Chaos Theory, the Butterfly Effect, El Nino, Hottest Year on Record. These are all now well-known and oft-used phrases and all allude to the interconnectivity of the Earth’s weather system.

A change in any one place will have an effect elsewhere.

We saw this very obviously in the 2015/16 El Nino event where warm water in a different place affected a lot of the world’s weather. That event developed in a matter of months.

The increase in carbon dioxide (CO2) largely through human industrialisation, has taken much longer but is now above 400 parts per million, a figure not known since the ice retreated.

Increased CO2 absorbs more heat into the atmosphere so that it gets warmer.

Some of that heat is transferred to the ocean and the combination of warmer atmosphere and ocean surface layers melts ice.

This is becoming a major problem and probable driver of extreme weather events.

It is received wisdom that the Arctic Ocean freezes over every winter: the air above gets increasingly cold as the Arctic night descends.

In recent years, however, the ice has not covered the Arctic Ocean during the winter, as it used to.

No source of heat

The November 2016 Arctic sea ice extent was the lowest in the 38-year satellite record, according to the National Snow and Ice Data Center (NSIDC).

The record low was due to unusually high air temperatures, winds from the south, and a warm ocean.

Seven of the 11 months of 2016 have seen record-low Arctic sea ice, and the annual sea ice minimum in September was the second lowest on record.

There is traditionally no source of heat in an Arctic winter, with the sun always below the horizon, and air temperatures will drop typically to about -30C.

But if the ocean is uncovered, it will radiate heat; the Arctic Ocean surface water is going to be always warmer than -2C which is much warmer than the air above.

This warming from the ocean will cause convection up through the atmosphere, making smaller the volume of really cold Arctic air. More worryingly, it also diffuses the temperature difference between the Arctic and the rest of us.

It is this steep temperature gradient that generates the jet stream. It is the jet stream that creates the winter boundary – when it crosses a country, wintry weather arrives.

When the gradient is steep, the jet stream is as taut as stretched elastic.

In this condition, the weather changes often, so winter extremes are rare.

When this temperature gradient is diffuse, as is the case when Arctic ice cover is incomplete, the tautness goes from the jet stream, it no longer contains the cold as tightly, and is likely to become misshapen and slower-moving.

When extremes occur

As the jet stream meanders more, big loops bring warm air to the frozen north and cold air into normally warmer southern climes.

These loops can remain stuck over regions for weeks, rather than being blown eastwards as in the past.

This is when extremes occur, especially in winter cold, as has just happened in the US.

Over this last weekend, temperatures dropped rapidly to between -20C and -30C from Montana, through Kansas to Michigan.

This was air that had formed over the polar ice but had flopped down through the Canadian Arctic as the rather flabby jet stream looped down over North America.

Luckily for the frozen citizens, this is not yet a “block”, a stationary loop.

Already, warmer air has been blown in from the Pacific although it has brought rare Christmas season snow to Vancouver.