What we learn flying over the world's largest iceberg - and why it's not long for this world

One thousand feet above the world's largest iceberg, it's hard to believe what you're seeing.

It stretches all the way to the horizon - a field of white as far as the eye can see.

Its edge looks thin in comparison, until you make out a bird flying alongside and realise it is, in fact, a cliff of ice hundreds of feet high.

Scientists who have used satellites to track the iceberg's decades-long meanderings north from Antarctica have codenamed the iceberg A23a.

But up close, numbers and letters don't do it justice.

It's a seemingly endless slab of white, fringed by an aquamarine glow - the ocean at its base backlit by a sill of reflective ice below.

Monotonous yet magnificent; we're flying along the coastline of a nation of ice.

And it's also hard to believe you're seeing it at all.

Where it has run aground - 50 miles off the small island of South Georgia - seems impossibly remote.

We're 800 miles from the Falkland Islands and 900 miles from the icy wastes of Antarctica.

With no runway on South Georgia, there's only one aircraft that ever flies here.

Once a month or so, a Royal Air Force A400 transport plane based in the Falklands carries out Operation Cold Stare - a maritime surveillance and enforcement flight over the British Overseas Territory that includes the neighbouring South Sandwich Islands.

It's a smooth, albeit noisy, two-hour flight to South Georgia.

But as the dramatic peaks of the island come into view, the ride - for us inexperienced passengers at least - gets scary.

Gusts off the mountains and steep terrain throw the plane and its occupants around.

Not that that stops the pilots completing their circuit of the island.

We are overflying some of its 500,000 square mile marine protected zone designed to protect the greatest concentration of marine mammals and birds on the planet that is found on South Georgia.

Only then do we head out to the iceberg, and even though it's only a few minutes flying from South Georgia it's at first hard to see. It's so big and white it's indistinguishable from the horizon through the haze.

Until suddenly, its edge comes into view.

It's immediately apparent the A23a is not too long for this world. Large icebergs hundreds of metres across have already broken off and are drifting closer to South Georgia.

All along its edges, cracks are appearing and arches at its base caverns are being eroded by the warmer ocean here, undercutting the ice, weakening it further.

The iceberg might present a problem for some of South Georgia's super-abundant penguins, seals and seabirds. A jumble of rapidly fragmenting ice could choke up certain bays and beaches in which colonies of the animals breed.

The trillion tonnes of fresh water melting out of the iceberg could also interfere with the food webs that sustain marine life.

However, the breeding season is coming to an end and icebergs are also known to fertilise oceans with sediment carried from the Antarctic continent.

The impact on shipping is more relevant. There's not much of it down here. But fishing vessels, cruise ships and research teams ply these waters and smaller lumps of ice called "growlers" are a regular risk.

A23a will create many.

Icebergs this big are too few for scientists to know if they are becoming more frequent or not.

But they are symptomatic of a clearly emerging trend. As our climate warms, Antarctica is slowly melting.

It's losing around 150 billion tonnes of ice a year - half of it breaking off the continent in the form of icebergs calving from glaciers, the rest melting directly from its vast ice sheets as temperatures gradually rise.

The pace of A23a's disintegration is far, far faster. It will disappear in months, not millennia.

But watching its edges crumble and slide into the South Atlantic, you can't help seeing it as the fate of a whole continent in miniature.