Changes in incoming energy can start a climate response. Growing reflective ice can amplify cooling; melting ice can amplify warming.
Solar output is the energy emitted by the Sun. Changes in output alter the incoming energy available to Earth. Solar activity varies, including over an approximately 11-year cycle, but its effect must be judged by the measured size and direction of the energy change.
Orbital changes are different: they redistribute sunlight by latitude and season rather than changing the Sun's output. Variations in eccentricity, axial tilt and precession helped pace Quaternary glacial cycles over tens of thousands of years. Cooler northern summers can allow more winter snow to survive and ice sheets to grow.
Albedo is the fraction of incoming sunlight reflected by a surface. Expanding snow and ice raise albedo, so less solar energy is absorbed. Cooling then favours further ice growth. This is a positive feedback because it amplifies the initial cooling. In reverse, warming melts reflective ice, darker surfaces absorb more energy, and warming is reinforced.
Feedback describes a loop back to the initial change. Positive does not mean beneficial, and it need not mean warmer: an amplifying cooling loop is also positive. A negative feedback opposes the initial change, as a warmer Earth emits more longwave radiation to space.
Natural mechanisms matter for past variability, but timescale and observations constrain their role today. The slow orbital cycle and measured solar changes cannot explain the rapid contemporary warming pattern. Their existence is not evidence that human greenhouse gases have little effect.
Step by step
Identify the initial change
Separate altered solar output from orbital redistribution.
Follow the surface response
Cooling grows ice; warming reduces it.
Close the feedback loop
Changed albedo alters absorption and reinforces the original temperature change.
Worked example: An amplifying cooling loop
In an illustrative sequence, reduced summer melting leaves more snow. Higher albedo lowers absorbed sunlight, keeping summers cooler and helping more snow survive. Every link reinforces cooling, so the loop is positive even though temperature falls.
Watch out for this
Positive feedback always increases temperature.
It strengthens the initial change. It can accelerate cooling or warming.
Check your understanding
Which sequence is a positive ice-albedo feedback?
- Warming melts ice, absorption rises, and warming increases.
- Warming increases emitted longwave radiation and reduces the imbalance.
- One cold afternoon is followed by a warm afternoon.