The United States has joined China in formally ratifying the Paris agreement on climate change, which could see the deal brought into force before the end of the year.
Video report by ITV News China correspondent Debi Edward reports:
The announcement - which has prompted calls for the UK to follow suit - came as leaders from the world's biggest economies, the G20, started to arrive in Hangzhou for a summit on Sunday and Monday.
US President Barack Obama and Chinese President Xi Jinping submitted their plan to join the agreement to UN Secretary-General Ban Ki-moon, who travelled to China to witness the announcement.
The standing committee of China's parliament earlier voted to adopt the "proposal to review and ratify the Paris Agreement" at the closing meeting of a week-long session.
Obama and Xi committed to cooperate on two other global environmental agreements this year.
The first is an amendment to the Montreal Protocol related to air-conditioning in refrigeration and the second aimed to reduce carbon emissions from flying.
Why G20 action is vital in climate change battle
G20 nations are responsible for around 80% of global carbon emissions, with China being the largest, and the US the second largest emitter.
Nearly 200 countries agreed in Paris last December to slash greenhouse emissions and keep global temperature increases "well below" 2C.
While 180 countries have signed the agreement, some 55 nations - who are responsible for at least 55% of global emissions - need to formally ratify the treaty to put it into legal effect.
Praise for US and China
The combined weight of the world's two biggest economies behind the Paris Agreement earned the nations praise from the UN's climate chief.
Campaigners urge the UK to follow suit
There have been calls for countries to ratify the Paris deal as soon as possible, so it can be adopted earlier than the planned 2020 start date, to meet the urgent need to tackle climate change.
Campaigners have urged the UK to follow suit.