The idea of planet-wide environmental boundaries is gaining ground
PULL a spring, let it go, and it will snap back into shape. Pull it further and yet further and it will go on springing back until, quite suddenly, it won't. What was once a spring has become a useless piece of curly wire. And that, in a nutshell, is what many scientists worry may happen to the Earth if its systems are overstretched like those of an abused spring.
The nine areas of concern were: climate change; ocean acidification; the thinning of the ozone layer; intervention in the nitrogen and phosphate cycles (crucial to plant growth); the conversion of wilderness to farms and cities; extinctions; the build up of chemical pollutants; and the level of particulate pollutants in the atmosphere. For seven of these areas the paper's authors felt confident enough to put numbers on where the boundaries actually lay. For chemicals and particulates, they deferred judgment.