Scientific American: Five years go an impressive, international group of scientists unveiled nine biological and environmental boundaries that humankind should not cross in order to keep the earth a livable place. To its peril, the world had already crossed three of those safe limits: too much carbon dioxide in the atmosphere, too rapid a rate of species loss and too much pouring of nitrogen into rivers and oceansprimarily in the form of fertilizer runoff. Now we have succeeded in transgressing a fourth limit: the...