The Canadian capital's iconic Rideau Canal Skateway -- the largest outdoor rink in the world and a UNESCO heritage site -- may not open this winter for the first time in five decades, due to a lack of ice.

Ottawa is in the grips of its third-warmest winter ever recorded, according to Environment Canada, with temperatures hovering just below freezing through most of December and January.

And they're now forecast to climb.

"Mother Nature has presented us with a big challenge this year," Bruce Devine, National Capital Commission senior manager responsible for the skateway, told AFP.

"Mild temperatures have made it difficult to make good, solid ice that can support the weight of our equipment and skaters," he said.

For the canal to freeze up, temperatures must hold steady at -10 to -20 degrees Celsius (14 to 16 Farenheit) for almost two weeks.

"Currently in several spots the ice is porous and of not very good quality," he said.

Although he remains optimistic for what would be the latest opening on record, others worry the skateway might not open at all this year.

A brief cold spell is forecast to be coming. "The weather will be more conducive (for skating) next week," according to meteorologist Peter Kimbell. "But will it be enough, I don't know."

'Got skates for Christmas'

Chantal Dussiaume, 57, was pessimistic as she eyed ice conditions: "I got skates for Christmas, so it is a big disappointment."

"I've heard so much about the canal and it would have been nice to go skating on it," said Lani Simmons, 46, visiting from Bermuda.

The winding 7.8-kilometre long (4.8 miles) skateway though the heart of Ottawa is the size of 90 Olympic rinks, according to Guinness World Records, which in 2005 confirmed it was the biggest anywhere, and in recent years attracted an average of 22,000 visitors per day.

They would leisurely skate up and down it, taking in views of parliament and quaint neighbourhoods, stopping at warming huts or food stands along the way offering snacks and hot beverages.

Commuters were also seen skating to work with a briefcase in hand.

"It's part of the DNA of local residents and attracts a lot of visitors from afar," said Devine.

The skateway normally opens at the end of December for 30 to 60 days of skating. But its opening has shifted later and later over the years, and for shorter periods.

"It's really too bad for all the businesses and all the people who find joy" in skating, said student Clara Harman-Denhoed, 22. But the situation highlights "the impact that climate change has on us here."

That view was echoed by Canada's environment minister, Steven Guilbeault, who said Tuesday, "This is yet another example of how our climate is changing in Canada."

'Blockbuster' winter attraction

"I don't think anyone has seen it this bad," said Davey Wright, who operates shacks on the canal selling flat fried dough dusted with sugar or cinnamon.

Local businesses, after suffering through pandemic lockdowns and a disruptive trucker-led protest last year, are taking a big hit.

The city was forced to scale back outdoor activities for its annual Winterlude festival, which runs until February 20, while hotel association president Steve Ball said bookings are way down.

"Skating on the canal is our blockbuster (attraction) and what people talk about, why they come back," he explained.

The National Capital Commission has ordered a flurry of climate adaptation studies to try to keep the skateway open earlier in the season and for longer periods.

Shawn Kenny, a professor at Carleton University in Ottawa who studies climate impacts on ice, has tried various fixes inspired from ski hills and seasonal ice roads in the Arctic, for example, using snow fans to blow ice crystals onto the canal to kickstart ice formation, and clearing snow that acts as insulation.

He is currently testing thermosyphons, used in the far north to prevent permafrost from thawing beneath railways, roads, pipelines and buildings, to regulate canal ice temperatures.

But he warned that adaptation may not be enough in the future: "Eventually we'll get to a stage where it might not be possible to open the skateway."