问答题 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. 1 And that, in a nutshell, is what many scientists worry may happen to the Each if its systems are overstretched like those of an abused spring.
One result of this worry was the idea of planetary boundaries. In the run-up to that year"s climate conference in Copenhagen, 2 a group of concerned scientists defined in a paper in Nature, what they thought of as a safe operating space for human development—a set of nine limits beyond which people should not push their planet.
The eight areas of concern were: climate change; ocean acidification; the thinning of the ozone layer; intervention in the nitrogen and phosphate cycles; the conversion of wilderness to farms and cities; extinctions; the build up of chemical pollutants; and the level of particulate pollutants in the atmosphere. 3 For seven of these areas the paper"s authors felt confident enough to put numbers on where the boundaries actually lay, but for chemicals and particulates, they deferred judgment.
Planetary boundaries provide a useful way of thinking about environmental change, because in many cases they give scope for further change that has not already happened. But the concept has numerous drawbacks. 4 The actual location of the boundaries is, as their proponents acknowledge, somewhat arbitrary, partly because of the incomplete state of current knowledge, but it may remain so however much anyone knows. Some boundaries might be transgressed without irreversible harm occurring. Some may have been drawn around the wrong things altogether. And some academic opinion holds that spectacular global change could come about without breaking through any of them.
Another problem for the idea of planetary boundaries is the assumption that they are independent of each other. That seems unlikely, and if they are not then a crisis might arise even if no single boundary were transgressed. On June 7th, Nature published a review of evidence which suggested that this may be happening. 5 It suggested that the Earth may be approaching a "tipping point" past which simultaneous changes—to land use, climate and more—driven by an ever larger, ever richer human population, push the system into a very different state, with climate zones changed permanently, ecosystems functioning differently, and so on.