Without more-ambitious pledges, the world is projected to warm 2.7 degrees Celsius (4.9 degrees Fahrenheit) by the end of the century compared with the end of the 1800s — far above the Paris climate accord’s goal of limiting warming to “well below” 2 degrees Celsius (3.6 Fahrenheit) compared with preindustrial levels, and, if possible, to 1.5 degrees Celsius (2.7 degrees Fahrenheit).