The US and China are expected to ratify the Paris climate change agreement at the G20 summit this weekend.
Only 23 nations have so far formally ratified the deal, which will only enter into force when at least 55 countries representing 55% of greenhouse gas emissions do so.
China and the US, the world’s two biggest contributors, together account for nearly 40% of global emissions.
The deal commits countries to limit global increase in temperatures to “well below” 2C.
The UK’s Labour Party recently urged Prime Minister Theresa May to formally ratify the climate deal before the end of the year following which the government said it is committed to doing so “as soon as possible”.