Global warming

⭐ HIGH PRIORITY

SPEC 4.7.3.5 • PAPER 2

One of the shortest spec sections in Module 7 — but one of the highest-stakes in the exam. Two named gases, biological consequences, and one critical examiner trap to avoid.

The Big Idea

Spec 4.7.3.5 is deliberately short. The exam wants you to know three things — and nothing more:

Spec Statement (Verbatim)

"Levels of carbon dioxide and methane in the atmosphere are increasing, and contribute to 'global warming'. Students should be able to describe some of the biological consequences of global warming."

What You Need To Know

The two named gases — carbon dioxide and methane.

Their levels are increasing — and they contribute to global warming.

Biological consequences — describe some of them.

Don't Over-Explain

The Jun 2024 examiner report specifically warned: descriptions of long-wave / short-wave radiation, ozone, or the detailed greenhouse mechanism are beyond the spec. Stay simple: gases rise → atmosphere warms → biological effects.

The Two Named Gases

The spec specifies exactly two greenhouse gases. Other real-world greenhouse gases (water vapour, ozone, nitrous oxide) are not on the GCSE spec and won't earn marks.

1 — Carbon Dioxide (CO₂)

Burning fossil fuels (coal, oil, natural gas).

Deforestation — burning trees releases CO₂; fewer trees take in CO₂ for photosynthesis.

Decay or burning of peat (link to lesson 7.09).

Respiration by all living organisms (normally balanced by photosynthesis).

2 — Methane (CH₄)

Livestock — especially cattle (digestive processes).

Rice paddies — anaerobic decay in flooded soil.

Landfill — anaerobic decay of organic waste (link to lesson 7.07).

Peat bogs when destroyed — methane stored in peat is released (link to lesson 7.09).

Memorise Both

Mark schemes specifically credit "carbon dioxide" and "methane" by name. Writing "greenhouse gases" alone or naming only one gas costs marks.

Why Levels Are Rising

Atmospheric levels of both gases have risen sharply since industrialisation. The drivers cross several Module 7 lessons.

DriverEffect on Atmospheric GasSynoptic Link

Burning fossil fuelsReleases CO₂ stored over millions of years—

DeforestationLess photosynthesis (less CO₂ uptake); burning trees releases stored CO₂Lesson 7.09 (4.7.3.4)

Peat bog destructionReleases stored CO₂ AND methaneLesson 7.09 (4.7.3.3)

Intensive livestock farmingCattle produce methane via digestion—

Landfill wasteAnaerobic decay produces methaneLessons 7.07, 7.09

Context Numbers

Atmospheric CO₂ has risen from about 280 ppm pre-industrial (~1800) to over 420 ppm today — a rise of roughly 50%. You don't need exact values for the exam, but you should know levels are "increasing".

What "Global Warming" Means

Spec-Level Definition

Global warming is the increase in average global atmospheric temperature caused by increasing concentrations of greenhouse gases (CO₂ and methane). The gases trap heat that would otherwise escape to space.

Distinct Terms

The Jun 2024 examiner report explicitly stated: "the 'greenhouse effect' and 'climate change' are not equivalent to 'global warming'". They're related but distinct concepts. The spec uses "global warming" — so do you.

Biological Consequences (1)

The spec says "describe some of the biological consequences" — you don't need all of them, but you should know several. Mark schemes typically accept the following:

1 — Loss Of Habitat

Warming melts polar ice → ice-dependent species (e.g. polar bears, seals) lose habitat. Warmer oceans bleach and kill coral reefs → reef ecosystems collapse.

2 — Changes To Species Distribution

Species shift to higher latitudes (towards the poles) or higher altitudes seeking cooler conditions. Cold-adapted species lose range; warm-adapted species expand. (Synoptic with lesson 7.08 for HT.)

3 — Migration Pattern Changes

Birds migrating earlier or later than usual. Insects emerging at different times. This can cause timing mismatches — e.g. caterpillars hatching before bird chicks need them, or pollinators emerging before flowers bloom.

Biological Consequences (2)

4 — Sea-Level Rise & Flooding

Polar ice melting + warmer water expanding → sea levels rise → coastal habitats flooded; salt water intrudes into freshwater habitats; low-lying islands lose land area.

5 — Extreme Weather

More frequent and intense droughts, floods, wildfires, and storms. These directly damage habitats and kill organisms. Reduces species' ability to recover between events.

6 — Impacts On Food Production

Crop yields fall where conditions become too hot or dry. Fisheries shift as ocean temperatures change. Food security threatened (synoptic link → lesson 7.12).

7 — Extinction Risk

Species that cannot adapt or cannot disperse fast enough to new suitable habitats face local — or total — extinction. Particular risk for specialists, slow-reproducing species, and species on islands or mountain tops.

For 4–6 Mark Questions

Name 2–3 specific consequences for full marks. Don't just say "the climate changes" — name habitat loss, distribution shifts, migration timing, etc.

Scientific Consensus & Evidence [WS 1.6, WS 1.3]

The spec includes two working scientifically skills here — about how scientists are confident in global warming, and why some evidence remains uncertain.

WS 1.6 — Scientific Consensus

The scientific consensus on global warming is based on:

Systematic reviews of thousands of peer-reviewed publications.

Multiple independent lines of evidence (temperature records, ice cores, sea-level data, species movements).

International bodies (e.g. IPCC) compiling and synthesising this evidence.

WS 1.3 — Why Evidence Can Be Uncertain

Some uncertainty remains because:

Climate is a complex system — many interacting variables.

Predicting future effects requires computer models with assumptions.

Local effects vary — global averages can hide regional differences.

Long timescales — full effects take decades to be measurable.

What These Terms Mean

Peer review — before a scientific study is published, it is checked and critiqued by other scientists working in the same field. This filters out errors, weak methods and false conclusions.

Systematic review — a structured method for finding, combining and analysing all relevant studies on a question. Reduces the influence of any single study and produces a more reliable overall conclusion.

Important Distinction

"Uncertainty in details" is not the same as "scientists disagree about whether warming is happening". The basic fact of warming caused by human-released CO₂ and methane is supported by overwhelming consensus. The uncertainty is about how fast, how much, and where exactly consequences will hit.

Common Errors

Wrong

"Greenhouse gases include CO₂, methane, water vapour and ozone."

Right

Spec lists only CO₂ and methane. Other gases (water vapour, ozone) are real greenhouse gases but not on the GCSE spec — won't earn marks.

Wrong

"Global warming traps heat using long-wave radiation reflected off the ozone layer..."

Right

Mechanism detail (long/short-wave radiation, ozone) is beyond the spec. Stay at: gases trap heat → temperature rises.

Wrong

"Global warming will affect the climate."

Right

Vague. Name specific biological consequences: habitat loss, distribution shifts, migration changes, sea-level rise, extreme weather, food production impacts.

Wrong

"Scientists disagree about whether global warming is real."

Right

There is strong scientific consensus from systematic reviews of thousands of peer-reviewed publications. Uncertainty is about details, not the basic fact.

Wrong

"CO₂ is the only greenhouse gas."

Right

The spec names both CO₂ and methane. Forgetting methane often costs the methane mark in mark schemes (e.g. Jun 2024 P2 Q2.5).

Examiner Alerts

Alert 1 — Name Both Gases

Mark schemes (e.g. Jun 2024 P2 Q2.5) award separate marks for CO₂ and for methane. Forgetting methane is the single most common error in this topic. "A significant number of students omitted any reference to methane" — Jun 2024 examiner report.

Alert 2 — Don't Over-Explain Mechanism

Jun 2024 examiner report flagged students wasting time on long-wave radiation, ozone, detailed greenhouse-effect descriptions. Beyond the spec. The exam wants: gas levels rising → atmospheric temperature rises → consequences. Three steps. That's it.

Alert 3 — "Climate Change" vs "Global Warming" — Nuance

Mark schemes (e.g. Jun 2022 P2 Q4.3) accept "global warming", "climate change", and "traps heat" as interchangeable for the warming mark point. You won't lose marks for writing "climate change". However, the Jun 2024 examiner report notes the terms are conceptually distinct: greenhouse effect = the trapping mechanism (natural process); global warming = the temperature rise; climate change = broader consequences. Use the spec term — "global warming" — where you can, for precision.

Alert 4 — Name Specific Consequences

"The climate changes" or "things get worse" scores zero. Mark schemes accept specific named effects: habitat loss; species distribution change; migration; sea-level rise; extreme weather; food production impacts.

Alert 5 — "Released", Not "Contains"

If a question tells you that something "contains" CO₂ or methane (e.g. peat bog questions), you must add the value by saying the gas is released when the source is disturbed. Repeating "contains methane" earns zero marks. "Further added value was required, in that these gases would be 'released'" — Jun 2024 examiner report.

Practice — Recall

(a) Name the two greenhouse gases specified by the AQA spec that contribute to global warming. 2 marks

(b) Give three biological consequences of global warming on living organisms. 3 marks

(c) State why the scientific consensus on global warming is considered reliable. 1 mark

(a)

Carbon dioxide (1); methane (1).

(b)

Any three from: loss of habitat (e.g. ice melts); changes to species distribution (species shift to higher latitudes/altitudes); changes to migration patterns (e.g. timing mismatches); sea-level rise (flooding habitats); extreme weather (drought, floods, fire, storms); impacts on food production; extinction for species that cannot adapt or move. (1 mark each)

(c)

The consensus is based on systematic reviews of thousands of peer-reviewed publications. (1)

Practice — Explain

A landfill site near a wetland nature reserve has been operating for 30 years. Anaerobic decay of organic waste in the landfill releases large quantities of methane into the atmosphere. The reserve is home to a butterfly species adapted to cool, damp conditions.

Explain how methane released from the landfill could affect the distribution of the butterfly species.4 marks

1

Methane is a greenhouse gas — it traps heat in the atmosphere. (1)

2

Increased methane levels contribute to global warming — average atmospheric temperature rises. (1)

3

The wetland becomes warmer / drier than the cool, damp conditions the butterfly is adapted to. The wetland habitat may shrink as conditions become unsuitable. (1)

4

The butterfly's distribution shifts — it may move to higher latitudes or higher altitudes (seeking cooler conditions), or its range may shrink. If it cannot disperse fast enough, the local population could become extinct. (1)

Tip: a chain answer like this scores best — gas → warming → habitat change → distribution change. Don't skip steps; don't add greenhouse-mechanism detail.

Practice — 6-Marker (Level)

A meadow in southern England contains a wildflower species that is pollinated by a specific bee species. The bees historically emerged in late April, just as the wildflowers began to bloom. Over the past 40 years, average spring temperatures in this region have risen by 1.5 °C, and scientists have observed that the bees now emerge in early April.

Evaluate the biological consequences this warming could have on the wildflower, the bee, and the wider meadow ecosystem. 6 marks (Level marked)

Level 3 (5–6): coherent evaluation with multiple named consequences, mechanism, and judgement.

A

Timing mismatch: the bees now emerge ~3 weeks earlier than the wildflowers bloom — a "migration / emergence pattern change". The bees may have no nectar source when they emerge; the wildflowers may have no pollinators when they bloom.

B

Impact on the wildflower: without effective pollination, fewer seeds form; the wildflower's reproduction declines. Distribution and population shrink. Local extinction possible if mismatch persists.

C

Impact on the bee: without nectar at emergence, bee colonies starve; bee populations decline. The bee may itself shift to higher latitudes or altitudes seeking matched conditions.

D

Wider ecosystem: other plants pollinated by the same bee species are also affected. Other insects/birds that depend on the wildflower (for food, shelter) lose a resource. Biodiversity in the meadow decreases. Food chains disrupted.

E

Judgement: the 1.5 °C rise has disrupted a co-evolved relationship — both species suffer, and the impacts cascade through the ecosystem. This is a named biological consequence of global warming (changes to migration/emergence pattern + distribution shift + food web disruption).

Level 2: describes consequences for the wildflower OR the bee but not the wider ecosystem. Level 1: lists effects without linking to warming or evaluating impact.

Recap & Next Up

2

Named gases

6+

Consequences

3

Steps in the chain

Three Things To Take Away

1

Two named gases — CO₂ and methane. Both contribute to global warming. Mark schemes credit each by name; don't forget methane.

2

Three-step chain — gases rising → atmospheric temperature rises → biological consequences. Don't over-explain the greenhouse mechanism.

3

Name specific consequences — habitat loss, distribution shifts, migration timing, sea-level rise, extreme weather, food production impacts. "The climate changes" scores zero.

Next: Lesson 7.11

Trophic levels & Pyramids of biomass & Transfer of biomass (4.7.4.1–7.4.3) — biology only. Producers, primary/secondary/tertiary consumers, apex predators, decomposers, and biomass calculations.