Leaders and diplomats from more than 160 countries are gathering at the United Nations' New York headquarters on April 22 to sign the Paris Agreement – the landmark climate deal hammered out at the culmination of last year’s talks.

The ceremony marks the start of a year-long opportunity for countries to sign the agreement, although most of the world will sign on the opening day. But the process doesn’t end there – nations will still need to ratify the treaty domestically. Only when at least 55 countries, accounting for at least 55% of global greenhouse emissions, have done so will the Paris deal become international law.