A4 Amplified Feedback Anomalies

Topic

Climate feedbacks are mechanisms through which an initial disturbance to the climate system is either amplified or dampened: a positive feedback amplifies the initial disturbance, whereas a negative feedback dampens it. Under abnormal conditions, certain positive feedbacks are triggered in an extraordinary manner and become coupled with one another; consequently, one activated positive feedback reinforces another rather than acting in isolation. This coupling between positive feedbacks produces a non-linear amplification of the initial disturbance—an amplification that models assuming a proportional response fail to anticipate. Prominent examples of this non-linear amplification include Arctic amplification, marine heatwave events, and the feedback loop between dry soil and extreme temperatures.

Arctic amplification is the first of these extraordinarily triggered positive feedbacks; it occurs because the loss of Arctic sea ice reduces the region's surface albedo. A dark ocean surface replaces the white ice surface, and this dark ocean surface absorbs more solar radiation than the white ice previously did. Compounding this surface albedo feedback is the polar lapse-rate feedback: in the Arctic, warming is greater near the surface than at higher altitudes—contrary to what occurs in the tropics—and this difference further amplifies the Arctic's thermal response to the same initial disturbance. The combined effect of the surface albedo feedback and the polar lapse-rate feedback is that Arctic warming proceeds two to four times faster than the global average.

Marine heatwave events represent the second of these extraordinarily triggered positive feedbacks; these are periods of abnormally high surface ocean temperatures that can persist for weeks or months. Its intensity is amplified by a local feedback loop between sea surface temperature and cloud cover: warmer surface waters suppress the ocean's mixed layer and reduce stratiform cloud cover over those waters, while reduced stratiform cloud cover allows more incoming solar radiation to reach the sea surface. This increased solar radiation further raises the sea surface temperature, which in turn further suppresses the ocean's mixed layer and reduces stratiform cloud cover, closing a feedback loop that sustains and prolongs the marine heatwave event as long as the triggering conditions remain unchanged.

The feedback between dry soil and extreme temperature is the third of these extraordinarily activated positive feedbacks; it couples soil moisture with near-surface air temperature: dry soil has less water available for evaporation, thereby losing the evaporative cooling capacity of moist soil, and this loss of evaporative cooling drives near-surface air temperatures to extreme levels. Higher near-surface air temperatures, in turn, dry the soil out even further, meaning that dry soil and extreme temperatures reinforce each other in the same way that sea surface temperature and cloud cover do during marine heatwave events.

The three positive feedbacks described—Arctic amplification, marine heatwaves, and the feedback between dry soil and extreme temperature—share the defining characteristic of amplified climate feedbacks: in each case, the climate system's own response to a small initial perturbation becomes the cause of a larger perturbation, rather than dampening it. That is why these extraordinarily triggered positive feedbacks produce anomalies—an Arctic warming several times faster than the planet, self-sustaining episodes of extreme ocean temperatures, and soils drying and heating in a single cycle—that models assuming a response proportional to the initial disturbance do not anticipate, and which can only be explained by recognizing that climate feedbacks, under abnormal conditions, cease to behave as independent mechanisms and instead become coupled with one another.

ID:806