Less than six months after U.S. Secretary of State John Kerry labelled Canada's oilsands "one of the dirtiest sources of fuel on the planet," the United States government is changing its tone.

The newly appointed special envoy for climate change Jonathan Pershing said the issue of whether or not Canada should continue to exploit the oilsands "is not so black and white."

Pershing said there is a more "nuanced story" to be told about the oilsands and that it's a question of how Canada chooses to develop its natural resources.

"There's some very interesting research that says you can actually have a negative barrel of oil in terms of greenhouse gas emissions, if what you do is collect more carbon dioxide from the atmosphere than is emitted by that barrel," Pershing told CBC News Network's Power & Politics.

But without efforts to offset the impact of carbon pollution, Pershing said development of the oilsands "doesn't work."

Canada 1st stop for U.S. climate envoy

Pershing is now the United States' climate czar. He is a scientist and was a member of the Nobel-prize winning intergovernmental panel on climate change.

Within two weeks of taking up his new position on April 1, Pershing flew to Ottawa to meet with Environment Minister Catherine McKenna. It was his first official visit as climate envoy.

Canada and the U.S. recently agreed to take joint steps to fight climate change and Pershing believes those efforts will be "powerful."

"Our relationship and what we could do jointly — it dwarfs things we could do with almost anyone else…. We have capacity, we have a political will, we have at the moment a set of technology opportunities.... If we can't do it, how will the rest of the world figure this out?"

World leaders will have the opportunity to discuss climate change again next week. Prime Minister Justin Trudeau is attending the Paris Agreement signing ceremony on Friday, April 22, in New York, where 130 countries are expected to be there to mark the first major step towards implementing the historic climate agreement.

Removing fossil fuel subsidies

Pressed by Power & Politics host Rosemary Barton about his own academic research into eliminating subsidies for oil and gas companies, Pershing said the move "clearly seems to be a big part of the solution."

Citing research from the International Energy Agency, he said the case has been made that one of the most significant things the world can do to tackle climate change is to remove subsidies.

"I think it's a very compelling analysis," Pershing said. "Almost all the economic analysis does not point to subsidies as being an efficient way to go."

You can watch the full interview with U.S. special envoy for climate change Jonathan Pershing in the viewer below: