E5 Climate Feedbacks

Topic

Climate feedbacks are mechanisms that amplify or attenuate the climate system's response to an initial forcing. The standard framework for describing this response breaks it down into three components: the radiative forcing that perturbs the climate system, the total gain of the climate feedbacks responding to that perturbation, and the surface temperature response the climate system reaches once it returns to a new equilibrium. This equilibrium surface temperature response is equal to the radiative forcing divided by the total climate feedback gain, with the sign inverted: the greater the radiative forcing perturbing the climate system, the greater the equilibrium surface temperature response; conversely, the greater the magnitude of the total climate feedback gain—a total gain that must be negative for the climate system to be stable—the smaller the equilibrium surface temperature response to that same radiative forcing.

Within the total climate feedback gain, the Planck feedback is the fundamental stabilizing mechanism, and it is always negative: as surface temperature rises, the climate system emits more infrared radiation into space, and this increased infrared emission cools the climate system, counteracting the rise in surface temperature that caused it. Without this negative Planck feedback, none of the other climate feedbacks could, on their own, return the climate system to a new equilibrium following a radiative forcing, because the other climate feedbacks modify the strength of this negative Planck feedback rather than replacing it.

Three other climate feedbacks modify the strength of the negative Planck feedback. The water vapor feedback is positive: a warmer atmosphere retains more water vapor, and the additional retained water vapor is itself a greenhouse gas that amplifies warming rather than attenuating it. The ice-albedo feedback is also positive, for the same reason described regarding Arctic amplification: less ice means a darker surface, and a darker surface absorbs more solar radiation, thereby amplifying warming. The lapse-rate feedback, on the other hand, can be positive or negative depending on the region; it alters how temperature changes with altitude across different atmospheric layers, and this shift in the vertical temperature structure can either reinforce or weaken the negative Planck feedback, depending on the location on the planet where it is measured.

Clouds represent the greatest source of uncertainty among all climate feedbacks because they exert opposing effects depending on the type of radiation: clouds reflect solar radiation back into space—an effect that cools the climate system—but they also trap infrared radiation that would otherwise escape into space—an effect that warms the climate system. Which of these two opposing cloud effects dominates depends on the cloud type, its altitude, and the region of the planet where it forms; this dependency makes cloud feedback the most difficult of all climate feedbacks to quantify precisely.

Equilibrium climate sensitivity integrates all these climate feedbacks—the negative Planck feedback, water vapor feedback, ice-albedo feedback, lapse-rate feedback, and cloud feedback—into a single total climate feedback gain: equilibrium climate sensitivity is equal to the radiative forcing produced by a doubling of carbon dioxide concentration (approximately 3.7 watts per square meter) divided by that same total climate feedback gain, with the sign inverted. According to the most recent international scientific assessment of the climate, the most likely value for equilibrium climate sensitivity is three degrees Celsius—falling within a probable range of 2.5 to 4 degrees. That same radiative forcing from a doubling of carbon dioxide—which was used to begin defining the climate system's response in the first paragraph—is ultimately what equilibrium climate sensitivity translates into a concrete rise in surface temperature once all climate feedbacks have fully played out.

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