Since the early 2000s, climate models have consistently identified that global warming will gradually push jet streams poleward. In 2008, this was confirmed by observational evidence, which proved that from 1979 to 2001, the northern jet stream moved northward at an average rate of 2.01 kilometres (1.25 mi) per year, with a similar trend in the
Southern Hemisphere
jet stream.
[46]
[47]
Climate scientists have hypothesized that the jet stream will also gradually weaken as a result of
global warming
. Trends such as
Arctic sea ice decline
, reduced snow cover,
evapotranspiration
patterns, and other weather anomalies have caused the Arctic to heat up faster than other parts of the globe, in what is known as the Arctic amplification. In 2021-2022, it was found that since 1979, the warming within the
Arctic Circle
has been nearly four times faster than the global average,
[48]
[49]
and some hotspots in the
Barents Sea
area warmed up to seven times faster than the global average.
[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
.
The hypothesis above is closely associated with
Jennifer Francis
, who had first proposed it in a 2012 paper co-authored by Stephen J. Vavrus.
[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
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.
[54]
However, a 2012 review in the
Journal of the Atmospheric Sciences
noted that "there [has been] a significant change in the vortex mean state over the twenty-first century, resulting in a weaker, more disturbed vortex.",
[55]
which contradicted the modelling results but fit the Francis-Vavrus hypothesis. Additionally, a 2013 study noted that the then-current
CMIP5
tended to strongly underestimate winter blocking trends,
[56]
and other 2012 research had suggested a connection between declining Arctic sea ice and heavy snowfall during midlatitude winters.
[57]
In 2013, further research from Francis connected reductions in the Arctic sea ice to extreme summer weather in the northern mid-latitudes,
[58]
while other research from that year identified potential linkages between Arctic sea ice trends and more extreme rainfall in the European summer.
[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
.
[61]
[62]
In 2015, Francis' next study concluded that highly amplified jet-stream patterns are occurring more frequently in the past two decades. Hence, continued heat-trapping emissions favour increased formation of extreme events caused by prolonged weather conditions.
[63]
Studies published in 2017 and 2018 identified stalling patterns of Rossby waves in the northern hemisphere jet stream as the culprit behind other almost stationary extreme weather events, such as the
2018 European heatwave
, the
2003 European heat wave
,
2010 Russian heat wave
or the
2010 Pakistan floods
, and suggested that these patterns were all connected to Arctic amplification.
[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. As these gradients are the reason that cause west to east winds through the thermal wind relationship, declining speeds are usually found south of the areas with geopotential increases.
[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. 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."
[67]
In a 2017 study conducted by climatologist Dr. Judah Cohen and several of his research associates, Cohen wrote that "[the] shift in polar vortex states can account for
most
of the recent winter cooling trends over Eurasian midlatitudes".
[68]
A 2018 paper from Vavrus and others linked Arctic amplification to more persistent hot-dry extremes during the midlatitude summers, as well as the midlatitude winter continental cooling.
[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.
[70]
A 2021 study found that a stratospheric polar vortex disruption is linked with extreme cold winter weather across parts of Asia and North America, including the
February 2021 North American cold wave
.
[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
.
[73]
However, because the specific observations are considered short-term observations, there is considerable uncertainty in the conclusions.
Climatology
observations require several decades to definitively distinguish various forms of natural variability from climate trends.
[74]
This point was stressed by reviews in 2013
[75]
and in 2017.
[76]
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.
[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 2010s and published in 2020s now suggests that the intensification of Arctic amplification since the early 2010s was not linked to significant changes on midlatitude atmospheric patterns.
[79]
[80]
State-of-the-art modelling research of PAMIP (Polar Amplification Model Intercomparison Project) improved upon the 2010 findings of PMIP2 - it did find that sea ice decline would weaken the jet stream and increase the probability of atmospheric blocking, but the connection was very minor, and typically insignificant next to interannual variability.
[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.
[83]
Additionally, a 2021 study found that while jet streams had indeed slowly moved polewards since 1960 as was predicted by models, they did not weaken, in spite of a small increase in waviness.
[84]
A 2022 re-analysis of the aircraft observational data collected over 2002?2020 suggested that the North Atlantic jet stream had actually strengthened.
[85]
Finally, a 2021 study was able to reconstruct jet stream patterns over the past 1,250 years based on Greenland
ice cores
, and found that all of the recently observed changes remain within range of natural variability: the earliest likely time of divergence is in 2060, under the
Representative Concentration Pathway
8.5 which implies continually accelerating greenhouse gas emissions.
[86]