Another Life: A phone-call from the headland across the bay sent us out of the house to look north, to identify "a big arc of light in the sky". It was, as we expected, aurora borealis, on another of this autumn's visitations. But it was also on a breathtaking scale: the entire sky, as it seemed, webbed with shimmering beams, writes Michael Viney.
A first encounter with this heavenly short-circuit, in anything like a clear sky, is awesome and transcendental: you remember when it was and where you were.
My own initiation came two Novembers ago when, in the sort of display common to much of northern Europe, the sky above the islands was veiled in gleaming mother-of-pearl, flanked by high, huge crimson "presences" that were not quite clouds.
This autumn a Dublin professor, alerted by the evening television news, seized the moment and drove up to the western rim of the mountains, clear of the city's light pollution. Following the high road through the forestry towards the Kilbride military camp, he stopped in a clearing that seemed "preternaturally bright".
He switched off his engine, got out, "and wow! There it was!" He craned back his neck to follow "massive beams like celestial searchlights that seemed to dart and shoot up to the zenith". Some of those in the west "turned a lurid, and indeed apocalyptic, red". He had been expecting something distant and horizon-bound, he wrote to me, not something so enveloping, "both cosmic and close." He drove home elated, a lifelong wish fulfilled.
It is a modern privilege to feel wonder at natural events whose causes we know. Aurora borealis results from a periodic bombardment of Earth's outer atmosphere by electromagnetic particles flying from the nuclear bonfire of the sun.
As this "solar wind" swirls round the planet, it flows and eddies along the force lines of the Earth's magnetic field, then curves down at the poles. Its charged particles excite, or ionize, the various gases of the atmosphere. They give off energy as light, flushed green from oxygen, red from nitrogen, and so on. This all happens high above Earth, in a luminous wall that teases our judgment of distance.
My professor friend knew he had to flee the city lights to be part of those shimmering fringes of Earth.
For some connection with the elemental it also helps to live above the ocean and look out 30 miles to an often stormy horizon. At this point in the year, a greeny tinge creeps into the northern blue, beckoning my mind's eye towards the Arctic Circle.
It's somewhere up there, between Greenland and Norway, that the north-travelling water of the Atlantic does this extraordinary somersault each winter.
There can't be anything actually to see or someone would have described it, but a whole mass of sea sinks to the ocean floor to join what is called North Atlantic Deep Water. This is ponded up behind a ridge between Greenland and Scotland, but periodically it overflows and rolls southwards.
Its long, dark journey, by way of the "conveyor belt" of global ocean circulation, ends in a slow upwelling to the surface of the North Pacific.
Here it joins the circulation of warm surface currents that notably, in the Atlantic, brings the Gulf Stream past our shores.
This majestic Gaian process, known as the thermohaline circulation, is primed by what happens in the North Atlantic in winter. Here the long fetch of often violent winds drags up to the surface the salty, warm water from as deep as 800 metres.
It is chilled from 10 degrees to 2 degrees Celsius, and this, coupled with its high salinity, increases the water's density and makes it sink to the ocean floor.
But now, as scientists warned 20 years ago, the freshwater ice dissolved by global warming is diluting the ocean's salinity and slowing the process down.
The volume of water flowing down through the pump has fallen by a fifth over the past 20 years and the current is weakening further every year.
There have been comparable events in Earth's past, at long intervals and due to natural cycles, but at the end of the last ice age turbulent physical changes in the North Atlantic were switching the current off and on within a decade or less.
We should know, fairly soon, if this is to happen again, and if withdrawal of the Gulf Stream is to lower our winter temperatures by 5 degrees, to match those of Canada. As I write this, the peak of Mweelrea outside the window has been whitened by the first snow of winter - a pure and elemental coincidence, of course.