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A climate feedback is a response to climate change that alters the initial warming or cooling. Positive feedback amplifies the change; negative feedback opposes it. Positive feedback alone does not prove an irreversible transition.

Loss of summer sea ice need not be irreversible at fixed external forcing. Open water can release excess heat in autumn, and thin ice grows faster than thick ice during winter because it offers less resistance to thermal conduction. Continued climate change can nevertheless shift the long-term ice cover downward. A one-summer perturbation and persistent forcing are different physical experiments.