Lapse rate feedback

Lapse rate feedback is a climate feedback mechanism in which the vertical temperature gradient of the atmosphere — the rate at which temperature decreases with altitude — changes in response to surface warming, altering the efficiency of outgoing longwave radiation and thereby amplifying or damping the initial warming. It is one of the three primary feedbacks driving Arctic amplification, though its sign and magnitude differ dramatically between the tropics and the polar regions.

The physics depends on atmospheric stability. In the tropics, warming increases upper-tropospheric temperature more than surface temperature (following moist adiabatic adjustment), steepening the lapse rate. A steeper lapse rate means the effective emission level of longwave radiation rises to colder temperatures, increasing the efficiency of radiative cooling. This is a negative feedback in the tropics, and it is one of the strongest stabilizing mechanisms in the climate system.

In the Arctic, the dynamics reverse. The Arctic atmosphere is frequently stable, with temperature inversions common in winter. The initial lapse rate is shallow. As the surface warms, the temperature increase is concentrated near the surface rather than distributed vertically. The effective emission level does not rise significantly, and the efficiency of longwave cooling increases only weakly. The result is that surface warming is trapped: the Arctic atmosphere cannot radiate away the additional heat as efficiently as the tropics. This makes lapse rate feedback a positive feedback in the Arctic, contributing roughly 20–30% of total Arctic amplification.

The asymmetry between tropical and polar lapse rate feedback is one of the most robust features of climate models and one of the most underappreciated. It explains why the tropics warm less than the global average despite receiving the most solar insolation, and why the Arctic warms fastest despite receiving the least. The lapse rate is not a passive background condition but an active determinant of regional climate sensitivity.

The feedback is sensitive to atmospheric moisture. In moist atmospheres, latent heat release from condensation dominates the vertical temperature profile, constraining the lapse rate toward moist adiabatic. In dry Arctic winter conditions, radiative processes dominate, and the lapse rate is more variable. This means lapse rate feedback is not a fixed parameter but a state-dependent function of humidity, temperature, and season.

_Lapse rate feedback is typically taught as a side note in discussions of Arctic amplification, with ice-albedo feedback receiving the majority of attention. This is a mistake. The two feedbacks are comparable in magnitude, and their interaction is critical: ice loss increases local humidity, which alters the lapse rate, which changes the vertical distribution of warming, which affects ice melt. Treating ice-albedo as the star and lapse rate as the understudy has produced a distorted picture of Arctic climate dynamics._