(Adds official report showing Canada heating up at twice the rate of other nations)
By David Ljunggren
OTTAWA, April 1 (Reuters) - Canada on Monday imposed a landmark carbon tax on four provinces which had defied Ottawa's push to combat climate change, prompting unhappy premiers to say they would challenge the measure.
Prime Minister Justin Trudeau, citing international commitments to fight global warming, had made clear for two years he would slap the tax on any of the 10 provinces that did not come up with their own plans by April 1.
The measure is opposed by Ontario, the most populous province, where Trudeau's Liberals need to do well to stand a chance of retaining power in a federal election this October.
Carbon pollution will initially cost C$20 ($15) a tonne, rising by C$10 a year until it reaches C$50 in 2022. It also applies in Manitoba, Saskatchewan and New Brunswick.
"As of today, it's no longer free to pollute anywhere in Canada," Environment Minister Catherine McKenna said on Twitter.
Official data regularly show Canada has little chance of meeting its climate change goals of reducing emissions by 30 percent from 2005 levels by 2030.
Canada will heat up twice as fast as the rest the world, with northern regions warming at an even greater pace, a Canadian environment ministry report said on Monday.
Smaller snow packs and a loss of glacier ice could mean water supply shortages in the summer, it added.
Trudeau says extreme weather events like forest fires and floods underline the need to combat global warming.
Although Ottawa says the money collected from the next tax will be returned to Canadians in the form of rebates, right-leaning parties portray it as a cash grab.
The Conservatives, who polls show could win the October election as a political scandal dogs Trudeau, promise to scrap the measure.
Ontario premier Doug Ford vowed to oppose what he called "the worst tax ever" in court.
"We're going to keep fighting this carbon tax with every single tool at our disposal," he said.
Trudeau's other efforts to combat climate change are also proving a challenge.
Last year Ottawa unveiled plans to overhaul environmental assessments of energy projects, paying more attention to greenhouse gas emissions. Critics say this will deter investment at a time when existing projects are in trouble.
Canada bought Kinder Morgan Canada Ltd's Trans Mountain crude pipeline for C$4.5 billion last year after the company expressed doubts it could proceed with plans to more than double the existing capacity.
$1=$1.33 Canadian Reporting by David Ljunggren in Ottawa Editing by James Dalgleish and Sonya Hepinstall