Explain the near time-invariance of the land–ocean amplification factor

Determine why the land–ocean amplification factor remains nearly time-invariant, or decreases only modestly, during historical and future scenario experiments and in observations of recent climate, despite evolving substantially in abrupt-4×CO₂ experiments.

Background

The paper analyzes the land–ocean amplification factor φ, defined as the ratio of mean land to ocean surface-air-temperature anomalies. Although φ changes substantially during abrupt-4×CO₂ experiments because land and ocean have different heat capacities, observations and historical and future scenario simulations show that it is nearly constant or declines only slightly over time.

The authors review several proposed explanations, including the time-invariant structure of the leading temperature-change mode, changes in atmospheric heat transport, and compensation between ocean heat uptake and atmospheric energy transport. They then show that their emulator can reproduce weak temporal variation under idealized exponentially increasing forcing through the decay of intrinsic ocean-adjustment modes relative to the forced response, but the quoted passage explicitly identifies the broader explanation for the historical and scenario behavior as unresolved.