The greenhouse effect: absorption, emission and energy balance
Separate reflected sunlight from thermal infrared absorption and explain why increased greenhouse gases slow energy loss.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish light energy, thermal infrared radiation and temperature.
- Recall the Core CO₂/methane climate effects and conservation of energy.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the warmed Earth emit in the required explanation?
What you will learn
- Explain the greenhouse roles of carbon dioxide and methane using absorption and emission of thermal energy.
- Distinguish reflection in the wider energy budget from greenhouse-gas absorption/emission and connect reduced outward loss with warming.
Follow incoming and outgoing energy separately
Some incoming sunlight is reflected by clouds, ice and other bright surfaces, while some is absorbed and warms the surface/atmosphere. The warmed Earth emits thermal infrared radiation. Carbon dioxide and methane absorb some outgoing infrared and the atmosphere emits infrared in different directions, including towards the surface and towards space.
Reflection and absorption/emission are distinct processes. Do not describe CO₂ as a perfect mirror that reflects all incoming sunlight, or as a source creating energy from nothing. The greenhouse explanation concerns absorption and emission of thermal energy, reducing the rate of energy loss to space under the changed conditions.
A warmer balance differs from a sealed energy trap
Increasing greenhouse-gas concentration can reduce the outgoing thermal energy loss at the same starting temperature. If absorbed incoming energy exceeds outgoing energy, the system gains energy and warms. A warmer system emits more energy, moving towards a new balance; energy is not trapped forever with no escape to space.
A supplied energy-flow calculation compares rates, not temperature values. An imbalance of ten arbitrary units per time does not itself mean a ten-degree temperature rise; temperature change also depends on time and the system's response. Keep the simplified model and the actual climate system separate, while explaining the required chain from greenhouse-gas increase to reduced outward loss and warming.
Sources for this lesson
Pause and explain
Which distinction is correct?
Worked example
In a hypothetical energy-rate model, absorbed incoming energy is 100 units/time and outgoing energy falls from 100 to 90 at the same starting temperature. Find the initial net gain and the direction of response.
Show the worked solution
- Initially 100−100=0 net gain in the given balance.
- After the change, 100−90=10 units/time are gained.
- The positive gain supports warming until the system responds; it does not give an exact temperature rise by itself.
Answer Initial net gain after the change: 10 energy units/time; warming response, with no exact temperature rise inferred.
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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.
Explain the energy route from sunlight to the greenhouse effect, including reflection in its appropriate role.
Give me a hint
Separate incoming sunlight, warmed-surface emission and atmospheric infrared interactions.
Compare my reasoning
- Some sunlight is reflected, while some is absorbed and warms the system.
- The warmed Earth emits thermal infrared, some of which CO₂ and methane absorb.
- The atmosphere emits infrared in different directions; increased greenhouse gases can reduce outward loss at the starting temperature.
Incoming sunlight can be reflected or absorbed; outgoing thermal infrared is absorbed/emitted by greenhouse gases, reducing outward energy loss under the changed conditions.
Look for these in your work
- Reflection is not substituted for all greenhouse absorption.
- Energy directions and warming link are explained.
A hypothetical absorbed incoming rate is 240 units/time and outgoing rate is 230 at a stated initial temperature. Find net gain and explain why it is not a predicted 10°C rise.
Give me a hint
Subtract rates; temperature response needs additional information.
Compare my reasoning
- Net gain is 240−230=10 energy units/time.
- Positive net gain means the system accumulates energy in the stated starting comparison.
- The rate units are not °C, so time, heat storage and system response are needed for a temperature prediction.
10 energy units/time gained; this supports warming but does not specify a 10°C rise.
Look for these in your work
- Units and subtraction are correct.
- Energy imbalance is separated from a numerical temperature forecast.
A diagram shows every outgoing infrared arrow returning permanently to Earth and labels CO₂ a creator of new energy. Correct both features.
Give me a hint
Atmospheric emission has multiple directions and energy remains conserved.
Compare my reasoning
- Greenhouse gases absorb infrared and the atmosphere emits in different directions, with energy still able to reach space.
- A warming imbalance is a difference between absorbed incoming and outgoing rates, not creation of energy.
- Redraw outward as well as downward emission and state that a warmer balance can follow.
Include emission towards space and the surface; warming follows changed energy loss, not energy creation or permanent total trapping.
Look for these in your work
- The direction error is corrected.
- The explanation conserves energy and retains possible outward loss.
Common mistakes
- Calling greenhouse gases mirrors reflecting all sunlight.
- Treating absorption and reflection as identical.
- Claiming no thermal energy ever escapes to space.
- Converting an energy-rate difference directly into the same number of degrees.
What can you explain now?
How do increased greenhouse gases contribute to warming in the stated model?
Compare with the explanation
They absorb outgoing thermal infrared and emit it in different directions, reducing energy loss to space at the starting temperature.
An imbalance with more energy absorbed than lost supports warming rather than creation of new energy.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch separate sunlight and thermal-energy arrows, explain CO₂/methane absorption and emission, and calculate a supplied initial energy imbalance without inventing degrees.
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.