National Geographic: For more than a decade international climate-policy discussions have revolved around a seemingly simple goal: Limit the rise in average global surface temperature to two degrees Celsius (3.6 degrees Fahrenheit). But a new paper argues that the two-degree target is not only increasingly unrealistic but also misleading. "More and more, it's a combination of fantasy and irrelevance," says David Victor, a professor of international relations at the University of California, San Diego, and the co-author...