Three years ago, the Eiffel Tower lit up with a written announcement that negotiators had completed the Paris Agreement. This landmark global deal to curb climate change includes pledges from nearly every country on earth, rich and poor alike, to control human-caused climate change. But the deal is not legally binding, so countries have latitude in whether and to what extent they fulfill their promises. Now, on the heels of dire scientific reports, negotiators are meeting in Poland to build global confidence that countries are actually taking the actions they pledged to in the Paris Agreement. At this year’s United Nations climate conference, being held Dec. 2–14 in the southern Poland city of Katowice, negotiators are drawing up a rule book. Governments are to follow it as they measure and report on their greenhouse gas emissions, allowing others to verify the numbers. Negotiators hope that such transparency and data will