In December 2015, global leaders met in Paris to hash out a climate deal. Negotiated under the aegis of the UN, it aims to hold the increase in the global average temperature to “well below” 2°C above pre-industrial levels. The agreement underpins the COP26 climate talks in Glasgow. At its heart is a “ratchet mechanism” which requires that every five years parties to the agreement come forward with more ambitious national climate goals. As a consequence, COP26 is the most important climate meeting since Paris (it was delayed by a year owing to the pandemic). Annotations below, written by our climate team, explain the Paris agreement’s most significant elements and point to our related coverage.