• Choose a category
• All
• Classic-Fiction

By Root 750 0

consume resources, including food, timber, water, and energy, harvested over vastly wider areas than the land that they physically occupy, and this greater footprint needs to be considered in any overall assessment to get a true picture. When peasants move to the cities, their land might just as easily be turned over to large-scale cattle ranching or plantations as allowed to revert to forest.72 Studies have suggested that this is particularly the case in Amazonia, where most deforestation is carried out by ranchers, so population density is less clearly linked with the fate of the forest.73 Moreover, even after people move out, the recovery of forests cannot always be left to chance—it needs active management and ecologically friendly government policies.74 Whether secondary forest can help avoid large-scale species extinctions also depends on the extent to which animals and plants accustomed to old-growth forests can successfully recolonize new areas.

But the overall conclusion seems to me irrefutable. Urbanization is good for sustainability because it reduces population growth and concentrates the overall human impact on the land in a smaller area. Handled properly, migration away from rural areas and into cities offers a huge opportunity for ecosystem protection and restoration. Our best hope for meeting the land use planetary boundary is therefore to encourage the trends toward rising prosperity and demographic transition in developing countries, in order to allow their forests and other important natural habitats to survive and regrow. Given the choice, most people around the world already find city life more attractive and varied than that in the countryside. Forget the “back to the land” self-indulgence of some disgruntled people in rich countries. Billions of people want to move to urban areas to achieve increasing prosperity and improve their standard of living. Let us be glad of that. They are unwitting “Greens,” whose efforts at self-improvement should be celebrated.

BOUNDARY FIVE

FRESHWATER

We have polluted the seas and appropriated land from other competing species—but water we have not so much stolen as imprisoned, behind concrete dam walls, within dark reservoirs, and behind the high levees that hem in once-mighty free-flowing rivers like the Mississippi and the Yangtze. Whole natural drainage basins, which once responded to the grand seasonal cycles of winter flood and summer drought, now react meekly to the whims of water managers seated in the control rooms that govern sluice gates in tens of thousands of large dams. The Colorado River may have gouged out the most spectacular cutting in the world—the Grand Canyon—but today the flow of this powerful torrent is as much a product of human hydrological engineering as it is of any natural force. If you live in Las Vegas, Los Angeles, or Phoenix, Arizona, the Colorado is more part of an enormous plumbing works that ends in your shower or bath, as it does for 30 million other people. Its ecological role has declined in tandem, for in an average year hardly a drop of the precious water in this 1,400-mile (2,200-km) river now succeeds in reaching the sea.

To put this in context: Worldwide, 60 percent of the 227 largest rivers have been fragmented by man-made infrastructure, and the total number of dams blocking the natural flow of the planet’s watercourses is estimated at 800,000.1 These impound approximately 10,000 cubic kilometers of water—a quantity so substantial that it measurably reduces the rate of sea level rise (by about half a millimeter a year for the last half-century2) and even changes the mass distribution of the planet sufficiently to alter its axis and slightly increase the speed of its rotation.3 The sheer scale of human engineering activity on rivers has been extraordinary: On average we have constructed two large dams per day over the last fifty years, half of those in China alone.4 Humans have affected the water cycle in less visible ways too: Deforestation and irrigation are altering water-vapor flows over the planet’s surface;5 changes