A Toronto Hydro vulnerability study published last year warned that climate change could result in more severe freezing rain storms, increasing the risk of major power outages.

The study, published in September, 2012, says warmer winter temperatures can increase the intensity and quantity of freezing rain and wet snow, which can damage tree branches and overhead wires. Toronto Hydro estimates that about 26,000 customers were still without power on Friday, six days after a massive ice storm swept through Southern Ontario.

Another 2,400 customers in Quebec and 13,300 in New Brunswick were also without power Friday as a result of last week's storm.

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The Toronto Hydro study describes freezing rain and ice storms as a "medium risk" for hydro systems in Toronto because they can lead to an accumulation of ice on trees and electrical equipment. While that buildup does not directly affect electrical equipment, the weight of the ice can cause conductors and poles to break, the report says.

Ice storms can also cause tree limbs to fall onto equipment and freeze access covers, making it more difficult for workers to access underground equipment.

Jennifer Link, a spokeswoman for Toronto Hydro, said the utility is aware of the risks of climate change and is constantly upgrading its infrastructure to better protect it from severe storms, including by replacing old cables and pruning trees that could present risks to overhead power lines.

Jim Burpee, president of the Canadian Electricity Association, said the biggest problems utilities face during major ice storms are trees falling onto overhead wires and ice accumulating on wires and poles.

"The issue with more frequent ice storms is what standards should you design for going forward," he said. "In the end, it all becomes a cost-benefit [analysis]. So what's the cost of the outage in terms of repairs and people not having power, and what do people believe is an acceptable period of time to go without."

Toronto Hydro CEO Anthony Haines said this week that it would be too expensive to bury power lines that are currently above ground in Toronto and doing so would make them vulnerable to other problems, such as flooding.

Mr. Burpee said many utilities are gradually upgrading to stronger overhead wires and selectively burying wires underground.

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Richard Kinchlea, chair of the Emergency Management and Public Safety Institute at Centennial College, said utilities rely on ratepayers to cover significant infrastructure upgrades. The Ontario Energy Board must approve requests to raise the rates, he said, adding that plans involving major upgrades – and therefore significant rate hikes – are frequently turned down.

A spokeswoman for Hydro One said the utility will review its response to the ice storm once all power has been restored and the cleanup is completed.

"Our first protocol right now is to restore power to those who are still affected," Nancy Shaddick said Friday. "Once we've been able to do that and able to clean up, we will undergo a review of the events and see if there's any procedures and protocol that we can improve upon."

With a report from The Canadian Press