Worldwatch Institute: The world is on the brink of a massive extinction event, according to the United Nations. Rapid releases of greenhouse gas emissions are changing habitats at a rate faster than many of the world’s species can tolerate. “Indeed the world is currently facing a sixth wave of extinctions, mainly as a result of human impacts,” said Achim Steiner, executive director of the U.N. Environment Programme in a statement. A study earlier this year in the Proceedings of the National …