China and the US have ratified the world's first comprehensive climate deal.
Here are the key questions.
:: What is the Paris Agreement?
At United Nations talks in Paris last December, the world's first comprehensive climate deal was produced. The Paris Agreement will see action to curb rising temperatures by all countries.
:: How many countries have signed up?
There are 180 signatories to the Paris Agreement but, until today, just 24 states had ratified the deal - accounting for 1.08% of the total global greenhouse gas emissions.
The move by China and the US bolsters the agreement - bringing the total greenhouse gas emissions accounted for by signatories to 39.06%.
:: Why do countries need to "ratify" the deal?
For the agreement to come into force, at least 55 countries representing 55% of the world's climate emissions must ratify the deal to drive down greenhouse gases. In doing so, they make the agreement legally binding.
:: We've known about climate change for decades, why are we only doing something now?
This deal has effectively been 20 years in the making. A first treaty, the Kyoto Protocol - which was adopted in 1997, only covered the emissions of developed countries - and the US never ratified it. It runs out in 2020 and the Paris Agreement will be its successor.
:: Have any previous environmental treaties actually worked?
Yes, though not for as wide-ranging an issue as climate change. The Montreal Protocol, for example, agreed in 1987 and ratified by all UN countries, has been successful in phasing out use of the chemicals which cause the ozone layer to be depleted.