President Donald Trump on his first day in office again withdrew the U.S. from a landmark global pact to fight climate change. So what is the Paris Agreement? And what happens to it now?

The Paris agreement is complex and works in a slow bureaucratic manner. It’s a mostly voluntary climate pact originally written in ways that would both try to reduce a worsening climate change problem and withstand the changing political winds in the United States.

When Trump announced the U.S. withdrawal from the climate agreement in 2017, the move reverberated around the globe. Nearly 200 nations had committed to reducing greenhouse gas emissions under the pact when it was created in 2015, and they had set ambitious targets to keep global temperature rise well below 2°C, and ideally below 1.5°C.