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.

Trump’s day-one actions on energy come as climate change-fueled fires ravage Southern California, following the globe’s hottest year on record.