Which of the following is an example of negative feedback in the context of…

Concept — A climate feedback is a loop: an initial change in the climate system alters some process, and that process then acts back on the change that produced it. In a positive, reinforcing feedback the return effect pushes in the same direction and amplifies the initial change. In a negative, balancing feedback the return effect opposes the initial change and damps it, holding the system nearer its starting state. The sign of a feedback describes the direction of the loop, not whether its outcome is desirable.

Application — The initial change here is a rise in atmospheric carbon dioxide, which strengthens the greenhouse effect and warms the planet. The ocean's uptake of carbon dioxide is the process that acts back on that same driver, so the loop can be traced step by step.

1. Atmospheric carbon dioxide rises, trapping more of the outgoing infrared radiation and raising surface temperature.

2. As the partial pressure of carbon dioxide in the air rises, more of it dissolves across the sea surface into the ocean, which stores far more carbon than the atmosphere does.

3. Mixing carries the dissolved carbon downward, and marine plankton fix a further share into organic matter that sinks into deeper water, the biological pump.

4. Atmospheric carbon dioxide is drawn down, weakening the extra greenhouse trapping that started the sequence, so the return effect opposes its own cause and the loop is damping.

Cross-check — Contrast this with the other processes named in the question.

• Warming increases water vapour in air: a warmer atmosphere carries roughly seven per cent more water vapour per degree Celsius by the Clausius-Clapeyron relation, and water vapour is itself a greenhouse gas, so the return effect adds to the warming. That loop reinforces the change, making it a positive feedback.

• Snow cover loss and ice-shelf melt reduce the sun's reflection: bright snow and ice reflect much of the sunlight falling on them, and as they retreat the darker ocean and land exposed absorb more solar energy, which drives further melting. This ice-albedo loop also reinforces the change, so it too is a positive feedback.

• Global temperature increases due to global warming: this states an outcome rather than a mechanism, with no return path acting back on a driver, so it describes no feedback loop at all.

Result — Among the processes listed, the removal of carbon dioxide from the air by the oceans is the one that feeds back against the change that produced it, so it is the example of negative feedback.