add water, contributing to sea-level rise. Melting floating ice is not the same direct addition of sea water.
    Two major sea-level contributionsOcean warming → water expands → Land ice melts → water added to ocean → Higher sea level → increased coastal risksOcean warming → water expandsLand ice melts → water added to oceanHigher sea level → increased coastal risks
    Floating-ice melt is not equivalent to melting a land ice sheet.
  • Sea-level rise can increase coastal flooding and erosion. Changes in rainfall and heat can affect water supply, crop production, habitats and species distributions.
  • Warming changes the likelihood or intensity of some extremes, including heatwaves and heavy rainfall. Do not claim every tornado, storm or single event has the same response or is solely caused by climate change.
  • Ocean uptake of CO₂ increases acidity, lowering pH and changing carbonate chemistry. This can make it harder for some organisms to build and maintain calcium-carbonate shells or skeletons.
  • Ocean acidification means becoming more acidic relative to before, not necessarily having pH below 7. It is a direct CO₂ chemistry effect as well as related to the broader climate problem.
  • Heat stress can make corals lose the algae that live with them in a mutually beneficial relationship (symbiosis). This exposes their pale skeletons: coral bleaching. Bleaching and ocean acidification are different processes.
    Two different ocean effectsDriver, Process, Possible harm; More dissolved CO₂, Acidification, Shell formation; Heat stress, Coral bleaching, Algae lostDriverProcessPossible harmMore dissolved CO₂AcidificationShell formationHeat stressCoral bleachingAlgae lost
    Ocean warming and acidification can both affect reefs, by different mechanisms.
  • Bleached coral can sometimes recover if stress ends, but prolonged or repeated stress can lead to death. Changes in reefs can affect many dependent organisms.

Mitigation and adaptation

  • Mitigation tackles the causes of climate change by reducing greenhouse-gas emissions or removing these gases. Examples include energy efficiency, low-carbon energy and transport, protecting forests and reducing methane releases.
  • Livestock and food-system changes can lower some methane emissions, but evaluate practicality, scale and other environmental effects rather than proposing one universal solution.
  • Carbon capture and storage aims to prevent some CO₂ reaching the atmosphere by capturing and storing it securely. Costs, energy needs, capacity and long-term leakage risks matter.
  • Adaptation reduces the harm from climate change. Examples include flood defences, better drainage, buildings suited to hotter weather and changes to farming. Some vulnerable communities may need to move, which has social and economic costs.
    Responses to climate changeType, Target, Examples; Mitigation, Causes, Cut emissions; Adaptation, Impacts, Flood defencesTypeTargetExamplesMitigationCausesCut emissionsAdaptationImpactsFlood defences
    A response can reduce risks without eliminating the need to tackle causes.
  • Reflecting more sunlight is a proposed solar-radiation-management approach with substantial uncertainties and risks. It does not remove CO₂ or directly solve ocean acidification and is not a simple substitute for emissions reduction.
  • Evaluate a response using scale, cost, effectiveness, risks, timescale and environmental implications. Tradeoffs can include land use, habitats, materials and who receives benefits or bears costs.
  • A clear explanation links activity → increased greenhouse gas → stronger infrared absorption/re-emission → warming → possible climate impacts. Include methane and other gases where relevant, not only CO₂.

Watch SC21 · Earth and atmospheric science · Topic 8 — Fuels and Earth science

Revise earth and atmospheric science with this narrated video. Use the player controls to pause, seek, adjust the volume or mute. Turn English captions on or off using the captions menu.

Open or download the video · English captions