YELLOWKNIFE, Northwest Territories, Dec 24 (Reuters) - Each winter, in the far reaches of Canada's north, a highway of ice built atop frozen lakes and tundra acts as a supply lifeline to remote diamond mines, bustling with traffic for a couple of months before melting away in the spring.
This year, the world's busiest ice road is running late. Unseasonably warm weather has set back ice formation on the Tibbitt to Contwoyto Winter Road, named after the first and last of hundreds of lakes on the route.
The road is still expected to open on schedule in late January, but if current weather patterns continue that could mean more work for crews trying to build the ice or cut the road's already short period of operation.
Since its first season in 1982, the road has been vital to a handful of mines scattered across Canada's Northwest Territories (NWT), cut off by a maze of water and spongy tundra, otherwise only reachable by air. Running 400 kilometers (248 miles), it links to three diamond mines, stretching as far as 600 km when it supplied a now-shuttered gold mine.
A shorter season could mean extra costs and inconvenience for moving what amounted last year to 9,000 truckloads of diesel, machines and mining supplies from the NWT's capital city, Yellowknife.
To climate scientists, this year's late freeze could be a harbinger of winters to come. It also raises the alarming prospect of thawing permafrost - the frozen layer of soil covering nearly half of Canada's landmass - which traps methane, a greenhouse gas, which would only hasten warming.
This year's warmer temperatures may be connected to the El Nino climate phenomenon, a periodic warming of Pacific Ocean waters that has far-reaching effects.
It is Yellowknife's second warmest December on record, said David Phillips, a senior climatologist at Environment Canada, Canada's national weather service. So far, the average temperature for this December is just above -15 Celsius, marginally cooler than the -13 Celsius for December 2005, but well above the mean of around -22 Celsius.
The NWT falls largely within the Mackenzie River Basin, an area where winter temperatures have warmed by 4.5 degrees Celsius over the last 68 years. "That's a sea change," said Phillips. "It is just runaway warming."
For Ron Near, a retired Royal Canadian Mounted Police officer who manages the road for a group of mining companies, slow ice formation is a transportation problem.
Despite the warmer weather, he said it is not "panic time," and said he expects the road to start operating by the end of January, with the heaviest loads waiting until a harder pack of ice at the beginning of March.
"It has affected us some, but we're still within guidelines of previous warmer years," he said. "It's just going to take considerably more management this year to have success."
Ice roads cross eight Arctic countries, and Canada alone has 5,400 km of them, critical to unlocking mineral wealth from remote, harsh regions.
In the NWT, a vast land covering more than 1.3 million square km with just 43,000 residents, diamonds were the biggest contributor to the economy last year.
It is no surprise that the territorial government has been pushing a partial all-season road on the southern end of the mine supply route, which could extend the ice road's duration to three months.
The C$170 million project may find favor with Canada's recently elected Liberal government, which has pledged to spend about C$10 billion annually on infrastructure for the next three years.
But it is a long way from the ambitious idea first mooted in the late 1950s by then-Prime Minister John Diefenbaker, who campaigned for a "road to resources" running through NWT's Mackenzie Valley and connecting to the Arctic coast.
More than a half century later, that vision for a Mackenzie Valley Highway remains elusive. There is a road in the south that extends as far as the town of Wrigley, and a C$300 million road is being constructed to connect the far north town of Inuvik with Tuktoyaktuk on the Arctic coast.
But there is no road connecting those two ends, a highway that proponents say would assert Canada's Arctic sovereignty, but would likely cost more than C$1.7 billion to build.
And advances on the Inuvik to Tuktoyaktuk project are slow because construction occurs only in winter to minimize permafrost damage. More than half the NWT permafrost is sporadic, or discontinuous. It is easily disturbed, which in turn produces ground thaw and instability.
Some 1,700 gigatonnes of carbon are held in permafrost soils globally in the form of frozen organic matter, researchers from the universities of Cambridge and Colorado said in September.
If that methane and carbon dioxide were released, it would increase the risk of catastrophic weather, or loss of agricultural land, causing up to $43 trillion in economic damage globally by 2200, the study calculated.
By mid-century, rising temperatures may reduce the land in Canada suitable for ice roads by 13 percent, or 400,000 square km, concluded researchers at the University of California, Los Angeles in a 2011 study published in Nature Climate Change.
The consequences of those warming temperatures pose an additional risk to mining companies in the NWT, where a half dozen planned mines are on hold due to multi-year low prices for gold, rare earths and other metals.
A taste of the trouble warm winters cause came in 2006, when the road closed after just 36 days. Miners spent more than C$100 million to charter flights for fuel and began talking seriously about options like hovercraft and blimps.
To make the most of winter's cold, lightweight groomers are now clearing snow that insulates and slows ice growth. Later, amphibious tracked vehicles, called Hagglunds, will tow ground-penetrating radar to measure ice thickness.
Crews may need to flood more of the road than normal to quicken the freezing process this winter to overcome the warmer weather, Near said.
The road, tracked by global positioning system technology, now allows longer trailers that haul heavier loads and even has 'express' lanes, so returning trucks with empty loads can exceed the 25 km per hour speed limit.
"We think about climate change all the time," said Near. But he said he "learned a long time ago you can't control the weather. You just have to be able to plan for it." (Reporting by Susan Taylor; Editing by Bruce Wallace and Bill Rigby)