Last year, for the first time in my life, I went to the North Pole. Together with a Frozen Planet film team, I flew to a Russian camp, set up on the sea ice 70 miles south of the Pole. After several days of bad weather, we took off again, this time in a helicopter and flew across the frozen Arctic Ocean to land on the ice at the geographic Pole.
The next day, leaving my tent to walk to the strip of ice that served as a landing strip, I stepped over a crack in the ice. It was no more than six inches across and snaked away into the distance. "We often get small cracks like that. There are currents below and the ice shifts," the Russian commander assured me. "No worries."
The next day, safely back on terra firma in Svalbard, we heard by radio from the Russians that crack had started to widen with alarming speed and they had only just managed to scramble across it to get to the airstrip. The annual melt had begun almost a month early. The Arctic is warming.
Scientists have even seen changes from space. Data from satellites collected over the last 40 years show a drop of 30 per cent in the area of the Arctic sea ice at the end of each summer. Not only that. The ice is almost half as thick as it was in the 1980s, a fact that only came to light when ice logs kept by the US and British submarines looking for places to surface during the Cold War were finally made public. The way things are going, there will be open water at the North Pole in summer within the next few decades.
Polar animals are already reacting to the changes. We saw the evidence for ourselves when we accompanied a Norwegian team from the University of Svalbard who were making their annual check on the health of polar bears. The bears' condition has been steadily deteriorating as the ice, which they need when hunting seals, diminishes. And cubs born to underweight mothers are much less likely to survive their first year.
The Loss of sea ice in the north affects not just polar bears but the whole planet. The frozen Arctic Ocean acts as a huge reflector, bouncing 85 per cent of the sun's heat back into space. This keeps the polar regions cool and moderates the whole of the earth's climate. But when the ice vanishes, the dark sea water that replaces it absorbs the sun's energy, so its temperature rises. This is why the Arctic - a region the size of North America - is warming twice as fast as the rest of the planet. As a consequence, increasing amounts of meltwater are now flowing into the polar sea. This could eventually disrupt the flow of ocean currents around the planet that transfer heat around the Earth and are critical in maintaining the climates we've known for centuries. The implications of that are hard to overstate.
The ice that lies on land is not salty, like sea-ice, but fresh, for it is the accumulated snowfall of centuries. In making Frozen Planet we documented evidence that this too is melting. And not only from the top but from beneath.
Meltwater, created by the warming air and accumulating on the surface, cascades down moulins - giant shafts that pierce the ice sheet. There it melts the underside of the glacier, so hastening the speed of the glaciers' descent to the sea. The meltwaters from Greenland's glaciers alone could cause a rise in global sea levels of up to half a metre by the end of this century.
The Antarctic has ten times more land ice than the Arctic. I first went to South Georgia, the mountainous, glacier-draped island just north of the Antarctic Circle, in 1988. At that time, the Cook Glacier that flows down to St Andrew's Bay reached the waterline. Last year, a Frozen Planet team discovered that the glacier had retreated by 400 metres. As a consequence, the Bay's king penguin colony now has far more beach front and is thriving as a result.
Further south, however, another species of penguin - the Adélie - is suffering. The southern part of the Antarctic Peninsula where they once flourished is now the most rapidly warming region in the whole of the southern hemisphere.
In 2010 it was announced that every single ice front here is in retreat. So the Adélies, which rely on ice-loving krill for food, find it hard to survive and colonies that I visited in 1992 have now disappeared altogether.
But it is the change to the permanent ice that fringes the coasts of Antarctica that is likely to have the most dramatic effects of all. One of these expanses, known as the Wilkins Ice Shelf, was roughly the size of Jamaica. In 2008 it began to fragment. Last year the British Antarctic Survey helped a Frozen Planet team to film all that remains - hundreds of giant tabular icebergs strewn across the ocean.
This collapse is only the most recent in a wave of similar events that have travelled southwards down the peninsula. Next in line could be some of the ice shelves that act as plugs for the immense body of ice lying on the continent itself. If any of these collapse, vast quantities of land ice and melt-water will slide into the sea and cause a major rise in sea levels around the globe.
When that will happen and by how much are difficult questions. But with over half of the human population living near the coast, the answers may be only too devastating.
The final episode of Frozen Planet airs tonight, 9pm, BBC1, BBC1 HD
This is an edited version of an article from the issue of Radio Times that went on sale 29 November 2011