Propelling the movement are signs that the problem is worsening at an accelerating rate. In an article this summer in the Proceedings of the National Academy of Sciences, 16 climate scientists from around the world argued that the planet may be much closer than previously realized to locking in what they call a “hothouse” trajectory—warming of 4C or 5C (7F or 9F), “with serious challenges for the viability of human societies.”