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(Gerard Wynn is a Reuters columnist. The views expressed are his own)
By Gerard Wynn
LONDON (Reuters) - A new, unpublished finding that the polar "jet stream" is slowing down provides compelling evidence of a link between rapidly melting Arctic sea ice and colder winters across the northern hemisphere and other extreme weather.
The possibility of far flung impacts from a rapidly warming Arctic underlines the danger of unpredictable, economically disruptive knock-on effects of rising greenhouse gas emissions.
The polar jet stream is created by difference in temperature between the cold high latitudes and warmer regions to the south; the steeper the temperature difference the faster the flow.
Arctic sea ice is melting three decades faster than predicted by models used in a U.N. climate panel report four years ago.
Ice floes about 2-3 metres thick floating in the open ocean around the North Pole now have a collective area in late summer about three-fifths of levels in the 1970s.
Without an insulating layer of ice the sea warms the Arctic air through autumn and winter, adding a large extra source of heat to the atmosphere and eroding the temperature difference with lower latitudes, and it seems slowing the jet stream.
The high altitude winds across North America have slowed by 20 percent or more in the past quarter century, according to Jennifer Francis at the U.S.-based Rutgers University.
That's important because the jet stream creates and steers storm systems and so helps set northern hemisphere weather patterns.
Four jet streams circle the Earth in wavy paths, changing the world's weather as they move.
Slower winds could affect the weather in two ways.
First, a slower jet stream takes a more meandering path which shifts more slowly. That can create more persistent weather conditions including floods or droughts.
Second, the meandering path tends to adopt a pattern which diverts cold air over the eastern United States and western Europe, in a paradoxical impact of global warming, reinforcing a negative phase of a well-known natural weather cycle called the North Atlantic Oscillation (NAO).
See this graphic, from the University of New Hampshire: link.reuters.com/cyv93s
Reuters Arctic TV package: here
Unpublished research by oceanographer James Overland at the at the U.S. National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory shows that a negative NAO hit new extremes in the last two winters, in a 150-year record.
See this table from the U.S. National Center for Atmospheric Research: here#naostatdjfm
That coincided with an unusally cold, 2009/2010 winter across the northern hemisphere in Western Europe, the eastern United States and east Asia.
Two years of observations are insufficient to prove a theory, no matter how extreme the data. Some scientists believe that natural effects flipped the NAO into an extreme negative phase, in a strongly divided scientific debate.
But the observation of a slowing jet stream provides a compelling link with melting ice.
The trend of melting sea ice over the Arctic Ocean is not irreversible. But most experts expect the ice to disappear completely each summer within decades, as a result of warming greenhouse gas emissions combined with natural effects, leading to an ice-free North Pole.
In her finding, Francis says that the west-east movement of wind in the polar jet stream across North America in autumn and winter has slowed to about 40 kilometers per hour compared with 55 km/hr in the mid-1980s, after a period of rather steady values since reliable measurements began in 1950.
The start of the slowdown coincided with that of sea ice decline. The findings, in preparation for publication, are based on observations and an established practice, called re-analysis, of running historical weather through forecast models to plug data gaps.
In an alternative, local interpretation of the wider impact of melting sea ice, Vladimir Petoukhov at Germany's Potsdam Institute for Climate Change Research suggested two years ago that warmer air over ice-free water north of Norway and Russia had created a high-pressure system.
That was because the warmer air over open sea sucked in surrounding colder air, he suggests. The resulting high pressure sent polar air southwest over Europe, deflecting warm Atlantic winds.
This year was the second biggest sea ice melt in the satellite record, so the odds should favour another cold winter, but a host of other factors from ocean sea surface temperatures to the phase in the El Nino weather cycle will also play a part.
Editing by William Hardy