Positive Feedback in IB ESS: Water Vapour and Ice–Albedo

25 Jul Positive Feedback in IB ESS: Water Vapour and Ice–Albedo

Positive feedback amplifies an initial change in a system. “Positive” describes the direction of feedback, not whether an outcome is desirable. Negative feedback counteracts change. Both may operate within the same system.

Trace the return link

A one-way chain is not enough to establish feedback. Show how the effects return to influence the original variable: change in A → change in B → further change in A in the same direction.

Water-vapour feedback

Warming allows air to contain more water vapour before saturation. As evaporation supplies additional vapour, the atmosphere generally becomes moister. Water vapour absorbs outgoing infrared radiation, strengthening the greenhouse effect and amplifying the initial warming. A greater capacity to hold moisture alone is not the complete loop: explain both the moisture supply and the radiative effect.

For another example, identify each variable and causal link. A falling fish population or a pollution event does not by itself demonstrate a closed feedback loop.

Positive feedback can amplify a decrease

In a simplified ice–albedo loop, an initial cooling can expand snow and ice cover. More incoming sunlight is reflected, reducing absorbed solar energy and reinforcing cooling. The reverse loop can amplify warming as reflective cover shrinks. Positive feedback means reinforcement of the initial change, including a decrease; it does not simply mean that every quantity rises.

Does amplification continue forever?

Limits and competing processes can restrict amplification. Avoid assuming every positive feedback causes irreversible collapse. Explain the specific system and evidence for any threshold or state change.

Check whether your explanation shows a loop

• Name the variable that changes first.
• Explain the effect on a second variable.
• Show how that effect returns to amplify the first change.
• State what your simplified model leaves out.

If the last link does not return to the starting variable, you have described a chain of effects rather than demonstrated feedback.

Original practice task

Draw the water-vapour loop above, label the arrows and explain why it amplifies change. Identify a limitation of your simplified diagram. This is an original exercise, not an IB exam question or a marked answer.

Read systems modelling in ESS for related revision.

Primary source: NASA on water-vapour feedback.

Practise with individual ESS support

In Marija’s online IB ESS SL and HL lessons, you can work on concepts, data interpretation and clear explanations. Request a free 20-minute trial lesson to discuss your priorities. For IA work, the investigation and submitted writing remain your own.

Official reference: IB ESS guide, first assessment 2026. Updated 19 October 2026.