The diplomats and world leaders now gathered in Egypt for the annual United Nations climate summit are tasked, in some sense, with holding the global average temperature below 1.5 degrees Celsius of warming. That's one of the key objectives around which the 2015 Paris Agreement was formed, and so it has become a shorthand for the success of every subsequent climate summit.
Talks in Glasgow last year at COP26 ended with the conference leader saying the limit of 1.5C is "alive but its pulse is weak." Ahead of COP27 in Sharm El-Sheikh, UN-backed scientists on the Intergovernmental Panel on Climate Change contributed to the darkening prognosis by projecting that the world is likely to pass the 1.5C mark in the 2030s. Obituaries for 1.5C have followed, even if politicians speaking from the ongoing summit in Egypt haven't quite given it up for dead.