Global leadership is urgently needed to ensure greenhouse gas emissions peak within 10 years, or the future costs of reining them in and adapting to a changing climate will rise dramatically, the head of the United Nations' Intergovernmental Panel on Climate Change said Monday.

The IPCC's Hoesung Lee spoke at an Ottawa session on Canada's climate challenge, where he dodged questions about the Trump administration's opposition to concerted global action on climate change, saying it is not his role to comment on policies of member states.

Based on the panel's scientific work, Dr. Lee said global temperatures are rising at a unprecedented pace, and that concerted action is required to avert the worst impacts of rising sea levels, threats to species and extreme weather events.

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"The important matter is the speed of change," the soft-spoken South Korean economist said in an interview after his speech.

"We know now that the global warming is unprecedented and at the same time, it may be happening much faster than the capacity to adapt. Which means that sooner or later, the concern about cascading risks and the irreversibility in certain ecosystems becomes a real concern."

U.S. President Donald Trump has questioned the science of climate change, famously tweeting at one point that it was a "hoax" perpetrated by the Chinese. The new administrator for the U.S. Environmental Protection Agency, Scott Pruitt, recently questioned whether carbon-dioxide emissions from fossil-fuel use is contributing to climate change, as IPCC reports have concluded.

Earlier this week, Mr. Trump's Energy Secretary, Rick Perry, attended a Group of Seven ministerial meeting, and blocked a proposed statement in which ministers would have reaffirmed their governments' commitment to the Paris accord, which was adopted by 195 countries in December, 2015.

The IPCC and other international bodies have said greenhouse gas emissions need to peak before 2030 – some say by 2020 – to give the world a reasonable chance of limiting average global temperature increases to less than two degrees Celsius, as countries pledged in the Paris agreement.

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"If we miss that early peaking, the implication is that it will be much costlier to achieve that two-degree goal," Dr. Lee said.

"It means temperatures will rise by more than two degrees with consequences attending that increase in temperatures. The probability of extreme weather events occurring would increase, and that should be a concern for policy makers and investors around the world."

Carbon pricing is a "very critical" tool for driving the transition to a lower-carbon economy, though other measures are required, he said. About 25 countries around the world are now adopting some form of carbon pricing, and there are related benefits – including cleaner air and healthier populations – that help offset the costs, he added.

The federal government is planning to introduce legislation later this year that will require provinces to adopt some form of carbon pricing, or have imposed a federal levy with the revenues returned to the provincial government.

Critics worry the Liberal government's determination to proceed with climate-change policies will hurt the Canadian economy, given the Trump administration is headed in the opposite direction.

In a report released Monday, the Conference Board of Canada said the economic cost to Canada of achieving its 2030 target would be manageable, though clearly provinces such as Alberta and Saskatchewan face steeper costs than others.

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"A lot of Canadian businesses are worried about what are the economic consequences of carbon pricing," said Craig Alexander, chief economist at the Conference Board, a non-profit research company. "What is it going to do to our competitiveness?"

He said the overall impact on economic growth and jobs would be negligible, though energy-intensive industries such as chemicals, forest products, and oil and gas would feel a bigger bite.

However, Mr. Alexander said his forecast could not take into account technological change that would occur as a result of putting a price on carbon. He added that there are also longer-term costs to a "business-as-usual" approach that must be considered.