Global climate change & greenhouse effect

What you will learn

explain the natural greenhouse effect and why Earth would be much colder without it,
distinguish the natural greenhouse effect from the enhanced effect caused by human activity,
name the main greenhouse gases and their major human sources,
describe evidence for climate change (temperature, ice, sea level, CO $_2$ records),
explain key feedback loops and compare mitigation with adaptation.

Worked example 0 Real-world example: an Australian heatwave

Melbourne records a January week with four days above $40^{\circ}\text{C}$ . Power demand spikes, bushfires start, hospital admissions for heat stroke rise.

A warmer average climate shifts the whole temperature distribution to the right — days that used to be rare ( $> 40^{\circ}\text{C}$ ) become more frequent.
Drier fuel + higher temperatures + lower humidity = more fire-danger days.
Power-grid, housing, and emergency services were all designed for the old distribution and are stressed by the new one.

Key idea: a small change in the mean produces a disproportionate change in the extremes — the impact is in the tails, not the averages.

1. The natural greenhouse effect

Earth receives energy from the Sun (mostly visible light). Earth re-emits energy as infrared (heat) radiation. Some infrared is absorbed by greenhouse gases in the atmosphere and re-radiated in all directions — including back toward the surface. This keeps Earth’s average surface temperature about $+15^{\circ}\text{C}$ instead of about $-18^{\circ}\text{C}$ .

Simplified greenhouse effect. Sunlight passes through the atmosphere and warms the surface; infrared emitted by the surface is partly absorbed by greenhouse gas molecules, which re-emit some of that energy back down.

Main greenhouse gases (GHGs): water vapour, carbon dioxide ( $\text{CO}_2$ ), methane ( $\text{CH}_4$ ), nitrous oxide ( $\text{N}_2\text{O}$ ), and various industrial gases. Their molecular structure lets them absorb infrared photons.

2. The enhanced greenhouse effect

Since the Industrial Revolution, human activity has added large amounts of GHGs to the atmosphere.

Source	Main GHG
Burning fossil fuels (coal, oil, gas)	$\text{CO}_2$
Deforestation	$\text{CO}_2$ (less uptake; burning releases stored carbon)
Livestock, rice paddies, gas leaks	$\text{CH}_4$
Fertiliser use	$\text{N}_2\text{O}$
Industrial refrigerants, solvents	CFCs, HFCs

Atmospheric $\text{CO}_2$ has risen from about 280 ppm (pre-1800) to over 420 ppm today. The extra gases trap more outgoing infrared, raising surface temperature — the enhanced greenhouse effect.

3. Evidence for climate change

Temperature records: global average temperature has risen about $1.2^{\circ}\text{C}$ since 1880, with most of the rise after 1970.
Ice cores: Antarctic and Greenland ice trapped air bubbles preserve a record of past CO $_2$ and temperature. Current CO $_2$ levels are higher than any in the last 800 000 years.
Sea level: rising at about 3-4 mm per year, from thermal expansion of seawater and melting land ice.
Glacier and sea-ice retreat: satellite records show systematic shrinkage.
Ocean acidification: oceans absorb about a quarter of human CO $_2$ ; pH has fallen from about 8.2 to 8.1 (a $26\%$ increase in hydrogen ions).
Biological shifts: earlier flowering, poleward range shifts, coral bleaching.

Each line of evidence can be questioned alone, but together they form a coherent picture.

Worked example 1 Reading a CO2 record

Ice cores show CO $_2$ oscillated between 180 and 280 ppm during ice-age cycles (800 000 years). Today CO $_2$ is about 420 ppm. By how much has CO $_2$ risen above the previous natural maximum, and as what percentage?

Rise above maximum: $420 - 280 = 140$ ppm.
Percentage above maximum: $\dfrac{140}{280} \times 100 = 50\%$ .
Current CO $_2$ is unprecedented in the ice-core record by a large margin.

Key idea: “natural variation” existed but was within a much narrower range than today.

4. Feedback loops

A positive feedback amplifies the initial change; a negative feedback dampens it.

Ice-albedo (positive): ice reflects sunlight; as ice melts, darker ocean/land absorbs more heat, warming further, melting more ice.
Permafrost thaw (positive): frozen soil stores methane and CO $_2$ . Warming thaws it, releasing gas, warming further.
Water vapour (positive): warmer air holds more water vapour, itself a GHG.
Cloud effects: complicated — can be positive or negative depending on cloud type.
CO $_2$ uptake by plants/oceans (negative): higher CO $_2$ can boost plant growth slightly and push more CO $_2$ into oceans, partly damping the rise — but not fast enough to offset emissions.

Ice-albedo positive feedback. Warming melts ice, exposing darker surfaces that absorb more sunlight and accelerate warming.

5. Responses: mitigation vs adaptation

Mitigation: reducing the cause — cutting GHG emissions.

Switching from coal/gas to renewables (solar, wind, hydro).
Efficiency improvements, electrification of transport.
Protecting and restoring forests; low-methane agriculture.
Carbon capture and storage (where viable).

Adaptation: adjusting to changes already happening.

Sea walls, flood defences, updated building codes.
Drought-tolerant crops, changed planting dates.
Upgraded cooling, early-warning systems for heatwaves.
Managed retreat from at-risk coastlines.

Both are needed: emissions already in the atmosphere will cause some warming regardless, and the choices made this decade largely determine how much more.

Practice: Year 9

Fluency

Greenhouse basics

Describe the natural greenhouse effect in four steps.
Name four greenhouse gases and one human source for each.
What is the difference between the natural and enhanced greenhouse effects?
State four independent lines of evidence for climate change.
Explain what “ppm” means and give current atmospheric CO $_2$ in ppm.
Describe the ice-albedo feedback loop in one sentence.

Reasoning

Apply the ideas

Why would Earth be about $-18^{\circ}\text{C}$ without any greenhouse effect?
Explain why water vapour is a greenhouse gas but is not the main focus of mitigation.
A politician argues that because CO $_2$ is only 0.04% of the atmosphere, it cannot affect climate. Evaluate this argument.
Describe how ice cores provide evidence of past atmospheric composition.
Explain the difference between mitigation and adaptation, giving one example of each.

Problem solving

Data and responses

Sea level is rising about 3.5 mm/year. Estimate the rise over 100 years and discuss implications for a coastal suburb with an average elevation of 1 m.
In 1960, atmospheric CO $_2$ was about 317 ppm. Today it is about 420 ppm. Calculate the percentage increase over this period.
A student installs rooftop solar. Categorise this as mitigation or adaptation and explain.
Rank these actions by likely CO $_2$ impact and justify: (a) switching to LED lights, (b) flying less, (c) eating less red meat, (d) buying an electric vehicle powered by renewable electricity.

Challenge

Reasoning

Harder reasoning

Describe the permafrost-methane feedback loop and explain why scientists are concerned it could accelerate warming beyond current projections.
Ocean acidification results from extra CO $_2$ dissolving in seawater to form carbonic acid. Explain why this is a problem for shell-forming organisms and why it is called the “other CO $_2$ problem”.
Some argue that natural climate variation (volcanoes, solar cycles, Milankovitch cycles) can explain current warming. Briefly outline why these cannot account for the post-1970 trend.
Evaluate the claim: “Australia should adapt rather than mitigate because it is a small emitter.” Discuss at least two counter-arguments.

Answers

Answer key

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Year 9 answers

Fluency

Greenhouse basics

(i) Sunlight reaches Earth’s surface. (ii) The warm surface emits infrared radiation. (iii) Greenhouse gas molecules absorb some of this infrared. (iv) They re-emit it in all directions, including back down, warming the lower atmosphere and surface.
E.g. CO $_2$ - burning fossil fuels; CH $_4$ - livestock, gas leaks, rice paddies; N $_2$ O - fertiliser use; water vapour - evaporation; CFC/HFC - refrigeration/industrial.
The natural effect keeps Earth habitable ( $\sim 15^{\circ}\text{C}$ average). The enhanced effect is the extra warming caused by human-added greenhouse gases raising their atmospheric concentrations.
Any four of: rising global temperature; ice core CO $_2$ and temperature records; shrinking glaciers and sea ice; rising sea level; ocean acidification; shifts in species ranges and flowering times.
“ppm” means parts per million, so 420 ppm means 420 molecules of CO $_2$ in every million air molecules. Current level about 420 ppm.
Warming melts ice/snow; darker exposed ocean or land absorbs more sunlight, causing further warming (positive feedback).

Reasoning

Apply the ideas

Without greenhouse gases, more of the infrared Earth emits would escape directly to space. The balance between absorbed solar energy and emitted infrared would then sit at a much colder surface temperature (around $-18^{\circ}\text{C}$ ).
Water vapour concentration is controlled by temperature (warmer air holds more); it cannot be directly managed. It acts as a feedback, amplifying warming triggered by CO $_2$ and other long-lived gases, which we can manage.
Weak argument. Small concentrations can still absorb strongly at specific infrared wavelengths. CO $_2$ ’s greenhouse effect is well measured in lab and field; doubling CO $_2$ roughly doubles the added radiative forcing.
Snowfall traps tiny air bubbles as it is buried and compressed into ice. Drilling cores and analysing the bubbles reveals CO $_2$ , methane, and isotope records going back hundreds of thousands of years.
Mitigation reduces the cause (e.g. switch to renewables). Adaptation adjusts to the impacts (e.g. build sea defences).

Problem solving

Data and responses

$3.5 \times 100 = 350$ mm $= 0.35$ m over 100 years. For a suburb at 1 m, a third of that elevation is lost, and storm surges plus high tides would already flood much of the area — plus feedback from rapid ice-sheet loss could make the rise larger.
Rise: $420 - 317 = 103$ ppm. Percentage: $\dfrac{103}{317} \times 100 \approx 32.5\%$ .
Mitigation — solar electricity replaces fossil-fuel generation, cutting CO $_2$ emissions.
Typical ranking (large to small): EV on renewables and flying less tend to be large; reducing red meat is sizable (methane, land use); LED lights give meaningful but smaller savings. Exact order depends on household baseline, but flying and vehicle fuel usually dominate.

Reasoning

Challenge

Arctic permafrost contains huge quantities of organic carbon. As air warms, permafrost thaws; microbes decompose the organic matter, releasing CO $_2$ and methane. These gases cause further warming and more thawing — a positive feedback that could lock in warming even if human emissions were stopped.
Dissolved CO $_2$ + water -> carbonic acid, which lowers ocean pH and reduces carbonate ions needed for shells and coral skeletons. Shell-forming organisms (plankton, molluscs, corals) are weakened, threatening food webs. Called the “other CO $_2$ problem” because acidification is distinct from, but caused by the same CO $_2$ that drives, climate warming.
Volcanoes cause short-term cooling from sulfate aerosols, not warming. Solar output has been roughly flat since 1970. Milankovitch cycles act on tens of thousands of years, not decades. None matches the timing or magnitude of the post-1970 rise, while CO $_2$ forcing does.
Counter-arguments: (i) Australia has high per-capita emissions, so “small” is misleading. (ii) Climate change is a collective action problem — if every country said “we are small”, global emissions never fall. (iii) Australia is particularly exposed to climate impacts (droughts, bushfires, Great Barrier Reef), so mitigation protects self-interest. (iv) Leadership and export economics — clean-tech and renewables trade will favour early movers.

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