Some 171 countries signed the Paris climate deal at United Nations headquarters on April 22 – a record number for a new international treaty.
About 15 nations, mainly small island states, had already ratified the agreement.
However, dozens of other countries were required to take this second step before the pact came into force.
UN Secretary-General Ban Ki-moon said: “Paris will shape the lives of all future generations a profound way – it is their future that is at stake.”
Speaking at the opening ceremony, Ban Ki-moon said the planet was experiencing record temperatures: “We are in a race against time I urge all countries to join the agreement at the national level.”
As the world marked the 46th Earth Day, UN climate chief Christiana Figueres explained what now needed to happened.
“Most countries, though not all, need to take the signed document and go back home and go to ratification procedures that in most countries requires parliamentary discussion and decision.”
Even though the US and China represent around 38% of global emissions, getting to the 55% figure will not be that easy.
The EU, which represents just under 10% of global CO2, will take a considerable amount of time as each of the 28 members has to ratify it themselves.
That is unlikely to begin until the EU can agree how much of the carbon cutting each country will have to undertake.
Small island states were upset with this approach.
President Barack Obama is also keen to see the new agreement take effect before he leaves office next January. A little known clause in the treaty means it would take four years if a new leader, less committed to climate action, wanted to take the US out of the agreement.
Other countries are also aware of this and are watching the US election process very closely.
China said it would “finalize domestic procedures” to ratify the Paris Agreement before the G20 summit in China in September.
There is obvious delight here in New York at the record turnout of countries and leaders to sign the agreement. However, some attendees are cautioning that this is merely the first rung on a very difficult ladder.