HKDSE Geography｜Climate Change and the Energy Budget Lesson-Prep Guide (for tutors)｜Solar constant, albedo, greenhouse effect, sea level and the 1.5°C target

2026-11-03

Tutor lesson-prep guides · Secondary Geography

Students can chant that greenhouse gases cause warming, yet cannot say where the energy enters or leaves, and cannot explain why half a degree between 1.5°C and 2°C nearly wipes out coral reefs. This lesson uses three numbers (1361, 340, 238) and one budget line to turn the slogan into a chain of causes.

Objectives — Students can: (1) divide the solar constant of 1361 W/m² by 4 to get about 340 W/m² averaged over the surface, multiply by (1 minus 0.3) to get about 238 W/m² absorbed, and explain why the division by four is geometry, not convention; (2) explain that the natural greenhouse effect lifts the mean surface temperature from about -18°C to about 15°C, a gap of 33°C, and that the enhanced effect changes the outgoing side, not the incoming one; (3) use the 0.5°C between 1.5°C and 2°C against coral loss of roughly 80% versus more than 99% to argue that climate consequences are not linear.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Radiation and wavelength	Which has the shorter wavelength, solar or terrestrial radiation?	Solar, which is why it penetrates the atmosphere
Greenhouse gases	Name three greenhouse gases.	Carbon dioxide, methane, water vapour
Energy balance	What is the long-term energy balance of Earth?	Input equals output, otherwise temperature keeps changing
Sea level	What are the two main sources of sea-level rise?	Thermal expansion of sea water and melting land ice

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0–8 min	Income and spending	Budget diagram: incoming, reflected, outgoing longwave	Draw first, name later
8–18 min	Solar constant divided by four	1361 / 4 = 340 W/m²	Geometry, not a fudge
18–30 min	Albedo	340 x (1 – 0.3) = 238	Reflected energy does not heat
30–42 min	The natural {GAP}°C	-18°C to 15°C	Without it the planet freezes
42–52 min	Enhanced greenhouse	Outgoing side blocked, income unchanged	The sun did not change
52–60 min	1.5°C versus 2°C	0.5°C apart, coral 80% versus 99%	Exit check

03 Teaching script (read it out)

Scene 1: Make them calculate before you explain

Ask: the sun delivers 1361 W/m² to the disc Earth presents, so why is the surface average only 340? The answer is geometry: the disc is a circle, the surface is a sphere, and the sphere has 4 times the area, so the same energy is spread over 4 times as much surface. Students who skip this step treat the solar constant as ground-level sunshine, and every later inference about the greenhouse effect inherits the error.

Ask: why divide by 4 and not by two? (Earth is a sphere, not a disc)

Scene 2: Turn albedo into subtraction

Incoming 340, albedo 0.3: three tenths bounce off cloud, ice and sand, leaving 238 W/m² to do the heating. Then run the same line for a desert and an ice sheet: high albedo means low absorption, so once ice melts the albedo falls and absorption rises. That is the ice-albedo positive feedback, and subtraction explains it better than the phrase “vicious cycle”.

Ask: after a glacier melts, does albedo rise or fall? (It falls, so absorption rises)

Scene 3: Close with 33°C and half a degree

With no greenhouse gases the balanced surface temperature would be about -18°C; with them it is about 15°C, a difference of 33°C. So the greenhouse effect itself is necessary, and the problem is its enhancement. Finish with 1.5°C against 2°C: only 0.5°C apart, yet coral loss climbs from about 80% to more than 99% because ecosystems have thresholds. Students should leave able to say one sentence: climate consequences are not linear.

Ask: why does 0.5°C cause disproportionate harm? (Thresholds and positive feedback)

04 Question types and worked steps

Type	Example	Steps	Answer
Energy budget calculation	Solar constant 1361 W/m², albedo 0.30	Divide by 4, then multiply by (1 – albedo)	340 and 238 W/m²
Data description	A 1900-2020 global temperature anomaly graph	State trend, size and acceleration; use annual means not single years	About 1.1°C, accelerating recently
Cause chain	Explain how the enhanced greenhouse effect raises sea level	Two routes: thermal expansion and meltwater, then coastal impacts	Both routes needed for full marks
Comparison	1.5°C versus 2°C scenarios	Compare one indicator across two scenarios and name the non-linearity	Coral 80% versus over 99%

Sanity check (teach this): 1361 is power per square metre, not the planet total; dividing by 4 is geometry; 340 times 0.7 gives 238, since albedo 0.30 reflects three tenths; the 33°C gap is with versus without any greenhouse effect, not pre-industrial versus now; enhancement cuts outgoing longwave, it never adds incoming; sea-level rise always has both a thermal expansion and a meltwater route; 1.5 and 2 differ by 0.5°C in the global mean, while land warms faster; the coral figures are a range, so answers should say about and more than, never an exact value.

05 Independent practice (3 questions)

Question	Answer	What to watch
Calculate: albedo rises to 0.35, what is the absorbed value?	221 W/m²	340 x 0.65 = 221, which is 17 less than 238
Describe: what characterises a 1.1°C global rise?	Land warms more than ocean, high latitudes more than low	Description marks come from spatial detail
Explain: why is 2°C far more dangerous than 1.5°C?	Thresholds and positive feedback; coral moves from 80% to over 99%	The word non-linear must appear

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating the solar constant as surface energy	A student writes that Earth receives 1361 W/m²	Tutor line: “1361 hits one square metre of the disc. Spread it over the sphere and the ground gets 340.”
Assuming the greenhouse effect is wholly bad	A student writes that the greenhouse effect must be stopped	Tutor line: “Without it the mean is minus eighteen. The problem is the enhancement, not the existence.”
Reporting 33°C as warming since industrialisation	A student writes that the planet has warmed 33°C	Tutor line: “Thirty-three is with versus without greenhouse gases. Since industrialisation it is about 1.1. Keep the two apart.”
Naming only ice melt for sea level	A student answers with melting glaciers only	Tutor line: “Warm water expands. That route matters as much as meltwater, so write both.”

07 Homework (paper, section and question numbers)

Secondary Geography chapter index — Redraw the energy budget diagram and copy the three core numbers
S5 Geography: weather and climate — Use the lapse rate method there to explain why mountain glaciers melt faster
Secondary Biology chapter index — Pick an ecology chapter and write one paragraph on the biology of coral bleaching

08 Exit check (last 5 minutes)

Question	Answer	Decision
Why is the average 340 and not 1361?	The sphere has four times the disc area, so 1361 / 4 = 340	Answering 1361 means geometry is missing
What is the natural greenhouse gap?	About 33°C, from -18 to 15	Answering 1.1°C confuses it with recent warming
How far apart are 1.5°C and 2°C?	0.5°C, but the consequences are not linear	Answering “almost the same” misses thresholds

09 Tutor FAQ

Which year group?

S6 Geography, climate change and environmental issues; S5 can extend into it.

How many figures must be memorised?

Three: 1361, 340 and 238; the rest is derived.

Is there calculation practice?

Three items with answers, including a changed-albedo calculation.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.