Every time severe winter weather hits the US or Europe, reporters like to say that global warming could be to blame. The paradox is as follows: as the arctic sea ice melts and the polar atmosphere warms, the swirling winds that restrict the cold arctic air weaken, causing it to continue flowing south. But this idea, popularized a decade ago, has long been received with skepticism by many atmospheric scientists who found the proposed link unconvincing and saw little evidence of it in simulations of the climate.
Now, the most comprehensive model investigation into this connection has delivered the worst blow to date: Even after the massive sea ice loss expected in the middle of the century, the polar jet stream will only weaken by tiny amounts – at most 10% of its natural fluctuations. And in today’s world, the impact of ice loss on winter weather is negligible, says James Screen, a climatologist at the University of Exeter and co-head of the study, who presented his findings at the annual meeting of the European Geosciences Union last month. “To say that the loss of sea ice had an impact in a particular extreme event, or even over the past 20 years, is a stretch.”
The idea that the loss of Arctic sea ice could affect mid-altitude winter weather continued in a 2012 article by two climate researchers, Jennifer Francis, now at the Woodwell Climate Research Center, and Stephen Vavrus of the University of Wisconsin, Madison, by. It started with a simple observation: the Arctic is warming almost three times faster than the rest of the world. At the time, sea ice loss was believed to be the main accelerator for this amplification: since light, reflective ice is replaced by dark, sunlight-absorbing water, the Arctic warms, resulting in more ice loss and thus more warming.
Warming, Francis and Vavrus suggested, would inflate the height of the polar troposphere – the lowest layer of the atmosphere and home to the weather. This would reduce the pressure differentials between polar air and medium latitude air that drive the polar jet stream that separates the air masses and keeps cold air around the pole. The jet would grow weaker and more ferocious, sending cold air in further south. In their paper, Francis and Vavrus argued that such a trend was visible in weather records and worsened as the Arctic warms and ice is lost.
A lot has changed since then, says Francis now. “Like all things, when you immerse yourself in them, they get more complicated.” Most significantly, the 25-year trend that she and others found in observations from the late 1980s to the early 2010s weakened after another decade of observations. Although sea ice loss continues, there is little evidence of colder winters, more cold extremes, or more frequent weakening or rippling of the jet stream in Eurasia or North America. The new computer modeling matches the observations, says Doug Smith, climate researcher at the UK’s Met Office and another co-lead of the modeling effort. “There is no inconsistency.”
In the year-long study known as the Polar Amplification Model Intercomparison Project (PAMIP), the researchers each performed more than a dozen climate models 100 times. One series of model runs simulated the Arctic atmosphere without significant sea ice loss using sea temperatures and sea ice extent from 2000. The other kept sea temperatures the same, but reduced ice cover to 2 ° C by the extent of global warming expected in decades if the Arctic could be ice-free in summer. If the oceans stay the same, the impact of sea ice loss – if any – should be highlighted.
The models found not only a tiny effect of sea ice loss on the polar ray, but also no coherent indication of a second proposed effect of the reduced sea ice: more frequent disturbances of the stratospheric polar vortex – a second group of swirling winds much higher. Such disturbances, which occur on average every two years, ultimately cause cold air in the atmosphere to rush south and cause extreme winter storms, including the cold that hit Texas last winter.
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