Humans have converted more than 40 percent of the earth's land to cities or farms. Roads and structures fragment most of the rest.
Humans appropriate more than half the world's fresh water. Ancient aquifers in the world's bread baskets, including the Ogallala in the Great Plains, are being drained.
Only 2 percent of major U.S. rivers run unimpeded. California's Sacramento-San Joaquin River Delta has been entirely re-engineered. The last time the Colorado River reached the Sea of Cortez was in 1998. The Nile, Indus and Ganges rivers have been reduced to a trickle.
Humans surpass nature as a source of nitrogen emissions, altering the planet's nitrogen cycle.
A quarter of known mammal species, 43 percent of amphibians, 29 percent of reptiles and 14 percent of birds are threatened. African elephants may be extinct within a decade.
A third of world fisheries are exhausted or degraded. Forty percent of coral reefs and a third of mangroves have been destroyed or degraded. Most species of predator fish are in decline.
Ocean acidification, a product of fossil fuel burning, is dissolving calcifying plankton at the base of the food chain.
A garbage gyre at least twice the size of Texas swirls in the Pacific Ocean.
"We're changing the ability of the planet to provide food and water," Harte said.
Even scientists who doubt ecological collapse, such as Michele Marvier, chair of environmental studies at Santa Clara University, acknowledge that "humans dominate every flux and cycle of the planet's ecology and geochemistry."
Water and food
In December, the Interior Department said by mid-century the Colorado River will not support demand from the seven states it supplies, including California. The main reason is expected population growth from 40 million to as many as 76 million people.
Among the remedies considered: towing icebergs from the Arctic to Southern California.
"Phoenix continues to grow at one of the highest rates in the country," said Jerry Karnas, population and sustainability director of the Center for Biological Diversity, the only national environmental group campaigning to limit population growth. "There is no discussion about what the future Phoenix is going to do when the Colorado River is done."
Ecosystems can endure large stresses. But multiple stresses can act synergistically.
Take food. The World Resources Institute, an environmental think tank, estimates that by mid-century the world will need 70 percent more food, because as people grow wealthier they eat more meat, requiring more grain to feed livestock.
That will require converting more land to crops, even as urbanization destroys prime farmland. Farms are a big source of deforestation and a big emitter of greenhouse gases that cause climate change. Climate change reduces yields by increasing the frequency of droughts and floods. Lower yields will require conversion of more land to farms.
Still, nature has shown great resiliency, said Santa Clara University's Marvier. Peregrine falcons nest in San Francisco skyscrapers. Coyotes roam Chicago.
"We can't just continue dumping nitrogen into the ocean at the same rate and expect everything to be fine," Marvier said. "The good news, though, is that when we do clean up our act, we tend to see some pretty amazing bounce back."
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