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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.
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:
where is a change in polar temperature and is, for example, a corresponding change in a global mean temperature.
Common implementations define the temperature changes directly as the anomalies in surface air temperature relative to a recent reference interval (typically 30 years). Others have used the ratio of the variances of surface air temperature over an extended interval.
The glacial / interglacial cycles of the Pleistocene provide extensive palaeoclimate evidence of polar amplification, both from the Arctic and the Antarctic. In particular, the temperature rise since the last glacial maximum 20,000 years ago provides a clear picture. Proxy temperature records from the Arctic (Greenland) and from the Antarctic indicate polar amplification factors on the order of 2.0.
Suggested mechanisms leading to the observed Arctic amplification include Arctic sea ice decline (open water reflects less sunlight than sea ice), and atmospheric heat transport from the equator to the Arctic.
Jennifer Francis told Scientific American in 2017, "A lot more water vapor is being transported northward by big swings in the jet stream. That’s important because water vapor is a greenhouse gas just like carbon dioxide and methane. It traps heat in the atmosphere. That vapor also condenses as droplets we know as clouds, which themselves trap more heat. The vapor is a big part of the amplification story—a big reason the Arctic is warming faster than anywhere else."
Studies have linked rapidly warming Arctic temperatures, and thus a vanishing cryosphere, to extreme weather in mid-latitudes. In particular, one hypothesis links polar amplification to extreme weather by changing the polar jet stream. However, a 2013 study noted that extreme events in particular associated with sea ice and snow cover decline have not yet been observed for long enough to distinguish natural climate variability from impacts related to recent climate change.
Studies published in 2017 and 2018 identified stalling patterns of rossby waves, in the northern hemisphere jet stream, to have caused almost stationary extreme weather events, such as the 2018 European heatwave, the 2003 European heat wave, 2010 Russian heat wave, 2010 Pakistan floods - these events have been linked to global warming, the rapid heating of the Arctic.
According to a 2009 study the Atlantic Multi-decadal Oscillation (AMO) is highly correlated with changes in Arctic temperature, suggesting that the Atlantic Ocean thermohaline circulation is linked to temperature variability in the Arctic on a multi-decadal time scale. A study in 2014 concluded that Arctic amplification significantly decreased cold-season temperature variability over the Northern Hemisphere in recent decades. Cold Arctic air intrudes into the warmer lower latitudes more rapidly today during autumn and winter, a trend projected to continue in the future except during summer, thus calling into question whether winters will bring more cold extremes. According to a 2015 study, based on computer modelling of aerosols in the atmosphere, up to 0.5 degrees Celsius of the warming observed in the Arctic between 1980 and 2005 is due to aerosol reductions in Europe.
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