Explain Earth temperature through radiation balance
Distinguish absorbed sunlight, reflected sunlight and infrared exchange at the surface and planetary boundaries.
Section P2.3.3 · Radiation
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish absorption, reflection and emission.
- Use a complete received-minus-outgoing account.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In the supplied planetary model, 100 units arrive and 30 are reflected. How much is absorbed?
What you will learn
- Explain factors controlling incoming and outgoing radiation and Earth temperature.
- Distinguish the surface account from the Earth-plus-atmosphere boundary.
Name the boundary before balancing
Sunlight reaching Earth includes several wavelength regions. Some is reflected by clouds, atmosphere and bright surfaces; the rest can be absorbed. The Earth system emits thermal infrared toward space. At the Earth-plus-atmosphere boundary, compare absorbed solar input with outgoing thermal infrared; reflected sunlight was not absorbed and is not counted again as internal-energy gain.
At the surface boundary, absorbed sunlight and infrared from the atmosphere can provide input, while surface emission and other thermal routes provide output. Radiation exchanged between surface and atmosphere is internal to the larger system. Do not add that exchange as a second independent solar source in the planetary account. These distinctions follow the primary NASA radiation-budget explanations; no present-day global measurement is assigned by the schematic.
Explain factors and the initial response
A larger reflected fraction can reduce absorbed sunlight if incoming radiation is otherwise unchanged. Greenhouse gases such as water vapour and carbon dioxide absorb some surface-emitted infrared and emit infrared, including back toward the surface. They do not create energy or block all possible outgoing radiation. Changes can alter the outgoing rate at a given state and therefore the temperature needed for a new balance.
If the whole system initially absorbs more than it emits to space, it gains energy and can warm; the opposite imbalance can cool it. Warmer surfaces generally emit at greater rates, affecting the later balance. Clouds can change both reflection and infrared exchange, so one unspecified cloud change has no universal sign here. Numerical future climate, weather or global-temperature changes are not determined by this qualitative lesson.
Sources for this lesson
Pause and explain
Which greenhouse account preserves energy and the system boundary?
Worked example
In an original planetary teaching model, 100 units of solar energy arrive per interval, 30 are reflected and 68 units of thermal infrared leave to space. Find the net system gain.
Show the worked solution
- Absorbed solar input is 100 − 30 = 70 units.
- Outgoing thermal infrared is 68 units at the larger system boundary.
- Net gain is 70 − 68 = 2 units per supplied interval; reflected energy is not counted as absorbed.
Answer A 2-unit system gain per supplied interval, not a measured global climate value.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find net gain in the supplied model with solar 100 units, reflection 30 and outgoing thermal infrared 68.
Give me a hint
Use absorbed input at the same boundary as the outgoing value.
Compare my reasoning
- Absorbed input is 70 units.
- Outgoing thermal infrared is 68 units.
- Net gain is 2 units per specified interval.
2 units gained per supplied interval.
Look for these in your work
- Subtract reflection once.
- Use the same system boundary.
With the same 100-unit incoming amount, reflection rises from 30 to 35 while outgoing remains 68 initially. Find the new balance and qualitative initial tendency.
Give me a hint
Keep the stated outgoing value fixed only for the supplied initial comparison.
Compare my reasoning
- Absorbed input becomes 65 units.
- Net is 65 − 68 = −3 units per interval.
- The system initially loses energy; a cooling tendency is supported, not an exact temperature drop.
−3 units per interval; an initial cooling tendency.
Look for these in your work
- Use changed absorption.
- Do not extrapolate a complete climate forecast.
A diagram includes downward atmospheric infrared as a surface input. A student adds it again to solar input for Earth plus atmosphere. Explain the error.
Give me a hint
The surface and planetary boundaries include different transfers.
Compare my reasoning
- Downward infrared enters the surface account.
- It leaves another part of the same larger system.
- Adding it as independent planetary input double-counts an internal exchange.
The larger-boundary account must not count internal atmospheric exchange as a new external source.
Look for these in your work
- Name both boundaries.
- Preserve energy conservation.
Common mistakes
- Reflected sunlight is not absorbed input.
- Surface-atmosphere exchange is internal to the larger system.
- Greenhouse gases do not create energy or block every infrared path.
What can you explain now?
Is atmospheric infrared toward the surface a new independent energy source added to solar input for the whole Earth system?
Compare with the explanation
No.
It is an internal exchange at that larger boundary, though it is an input for the surface boundary.
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Make it stick
Explain albedo and atmospheric infrared while keeping absorbed solar input, internal exchange and outgoing planetary radiation distinct.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.