A mechanism answer explains how one change leads to another, step by step. For climate, the key mechanism is the enhanced greenhouse effect, and the marks go to a chain with no missing links.
This lesson follows reading datasets and prepares you to judge responses in comparing response mechanisms.
What does the chain look like?
- Shortwave radiation from the Sun passes through the atmosphere and is absorbed by the surface.
- The warmed surface emits longwave (infrared) radiation back upwards.
- Greenhouse gases absorb some of this radiation, such as carbon dioxide, methane and water vapour.
- They re-emit it in all directions, so part of it returns towards the surface.
- The surface and lower atmosphere stay warmer than they would without these gases.
- More gases mean more returned radiation, so the surface warms further until balance is restored.
The word “enhanced” refers to step 6: human activity has increased the concentration of some of these gases.
Worked example: a simple balance
This uses invented units to show the logic. It is a teaching model, not measured data.
Suppose the surface receives 60 units from the Sun and 40 units returned by the atmosphere. Total received: 60 + 40 = 100 units. At balance, the surface emits 100 units.
Now suppose more greenhouse gas raises the returned amount to 45 units.
Step 1: received = 60 + 45 = 105 units.
Step 2: the surface is receiving more than it emits (100), so it warms.
Step 3: a warmer surface emits more, until emission reaches 105 units and balance returns, at a higher temperature.
Written answer: “More greenhouse gas returns more infrared radiation to the surface. The surface then receives more energy than it emits, so it warms until its emission rises to match, and a new balance is reached at a higher temperature.”
The mistake to watch for
Mistaken answer: “Greenhouse gases make a hole in the ozone layer, which lets in more heat from the Sun.”
This confuses two processes and gets the direction wrong, because the effect is about heat held near the surface, not extra sunlight.
The correction is to name the radiation: shortwave in, longwave out, and gases absorbing and re-emitting the longwave. Keep the ozone layer for a separate answer about ultraviolet radiation.
Check yourself
1. Put in order: (a) gases re-emit radiation, (b) surface emits infrared, (c) Sun heats surface, (d) gases absorb infrared.
Show answer
c, b, d, a. The Sun heats the surface, the surface emits infrared, gases absorb it, then re-emit it.
2. In the balance model, the returned amount rises from 40 to 52 units with the Sun giving 60. What is the new total received?
Show answer
60 + 52 = 112 units, compared with 100 before. The surface warms until it emits 112.
3. Why is the greenhouse effect described as natural as well as enhanced?
Show answer
Greenhouse gases occur naturally, so some effect always exists and keeps the surface warmer than it would be otherwise. Human activity adds to the concentrations, which strengthens it. That added part is the enhanced effect.
Where this leads next
Next, see how people respond in comparing response mechanisms neutrally. The food-web and energy-flow explorer practises reading arrows as energy movement, a useful habit for any flow diagram.
Chain explanations are learnt fastest when someone interrupts you at each link. That is one thing a teacher in online one-to-one Geography tuition can do with your own written answers.