Polar amplification

Polar amplification is the phenomenon that any change in the net radiation balance (for example greenhouse intensification) tends to produce a larger change in temperature near the poles than the planetary average. On a planet with an atmosphere that can restrict emission of longwave radiation to space (a greenhouse effect), surface temperatures will be warmer than a simple planetary equilibrium temperature calculation would predict. Where the atmosphere or an extensive ocean is able to transport heat polewards, the poles will be warmer and equatorial regions cooler than their local net radiation balances would predict.   Polar amplification is the phenomenon that any change in the net radiation balance (for example greenhouse intensification) tends to produce a larger change in temperature near the poles than the planetary average. On a planet with an atmosphere that can restrict emission of longwave radiation to space (a greenhouse effect), surface temperatures will be warmer than a simple planetary equilibrium temperature calculation would predict. Where the atmosphere or an extensive ocean is able to transport heat polewards, the poles will be warmer and equatorial regions cooler than their local net radiation balances would predict.   In the extreme, the planet Venus is thought to have experienced a very large increase in greenhouse effect over its lifetime, so much so that its poles have warmed sufficiently to render its surface temperature effectively isothermal (no difference between poles and equator). On Earth, water vapor and trace gasses provide a lesser greenhouse effect, and the atmosphere and extensive oceans provide efficient poleward heat transport. Both palaeoclimate changes and recent global warming changes have exhibited strong polar amplification, as described below. Arctic amplification is polar amplification of the Earth's North Pole only; Antarctic amplification is that of the South Pole. An observation based study related to Arctic amplification was published in 1969 by Mikhail Budyko, the study conclusion has been summarized as, 'Sea ice loss affects Arctic temperatures through the surface albedo feedback.' The same year a similar model was published by William D. Sellers. Both studies attracted significant attention since they hinted at the possibility for a runaway positive feedback within the global climate system. In 1975 Manabe and Wetherald published the first somewhat plausible global climate model that looked at the effects of an increase of greenhouse gas. Although confined to less than one-third of the globe, with a 'swamp' ocean and only land surface at high latitudes, it showed an Arctic warming faster than the tropics (as have all subsequent models). Feedbacks associated with sea ice and snow cover are widely cited as the main cause of recent terrestrial polar amplification. However, amplification is also observed in model worlds with no ice or snow. It appears to arise both from a (possibly transient) intensification of poleward heat transport and more directly from changes in the local net radiation balance (an overall decrease in outward radiation will produce a larger relative increase in net radiation near the poles than near the equator). Some examples of climate system feedbacks thought to contribute to recent polar amplification include the reduction of snow cover and sea ice, changes in atmospheric and ocean circulation, the presence of anthropogenic soot in the Arctic environment, and increases in cloud cover and water vapor. Most studies connect sea ice changes to polar amplification. Some models of modern climate exhibit Arctic amplification without changes in snow and ice cover. The individual processes contributing to polar warming are critical to understanding climate sensitivity. It has been estimated that 70% of global wind energy is transferred to the ocean and takes place within the Antarctic Circumpolar Current (ACC). Eventually, upwelling due to wind-stress transports cold Antarctic waters through the Atlantic surface current, while warming them over the equator, and into the Arctic environment. Thus, warming in the Arctic depends on the efficiency of the global ocean transport and plays a role in the polar see-saw effect. Decreased oxygen and low-pH during La Niña are processes that correlate with decreased primary production and a more pronounced poleward flow of ocean currents. It has been proposed that the mechanism of increased Arctic surface air temperature anomalies during La Niña periods of ENSO may be attributed to the Tropically Excited Arctic Warming Mechanism (TEAM), when Rossby waves propagate more poleward, leading to wave dynamics and an increase in downward infrared radiation. Polar amplification is quantified in terms of a polar amplification factor, generally defined as the ratio of some change in a polar temperature to a corresponding change in a broader average temperature: