Science 7–10 · Year 10

Earth's energy balance: albedo, the greenhouse effect and surface temperature

Science understanding: Earth and space sciences (NSW Stage 5 focus area: Environmental sustainability)

Calculation and dataLow risk

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The idea

Earth's temperature settles where the sunlight it absorbs equals the infrared it gives off; an atmosphere that absorbs and re-emits part of that infrared warms the surface from the 255 K of a bare planet to the observed 288 K.

What you need

the NASA Earth Fact Sheet values: solar irradiance 1361.0 W/m^2, Bond albedo 0.294, average temperature 288 K
a calculator or spreadsheet
the class results of the albedo practical, for comparison

How to do it

Calculate the sunlight absorbed per square metre averaged over the whole globe: S (1 - albedo) / 4.
Calculate the temperature a bare planet needs to radiate that much (Stefan-Boltzmann law) and compare it with 288 K.
Add a one-layer atmosphere that absorbs a fraction epsilon of the surface infrared and find the epsilon that gives 288 K.
Keep that epsilon and change the albedo to 0.25 (less ice and cloud) and then 0.35; record the surface temperature each time.
Calculate the extra forcing from carbon dioxide, 5.35 ln(C / 278.3) W/m^2, for 420 ppm and for a doubling, and the equilibrium warming using the IPCC best estimate of 3 degrees C per doubling.
Compare the equilibrium warming for 420 ppm with the observed 1.09 degrees C (2011 to 2020 against 1850 to 1900) and explain the difference.

What you should see

Absorbed sunlight 240.2 W/m^2; bare-planet temperature 255.1 K (the Fact Sheet's listed black-body temperature, 254.0 K, is what an albedo of 0.306 gives). An absorptivity epsilon = 0.768 gives 288.0 K, the observed mean. With that atmosphere, albedo 0.25 gives 292.4 K and albedo 0.35 gives 282.1 K. Carbon dioxide at 420 ppm gives a forcing of 2.20 W/m^2 (3.71 W/m^2 for a doubling) and an equilibrium warming of 1.78 degrees C at 3 degrees C per doubling, more than the observed 1.09 degrees C because the ocean is still taking up heat and because other gases and aerosols, left out of this one-gas model, also act. The learner knows the model is right when epsilon = 0 returns 255.1 K.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The greenhouse effect is harmful in itself; without it the model surface would sit near 255 K, 33 K colder than today.
Greenhouse gases trap heat so none escapes; the energy still escapes to space, from higher and colder levels, so the surface warms until the balance is restored.
Albedo changes are too small to matter; lowering it from 0.294 to 0.25 warms the model surface by about 4 K.

Safety card

Low riskLearners carry it out

Hazards

No hazard is listed.

Controls

No control is listed.

Note

Screen-based model with no practical hazard; follow the school's guidelines for device use.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC5-ENV-01 SC5-WS-05 SC5-WS-06
Australian Curriculum v9AC9S10U04AC9S10I04AC9S10I05

Sources

The pages the author read to write this activity.

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