What Is This Quadratic Extrapolation?

No one has any idea what the actual damages are at, let's say, 5 or 6 degrees centigrade of global average warming. What happens at relatively low levels? No one really believes it's a straight line where we just keep adding sort of a bit of damages. The warmer it gets once you hit certain thresholds, the higher the costs of any additional warming. Hence not a straight line but one that curves slightly up.