The U.S. and China formally joined the landmark Paris climate agreement on Saturday, a momentous step toward global emissions reductions that Canada has yet to take, though Justin Trudeau continues to boast about his government's environmental leadership

Presidents Barack Obama and Xi Jinping pledged their nations to the historic deal on the eve of talks between the Group of Twenty nations, midway through a scorching year likely to break records as the hottest in modern human history.

"This is the single best chance that we have to deal with a problem that could end up transforming this planet in a way that makes it very difficult for us to deal with all the other challenges that we may face," Mr. Obama said in Hangzhou, where China is hosting the G20 summit for the first time.

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Twenty-six countries representing nearly 40 per cent of global emissions have now joined the agreement. The U.S. and China account for nearly all of that. To come into effect, 55 nations together responsible for at least 55 per cent of global emissions must formally join.

The accord is now on track to enter into force much earlier than 2020, as was originally envisioned.

The most sweeping climate deal ever achieved, the Paris agreement is a legally-binding document signed by 195 nations to keep global temperatures from exceeding 2 per cent above pre-industrial levels.

Average temperatures had risen 0.85 degrees by 2012. Climate scientists at the Met Office in the UK have predicted 2016 temperatures will rise to 1.1 degrees above the pre-industrial threshold.

The Paris agreement is one "on which rests the opportunity for a sustainable future for every nation and every person," Patricia Espinosa, the UN's climate chief, said in a statement. "The earlier that Paris is ratified and implemented in full, the more secure that future will become."

The U.S. president called other nations to follow his and Beijing's lead.

"We hope that our willingness to work together on this issue will inspire greater ambition and greater action around the world," Mr. Obama said.

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Hours earlier, however, Canada's prime minister signalled he is not yet ready to ratify the deal.

Speaking to reporters in Hangzhou, Mr. Trudeau said his government had done "exceptional work" to help attain global consensus on the deal late last year.

But he did not commit to immediate ratification.

"Our commitment to ratifying that deal early was made, and is continuing to be a hot item on the agenda for us," he said.

Mr. Trudeau's government initially pledged rapid movement, with a promise to meet provinces and territories within 90 days of the Paris agreement signing. That meeting took place in March.

Ottawa has since sought to work with premiers to fashion a Canada-wide plan for combatting climate change. Earlier this year, sources told the Globe and Mail the Trudeau government wanted to reach a deal within six months of the March meeting.

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But Mr. Trudeau has encountered resistance from places like Saskatchewan, whose premier Brad Wall has threatened to take Ottawa to court if it imposes a carbon price on provincial utilities. Territorial leaders have also said their constituents already pay too high a cost for energy, and cannot bear any increases.

Data obtained by the Globe this summer showed that if nothing changes, current federal and provincial climate policies will allow carbon emissions to soar 38 per cent over than the national target for 2030.

Mr. Trudeau on Saturday acknowledged more work remains to be done before Canada can ratify the Paris agreement.

"I look forward to having, continuing to have, extremely productive conversations with my provincial counterparts as we demonstrate Canada's genuine and deep leadership on the climate change file," he said.

In China, meanwhile, a slowing economy and a slow transformation away from heavy industry have prompted some observers to suggest its emissions have already begun to decline.

In a speech Saturday, Mr. Xi pledged to cut carbon dioxide emissions per unit of GDP by 18 per cent in the next five years.

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"Our response to climate change bears on the future of our people and the wellbeing of mankind," he said, according to the Associated Press.

China's leadership sees common purpose in battling emissions and sparking new forms of economic growth.

"In promoting green development we also aim to address climate change and overcapacity," Mr. Xi said in an earlier speech Saturday.

He added: "green mountains and clear water are as good as mountains of gold and silver."