Scientists say there are indications that climate change has begun to feed on itself.
They say at least some parts of the Arctic appear to have changed as the region has warmed.
Instead of storing carbon dioxide in the frozen soil, they believe, these warming areas are now releasing the carbon to the atmosphere. This will intensify the warming that is already happening.
The managing director of a Unep information and monitoring centre in Norway, Grid Arendal, Dr Svein Tveitdal, told journalists: "Climate change is going to mean tremendously big problems for the Arctic."
Carbon release
He said there was emerging evidence that the Arctic permafrost, the soil that used to stay frozen all year round, was now releasing carbon as it thawed.
About 14% of the carbon stored in the world's soils is estimated to be in the Arctic.
This probably amounts to several hundred Gigatonnes, and the release of the entire Arctic carbon store, if it happened, would add prodigiously to climate change (emissions of all greenhouse gases produced by human activities are about six Gt annually).
Dr Tveitdal said: "Permafrost has acted as a carbon sink, locking away carbon and other greenhouse gases like methane, for thousands of years. But there is now evidence that this is no longer the case, and the permafrost in some areas is starting to give back its carbon."
Scientists from the University of Wales at Bangor have already identified a temperature-sensitive enzyme which rapidly breaks down peat when it dries out, causing it to release carbon to the atmosphere.
A similar mechanism appears to be operating in the permafrost, with Grid Arendal warning that higher temperatures are allowing bacteria to break down the previously frozen organic material.
Damage to buildings
The permafrost is normally ideal for construction, because of its solidity. But studies at the University of Alaska at Fairbanks suggest that a warming of the permafrost from minus four to minus one degree Celsius decreases its load-bearing capacity by up to 70%.
Damage to buildings, roads, pipelines and other structures caused by thawing of the permafrost is already occurring in Alaska and Siberia.
Dr Tveitdal said that while the Intergovernmental Panel on Climate Change was now suggesting that global average temperature might increase by 2100 by up to 5.8 degrees C, in high latitudes the worst case could be almost twice that.
"In some areas like the Arctic you might have an increase of up to 10 degrees C this century," he said.
He told BBC News Online: "As the temperature increases, the snow cover will move northwards, and the soil that is then exposed will be very slow to revegetate. So it will absorb more solar energy.
"In Norway today, if you're investing in a ski resort, you'll be putting your money a bit higher up the mountain, because the skiing season is already getting shorter.
Danger to indigenous people
"We obviously have to learn to adapt - but the Arctic ice has lost 40% of its volume in the last four decades. It's not too easy to adapt from an iceberg to the open sea."
Dr Tveitdal is also concerned for the survival of the estimated 200,000 indigenous people, from 30 ethnic groups, in the Russian Arctic, and for similar groups elsewhere in the permafrost zone.
"Their existence is under threat," he said.
The Arctic Council, the bloc of eight countries with territory in the Arctic, is meeting in Rovaniemi in Finland in June, and will be updated on the thawing of the permafrost.