Al Gore has called on Barack Obama to veto the Keystone XL tar sands pipeline, describing it as "an atrocity".

The former vice-president said in an interview on Friday that he hoped Obama would follow the example of British Columbia, which last week rejected a similar pipeline project, and shut down the Keystone XL.

"I certainly hope that he will veto that now that the Canadians have publicly concluded that it is not safe to take a pipeline across British Columbia to ports on the Pacific," he told the Guardian. "I really can't imagine that our country would say: 'Oh well. Take it right over parts of the Ogallala aquifer', our largest and most important source of ground water in the US. It's really a losing proposition."

The proposed route for the Keystone XL pipeline takes it across the Ogallala aquifer, which supplies ground water for drinking and irrigation

Campaigners have cast Keystone XL as the most important decision of Obama' presidency. The State Department, which has say over the project because it crosses the US-Canadian border, is to announce its decision later this year.

But Gore said an even larger environmental decision loomed for Obama next month. The White House has indicated Obama could offer a long-awaited climate plan, the first concrete proposals since his inauguration in January when the president suggested it was a religious and patriotic duty to deal with the challenge

"This whole project [Keystone XL] is an atrocity but it is even more important for him to regulate carbon dioxide emissions," Gore said. He urged Obama to use his powers as president to cut carbon dioxide emissions from new and existing power plants – the biggest since source of global warming pollution.

"He doesn't need Congress to do anything," Gore said. "If it hurts the feelings of people in the carbon polluting industries that's too bad."

Gore was speaking from Istanbul, where he will soon lead a three-day training session on climate change for a global group of some 600 activists. Since the 2000 election, when Gore won the popular vote but lost the White House to George Bush, he has turned his public life over to action on climate change. The climaterealityproject.org gathering in Istanbul will be the 22nd time Gore has presented his regularly updated slide show on the science behind climate change to a group of global activists.

He planning an even bigger training exercise in Chicago at the end of July, where he hopes to deliver his new slide show to more than 1,000 activists. It will be the largest such session since Gore adopted education and training of climate-change activists as one of the main concerns of his post-political career, and the first such exercise in the American mid-west.

Al Gore's slideshow presentation in the film an Inconvenient Truth

The timing is critical – in climate terms, with atmospheric carbon dioxide reaching a new milestone of 400ppm – and on the political agenda, Gore argued. Last year's extreme weather – including superstorm Sandy and the punishing drought across the mid-west – has exposed the real-time costs of climate change. Extreme events inflicted $110bn in damages last year, according to the Obama administration.

Gore said he was also encouraged by the rise in climate activism by Democrats in Congress, singling out the Rhode Island senator Sheldon Whitehouse, who has made weekly speeches on climate change. "The conversation on climate is evolving rapidly partly because mother nature has joined the conversation and has a powerful voice, Gore said.

He said he believed those events were steadily moving public opinion on climate change to an historic tipping point – similar to the shift of opinion on such once controversial issues as civil rights and same sex marriage.

"People have the impression that is a Sisphyean task right now but times are changing," Gore said. "Just because the opponents of doing anything on global warming are trying to intimidate people to not even considering it, that is no reason for the rest of us to conclude that it is impossible. I don't think that it's impossible."

Gore said there was no way, in his view, to achieve climate action without continuing to keep the topic on the public agenda.

"I think we have to engage, difficult as it can seem to be, and build a critical mass to get beyond the critical tipping point," he said. "That is what it is all about. We have to win the conversation and change the law and put a price on carbon."