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 research, and therefore an impractical spur to policy action. “It emerged from a political agenda, not a scientific analysis,” said Mark Maslin, professor of climatology at University College London. “It’s not a sensible, rational target because the models give you a range of possibilities, not a single answer.” Policy makers tend to assume the two-degree target expresses a solid scientific view, but it doesn’t. The exhaustive reports published […]