Table 1 Key radiative and non-radiative feedbacks in polar regions that are related to the atmosphere, ocean, sea ice, ice sheets and land surfaces and can be measured using a feedback factor
Name | Description | Measure | Reference(s) | |
|---|---|---|---|---|
Radiative feedbacks | Planck (−) | Higher surface and atmospheric temperatures increase outgoing longwave radiation, avoiding runaway warming | Change of TOA flux due to temperature change at constant lapse rate divided by surface temperature change | |
Lapse rate (+ in Arctic, close to 0 in Antarctic) | In a warmer world and at high latitudes, stable stratification conditions in the lower troposphere result in a larger warming of the lower than of the upper troposphere, leading to a smaller increase in outgoing longwave radiation compared to vertically uniform warming, and thus to further warming | Change of TOA flux due to lapse rate changes divided by surface temperature change (normalized by Planck feedback) | ||
Surface albedo (+) | Melting ice and snow lowers surface albedo, leading to increased absorption of shortwave radiation and amplified warming | Change of TOA flux due to surface albedo change divided by surface temperature change (normalized by Planck feedback) | ||
Water vapor (+) | In a warming climate, the amount of water vapor in the atmosphere increases, which amplifies the greenhouse effect and leads to further warming | Change of TOA flux due to water vapor change divided by surface temperature change (normalized by Planck feedback) | ||
Cloud (+/−) Two examples are provided below | Warming of the atmosphere leads to changes in the amount and characteristics of clouds, modifying the radiative balance. The cloud contribution can be decomposed in several ways, two examples being given below | Change of TOA flux due to changes in cloud properties divided by surface temperature change (normalized by Planck feedback) | ||
Example 1: Cloud-sea ice (+ in non-summer months, close to 0 in summer) | Decreased sea ice extent in non-summer months results in greater cloud cover and increased downwelling longwave radiation, leading to further sea ice loss | Change of TOA flux due to changes in cloud amount and opacity resulting from varying sea ice concentration divided by surface temperature change | ||
Example 2: Cloud optical depth (−) | As the climate warms, the fraction of liquid water in mixed-phase clouds increases, resulting in higher cloud albedo, more reflection of shortwave radiation and reduced warming | Change of TOA flux due to changes in cloud optical depth divided by surface temperature change | ||
Non-radiative feedbacks | Ice production–entrainment (−) (mostly active in Southern Ocean) | Brine rejection during sea ice formation induces an ocean mixed layer deepening that brings to the surface warmer water from deeper levels, melting a part of the ice initially formed and inhibiting further ice production. | Ratio of the sea ice melt due to the entrainment of warmer water in the mixed layer to the initial ice formation | |
Ice production–ocean heat storage (+) (mostly active in Southern Ocean) | Anomalous sea ice production induces vertical exchanges of salt, a higher stratification, storage of heat at depth and finally lower oceanic heat fluxes that favor further ice production. | Ratio of the latent heat associated to ice production to the heat content change of the ocean subsurface layer | ||
Ice growth–thickness (−) | Thin sea ice grows more rapidly than thick sea ice due to its higher heat conduction, dampening the response to an initial decrease imposed by a perturbation. | Normalized difference in the thickness response to an energetic perturbation with and without thickness dependence of the ice growth rate | ||
Surface mass balance–elevation (+) (mostly active in Greenland Ice Sheet) | Increased air temperature leads to ice melting, which lowers the surface elevation of the ice sheet, hence leading to ice exposure to warmer air temperatures and further ice melting. | Ratio of the additional sea level contribution due to this feedback to the sea level contribution without feedback | ||
Ice shelf melting sea ice (−) (mostly active in Southern Ocean) | Ocean warming leads to ice shelf melting, which releases freshwater into the ocean and reduces vertical mixing. This results in sea ice expansion and reduced ocean warming. | Ratio of the additional change in sea ice extent caused by this feedback to the total change in extent without feedback | ||
Marine ice sheet instability (+) (mostly active in West Antarctic Ice Sheet) | An initial retreat in the grounding line position of a marine ice sheet on an upward-sloping bed towards the ocean leads to increased ice discharge, ice thinning and further retreat. | Ratio of the additional sea level contribution due to this feedback to the sea level contribution without feedback |
