working capital

For millions of years we have known a world whose resources seemed illimitable. However fast we cut down trees, nature unaided would replace them. However many fish we took from the sea, nature would restock it. However much sewage we dumped into the river, nature would purify it, just as she would purify the air, however much smoke and fumes we put into it. Today we have reached the stage of realizing that rivers can be polluted past praying for, that seas can be overfished and the forests must be managed and fostered if they are not to vanish.
But we still retain our primitive optimism about air and water. There will always be enough rain falling from the skies to meet our needs. The air can absorb all the filth we care to put in it. Still less do we worry whether we could ever run short of oxygen. Surely there is air enough to breathe ? Who ever asks where oxygen comes from, to begin with ? They should -- for we now consume about 10 per cent of all the atmospheric oxygen every year, thanks to the many forms of combustion which destroy it: every car, aircraft and power station destroys oxygen in quantities far greater than men consume by breathing.
The fact is, we are just beginning to press up against the limits of the earth's capacity. We begin to have to watch what we are doing to things like water and oxygen, just as we have to watch whether we are overfishing or overfelling. The realization has dawned that earth is a spaceship with strictly limited resources. These resources must, in the long run, be recycled, either by nature or by man. Just as the astronaut's urine is purified to provide drinking water and just as his expired air is regenerated to be breathed anew, so all the earth's resources must be recycled, sooner or later. Up to now, the slow pace of nature's own recycling has served, coupled with the fact that the "working capital" of already recycled material was large. But the margins are getting smaller and if men, in ever larger numbers, are going to require ever larger quantities, the pace of recycling will have to be artificially quickened.
All we have is a narrow band of usable atmosphere, no more than seven miles (11.3 km) high, a thin crust of land, only one-eighth of the surface of which is really suitable for people to live on, and a limited supply of drinkable water, which we continually reuse. And in the earth, a capital of fossil fuels and ores which we steadily run down, billions of times faster than nature restores it. These resources are tied together in a complex set of transactions. The air helps purify the water, the water irrigates the plants, the plants help to renew the air.
We intervene in these transactions. For instance, we cut down the forests which transpire water and oxygen, we build dams and pipelines which limit the movement of animals, we pave the earth and build reservoirs, altering the water cycle. So far, nature has brushed off these injuries as pinpricks. But now we are becoming so strong, so clever and so numerous, that they are beginning to hurt.