Polar amplification
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.
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).
[11] Feedbacks associated with sea ice and snow cover are widely cited as one of the principal causes of terrestrial polar amplification.
[17] 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.
[25] 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.
[30] It is observed that Arctic and Antarctic warming commonly proceed out of phase because of orbital forcing, resulting in the so-called polar see-saw effect.
[16] The Arctic was historically described as warming twice as fast as the global average,[34] but this estimate was based on older observations which missed the more recent acceleration.
[42] The first acceleration is attributed to the increase in anthropogenic radiative forcing in the region, which is in turn likely connected to the reductions in stratospheric sulfur aerosols pollution in Europe in the 1980s in order to combat acid rain.
[50][51] While the Arctic remains one of the coldest places on Earth today, the temperature gradient between it and the warmer parts of the globe will continue to diminish with every decade of global warming as the result of this amplification.
If this gradient has a strong influence on the jet stream, then it will eventually become weaker and more variable in its course, which would allow more cold air from the polar vortex to leak mid-latitudes and slow the progression of Rossby waves, leading to more persistent and more extreme weather.
[52] While some paleoclimate reconstructions have suggested that the polar vortex becomes more variable and causes more unstable weather during periods of warming back in 1997,[53] this was contradicted by climate modelling, with PMIP2 simulations finding in 2010 that the Arctic Oscillation (AO) was much weaker and more negative during the Last Glacial Maximum, and suggesting that warmer periods have stronger positive phase AO, and thus less frequent leaks of the polar vortex air.
[59] At the time, it was also suggested that this connection between Arctic amplification and jet stream patterns was involved in the formation of Hurricane Sandy[60] and played a role in the early 2014 North American cold wave.
[64][65] Further work from Francis and Vavrus that year suggested that amplified Arctic warming is observed as stronger in lower atmospheric areas because the expanding process of warmer air increases pressure levels which decreases poleward geopotential height gradients.
[66] In 2017, Francis explained her findings to the Scientific American: "A lot more water vapor is being transported northward by big swings in the jet stream.
[69] Another 2017 paper estimated that when the Arctic experiences anomalous warming, primary production in North America goes down by between 1% and 4% on average, with some states suffering up to 20% losses.
[71][72] Another 2021 study identified a connection between the Arctic sea ice loss and the increased size of wildfires in the Western United States.
[76] A study in 2014 concluded that Arctic amplification significantly decreased cold-season temperature variability over the northern hemisphere in recent decades.
[77] A 2019 analysis of a data set collected from 35 182 weather stations worldwide, including 9116 whose records go beyond 50 years, found a sharp decrease in northern midlatitude cold waves since the 1980s.
[78] Moreover, a range of long-term observational data collected during the 2010s and published in 2020 suggests that the intensification of Arctic amplification since the early 2010s was not linked to significant changes on mid-latitude atmospheric patterns.
[81][82] In 2022, a follow-up study found that while the PAMIP average had likely underestimated the weakening caused by sea ice decline by 1.2 to 3 times, even the corrected connection still amounts to only 10% of the jet stream's natural variability.
