After a summer of catastrophic flooding in Britain, it would be encouraging to see the conference season as an opportunity for British politicians to move from the highly rhetorical to the real in their climate change policies. The reaction of some Conservatives, however, to the relatively modest suggestion that airport expansion in Britain should be halted, or that those who pollute should pay, is a reminder that there are still people who suffer from the delusion that doing nothing on climate is an option, or that inaction will somehow guarantee that things stay the same. Eric Rode Frederiksen knows different.

Eric is rotund, white-haired - and clear-eyed on the subject of climate change. He was born 80 years ago in the village of Qassiarsuk in Subarctic Greenland, where an earlier Eric, the Nordic Viking Eric the Red, first established a colony in the 10th century.

One legend has it that Eric was so desperate to entice others to join him that he lied about the inhospitable nature of the vast island of rocky mountains and deep valleys. He sent word that he had found a "green land" fit for settlement. Fifteen ships did follow him, and the Viking settlement lasted nearly 500 years.

The legend does Eric a disservice. The slopes the Vikings settled were, indeed, green, and the accumulated topsoil testifies to long periods of fertility in the past. Eric and his people farmed sheep and grew potatoes, supplementing their diets with fishing and hunting. It was when the climate cooled, 400 years later, narrowing the margin of possible agriculture in southern Greenland to zero, that the Viking settlement failed and farming disappeared for 500 years. In the 20th century, Eric Rode Frederikse's father became the first Greenlander since the Vikings to give sheep farming another go. Today, 65 families are making a living as farmers in southern Greenland, the growing season is a month longer than before, and people have begun to plant little gardens. This summer, they tasted their first Greenland-grown broccoli.

As the Vikings failed in the south, in north Greenland the Inuit were thriving, equipped with effective technology and the hunting skills needed to live off the rich Arctic animal life. But two years ago, for the first time, the Greenland government had to fly dog food to the far north, emergency relief for the starving hunting dogs of the Arctic Circle. Today, Greenland's Inuit hunting communities are facing extinction.

The uneven effects of climate change are not lost on the Greenlanders. If the ice loss is a disaster for the Arctic hunters, Greenland as a whole is sitting, literally, on a gold mine. The gold is already being mined, and other resources are beckoning from the melting ice fields. Oil majors are quartering the territory, looking for fresh supplies. The aluminium giant, Alcoa, is negotiating to open three smelters in Greenland to take advantage of a bonanza in hydro-electric power. In settlements inside the Arctic Circle, where thousands of dogs still sit tethered and restless, waiting for ice that no longer comes, Greenlanders are about to be catapulted into an industrial age.

They have little choice but to adapt. In the Arctic, the mirror of life for the rest of the planet, things are now changing at a dizzying pace that far outstrips the cautious estimates of the Intergovernmental Panel on Climate Change, or even the expert assessments of the Arctic Council. For instance, the sea around Ilulissat, the biggest town in Arctic Greenland, always froze between December and the end of May. It has not frozen at all for the last three years. The IPCC predicted an ice-free Arctic by the end of the century; some scientists now predict it for 2020. Ten years ago, it was thought that the Greenland ice cap - 11,000 feet high in some places - would take centuries to melt. Now, the pace of melting of the ice cap, and the unpredictable interaction of the feedback loops such melting may trigger, makes any firm prediction hazardous. In Greenland they know human civilisation is already entering unknown territory.

Robert Corell, a US-based Arctic scientist and member of the IPCC, described what he had found three weeks ago on a visit to the ice cap. "I spent four months on the ice cap in 1968 and there was no melting at all," he told participants in the Symposium on Religion, Science and Environment in Greenland this week. "Now it's dramatic. There are thousands of moulins - holes that go down into the ice. You can hear water roaring and gurgling. Nobody knows now how quickly it will melt, but the palaeo-data tells us that at three degrees warmer than at present, the ice cap will melt. The projections for global temperature increases are now between three and four degrees."

Greenlanders have waited a long time for prosperity and, bizarrely, climate change may bring it, ending the need for the present annual subsidy from Denmark of $10,500 a year for each of Greenland's 55,000 inhabitants - equivalent to roughly half the Greenland national budget. Future economic promise is fuelling an independence movement; but for the rest of the world, Greenland's melting ice threatens catastrophe. So vast is the Greenland ice sheet that it works as a regional air conditioner. The albedo effect - the reflection of 80% of the sun's heat by the snow and ice - keeps temperatures in the region cool. But as the ice melts, the dark seas and the bare rock surface absorb heat, further accelerating melting and triggering sea level rises that will inundate, among other places, the Nile Delta, much of San Francisco and 40% of Bangladesh.

Arctic scientists are having to tear up their recent predictions and start again. "This is all unprecedented in the science," Corell explains. "Until recently we didn't believe it possible, for instance, for water to permeate a glacier all the way to the bottom. But that's what's happening. As the water pools, it opens more areas of ice to melting."

"For the last 10,000 years," Corell says, "we have been living in a remarkably stable climate that has allowed the whole of human development to take place. In all that time, through the mediaeval warming and the Little Ice Age, there was only a variation of 1C. Now we see the potential for sudden changes of between 2C and 6C. We just don't know what the world is like at those temperatures. We are climbing rapidly out of mankind's safe zone into new territory, and we have no idea if we can live in it."

isabel.hilton@theguardian.com