The deal commits all countries signing the pact to cut greenhouse gas emissions, with the goal of reducing the global temperature increase to under 2 degrees Celsius. A rise above that level, scientists say, could be catastrophic.
The deal needed 55 countries that accounted for at least 55 percent of global emissions to adopt it before it could take effect.
They said Europe is too complicated to agree quickly. They said we had too many hoops to jump through. They said we were all talk.
The deal is expected to be approved by the European Parliament next week, giving the agreement the numbers it needs to officially take effect.
"When the going gets tough, Europe gets going," said Miguel Arias Cañete, EU commissioner for climate action and energy.
The United States, the second-largest global producer of greenhouse gases per the EPA, joined the agreement last month. India, the fourth-largest producer, joined on Monday.
Only seven nations in the EU have completed planning for the deal, according to the International Business Times. In terms of emissions, that's enough to make the deal official. Taken as a whole, the EU is the third-largest greenhouse gas producer.
EU officials celebrated the announcement on Twitter.
"Today it happened."
So, how are we going to enforce this, exactly?
Questions remain about how exactly the Paris agreement will be enforced. The terms of the agreement says oversight will be "non-adversarial and non-punitive," Reuters reports.
Robert Watson, a British-American scientist, said the hope is "moral obligation" will compel countries to act.