Lesson 6 of 6 · Feedbacks, forcing & evidence
When a change changes its own cause
7 minute lesson · 3 checks along the way
Your goal for this lessonTrace a closed cause-and-effect loop and distinguish an amplifying feedback, an opposing response, and an initial forcing.
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A chain becomes a loop
Earth’s systems exchange energy and material, so a change in one can affect another. A occurs when the resulting changes feed back onto the original change. An amplifying, or positive, feedback strengthens it; an opposing, or negative, feedback reduces it. Positive and negative describe the direction of the response, not whether people consider the outcome desirable. A one-way chain of effects is not yet a feedback loop.
In a specified climate comparison, a is an imposed influence that alters Earth’s energy balance, such as increased atmospheric carbon dioxide from human emissions. Temperature and other conditions then respond, potentially activating feedbacks. The distinction depends on the question and model boundary. Naming a feedback does not, by itself, identify what started the change or determine how large the eventual response will be.
Check 1 of 3
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Follow sunlight over ice and water
is the fraction of incoming sunlight reflected. Snow-covered ice generally reflects more sunlight than open ocean. If warming reduces that bright cover while sunlight is available, more energy can be absorbed by the newly exposed darker surface. That can support further warming and additional ice loss. The causal path returns to the temperature change, making it an amplifying ice–albedo feedback.
The same relationship can amplify cooling when increasing bright cover reduces absorbed sunlight. It does not require all ice to disappear, and its strength changes with sunlight, surface conditions, and available ice. A black-and-white photograph can suggest contrasting surfaces, but exposure settings and viewing angle prevent its pixel brightness from serving as a calibrated albedo measurement. A ship-cut channel also cannot establish a climate trend.
USS Glacier breaking ice in Kainan Bay, Antarctica, during Operation Deep Freeze I in 1956. This historical black-and-white aerial photograph shows mechanically opened water among ice. It is not evidence that warming caused the opening, a calibrated albedo map, or a record of a long-term ice trend.
Look closely
Bright ice and dark openings are visible. Why should an explanation of climate feedback avoid treating this single image as proof of warming-driven ice loss?
See what to notice
The documented opening was made by an icebreaker, and the image provides neither a calibrated sunlight-reflection measurement nor a climate time series. It can motivate a question about different surfaces. Testing an ice–albedo feedback needs measurements of cover, sunlight, reflected energy, and temperature with their timing and other influences considered.
Work through a decision
Test whether the arrows close
Consider an invented diagram: temperature rises, snow cover decreases, reflected sunlight decreases, absorbed sunlight increases, temperature rises further. Trace each arrow aloud. It closes a loop and reinforces the initial warming. Now remove the final link: snow loss changes a transport route. That may be an important consequence, but without a demonstrated path back to the original change, this shorter chain does not establish a feedback.
To test the ice loop in nature, compare dated measurements of cover, incoming sunlight, reflected energy, and temperature over suitable regions and intervals. Look for competing influences, including clouds and ocean heat transport. An accurate observation of one arrow cannot prove the whole chain. A useful model must specify which processes it includes and be checked against more than the pattern it was designed to reproduce.
Check 2 of 3
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Several responses act together
As Earth warms, increased thermal radiation to space provides a fundamental opposing response. Other processes can amplify warming, including increased atmospheric water vapor and loss of reflective snow and ice. The net result depends on their combined strengths and timescales. An opposing response does not guarantee a return to the old temperature while a forcing persists, and an amplifying feedback does not automatically mean unlimited runaway change.
Bring the course together by tracing a change through at least three systems: altered rainfall, soil-water storage, and plant growth, for example. State which arrows are plausible mechanisms and which need measurements. Add a return arrow only if you can explain it. Earth becomes understandable through these testable connections, from rock and water budgets to and radiation, without assuming that every part always restores a fixed balance.
Check 3 of 3
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Put it together
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