With China and U.S. joining, some civil society trackers say they are confident the deal could hit the 55 per cent threshold before the end of the year.
On Wednesday, investors managing more than US$13 trillion of assets urged leaders of the Group of Twenty major economies to ratify the deal before the end of December.
The 130 investors also called for the G20 to double global investment in clean energy, develop carbon pricing and phase out fossil fuel subsidies.
"Governments must ratify the Paris agreement swiftly and have a responsibility to implement policies that drive better disclosure of climate risk, curb fossil fuel subsidies and put in place strong pricing signals sufficient to catalyse the significant private sector investment in low carbon solutions," said Stephanie Pfeifer, chief executive at Institutional Investors Group on Climate Change.
Ratification is expected to play out differently in the U.S. compared with China.
While China has "few uncertainties" at home for passing the deal, it could cause controversy within the U.S., according to Liu Shuang, an officer with Energy Foundation's low-carbon development programme.
But the Obama administration's commitment to international frameworks suggests the accord would be passed in a way that would make it difficult for his successors to undo, civil society trackers said.
The two countries started extensive cooperation at the leadership level in 2014. In a joint declaration that year, China promised its emissions would peak before 2030, while the US promised to cut emission by at least 26 per cent. That deal is widely regarded as paving the way for the Paris Agreement.
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