The US and China have officially agreed to ratify the Paris climate treaty.
US President Barack Obama and Chinese President Xi Jinping announced the deal during the G20 summit in Hangzhou.
President Jinping called on other nations to do the same before the end of the year.
Last year, negotiators from 179 countries and the EU agreed to limit temperature rise to below 2°C.
However only 23 nations have given their final approval. At least 55 countries or those accounting for 55% of emissions need to ratify the agreement before it can go into effect.
The move from the world’s two biggest polluters is likely to accelerate this – China accounts for around 25% of global emissions and the US is responsible for 15%.
President Obama said: “This is not a fight that any one country, no matter how powerful, can take alone. Someday we may see this as the moment that we finally decided to save our planet.”
President Jinping added: “Our response to climate change bears on the future of our people and the well-being of mankind.”