The single most important benchmark underpinning this week’s talks in Paris on climate change—two degrees Celsius—has guided climate-treaty discussions for decades, but scientists are at odds on the relevance of that target.

Many researchers have argued that a rise in the planet’s average global air temperature of two degrees or more above preindustrial levels would usher in catastrophic climate change. But many others, while convinced the planet is warming, say two degrees is a somewhat arbitrary threshold based on tenuous...