By Alastair Sharp and David Ljunggren TORONTO/OTTAWA (Reuters) - Canada's federal government on Monday vowed to bring in a minimum price on carbon emissions by 2018, prompting the energy-producing province of Alberta to demand approval of a key oil pipeline in return. Liberal Prime Minister Justin Trudeau, who took power last November promising to do more to protect the environment, said carbon pollution would cost C$10 a tonne in 2018, rising by C$10 a year until it reaches C$50 in 2022. In an announcement which could set off a prolonged bout of political arguments and horse-trading, he said the 10 provinces could either implement a carbon tax or a cap-and-trade market by then.
