The combined activities of our enormous population are…

““The combined activities of our enormous population are already producing breathtaking effects. Our planet is only 12,700 kilometers in diameter—about three times the distance between New York and Los Angeles—and we can easily travel halfway around it in less than a day. We have turned much of its land surface into a patchwork of cities, industrial parks, farms, and rangeland. We have laid on this land a web of roads, canals, and pipelines. We have dug out of it hundreds of billions of tons of material, moved this material around, processed it, and dumped it. Our factory ships and trawlers crisscross the world’s oceans to exploit every valuable fishery. Our planes and satellites weave themselves around its sphere. We are moving so much rock and dirt, blocking and diverting so many rivers, converting so many forests to cropland, releasing such huge quantities of heavy metals and organic chemicals into air and water, and generating so much energy, carbon dioxide, methane, and nitrogen compounds that we are perturbing the deepest dynamics of our global ecosystems. Between one-third and one-half of the planet’s land area has been fundamentally transformed by our actions: row-crop agriculture, cities, and industrial areas occupy 10 to 15 percent of Earth’s land surface; 6 to 8 percent has been converted to pasture; and an area the size of France is now submerged under artificial reservoirs. We have driven to extinction a quarter of all bird species. We use more than half of all accessible fresh water. In regions of major human activity, large rivers typically carry three times as much sediment as they did in pre-human times, while small rivers carry eight times the sediment. Along the world’s tropical and subtropical coastlines, our activities—especially the construction of cities, industries, and aquaculture pens—have changed or destroyed 50 percent of mangrove ecosystems, which are vital to the health of coastal fisheries. And about two-thirds of the world’s marine fisheries are either overexploited, depleted, or at their limit of exploitation. The decline of global fish stocks has followed a predictable pattern: like roving predators, we have shifted from one major stock to another as each has reached its maximum productivity and then begun to decline.30””

— Thomas Homer-Dixon