Failure in the battle to restrain global climate change will entail more costly and devastating impact than any effort to transition off fossil fuels, the head of United Nations Environment Programme warned Sunday.

UNEP executive director Erik Solheim is in Canada to mark World Environment Day on Monday in an event in Niagara Falls, Ont., with Prime Minister Justin Trudeau. He arrived just days after U.S. President Donald Trump announced he would pull the United States out of the Paris agreement because, the President said, it would result in the loss of American jobs.

Mr. Solheim said the world is already seeing costly effects of a changing climate – from flooding to drought in some of the world's poorest countries such as Somalia. And that impact will intensify and reach catastrophic levels if the world does not work together to reduce greenhouse gas emissions that result from the burning of fossil fuels such as coal, oil and natural gas, he said in a telephone interview from Toronto.

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"It was argued in the United States last week that there are enormous costs related to climate action. The opposite is of course true: There are enormous costs to non-action," said Mr. Solheim, a former Norwegian environment minister.

Mr. Solheim will join Mr. Trudeau and Environment Minister Catherine McKenna for a World Environment Day event, where the Canadian politicians plan to paddle a canoe on the Niagara River to highlight the benefits of connecting with and protecting nature.

Ms. McKenna insists Canada will continue to show leadership on climate change and the broader environmental agenda, both at home and internationally.

Critics within the environmental community have questioned that leadership after the government approved expansion of two oil-export pipelines last year that will facilitate expansion of the oil sands in Alberta, while conservative opponents argue Ottawa is moving too aggressively with measures such as carbon levies and regulation, especially in light of Mr. Trump's deregulation push.

Ms. McKenna argues there is economic opportunity for Canada in the global transition from fossil fuels to lower-carbon sources. "If the U.S. is going to step back, we're going to step in," she told CTV's Question Period in an interview that aired Sunday.

She rejected Mr. Trump's call for a new deal that would be fairer to the United States. "You can't renegotiate the Paris accord; it's in force," she said. The G7 environment ministers – including Scott Pruitt, head of the Environmental Protection Agency – are scheduled to meet in Rome this weekend.

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Mr. Solheim said it is important to recognize the economic costs of global warming, as well as from other types of pollution that can cause health impacts and the degradation of natural ecosystems.

In a report issued last week, the Winnipeg-based International Institute for Sustainable Development calculated that the known costs of various types of pollution in Canada – including climate change – is $39-billion a year, or roughly $1,100 for every Canadian.

The past two years have been the hottest on record and scientists forecast accelerating warming caused by the buildup of greenhouse gases in the atmosphere will bring extreme storms, intensive hurricanes and devastating droughts, especially in arid regions near the equator. As well, they predict rising sea levels that would flood low-lying coastal areas, from south Florida to Bangladesh, and inundate island states.

All that impact – which can be moderated through a concerted effort to reduce GHGs – carries enormous economic costs, Mr. Solheim noted.

"If you look at the economy, prevention is always much, much, much cheaper than carrying the costs" of adaptation to climate change, he said.

Despite the setback for the Paris accord from Washington, the former Norwegian politician said he remains optimistic that the world will cut GHG emission and avert the most catastrophic impact of climate change.

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"It's clearly a negative decision made by the President of the United States, but the impact will be much less than people tend to believe," he said.

He said other major emitting countries, including China and India, have served notice they won't back away from their commitments, as has Canada which, while a small percentage of global emissions, is among the largest on a per-capita basis.